Kawalan Penyakit Cili

Pepper Disease Control -It Starts with the Seed
Thomas A. ZitterProfessor, Department of Plant PathologyCornell University, Ithaca, NY 14853

Plant diseases can be a limiting factor in pepper production wherever the crop is grown. Moisture in the form of wind blown rain, saturated soils and high humidity plays a major role in the occurrence of both bacterial and fungal diseases. Insects that attack pepper serve to create wounds favorable for bacterial soft rot and spread several virus diseases. Clean seed, greenhouse sanitation, crop rotation, and cultural measures in the field are all key components for disease control, but it all starts with the seed! This is especially true for the first disease discussed, bacterial leaf spot. All major seed companies are incorporating disease resistance into most released varieties with emphasis placed on bacterial leaf spot, Phytophthora blight, and assorted virus diseases.

Bacteria
Bacterial Leaf Spot (BLS)

Bacterial leaf spot is caused by two major groups of bacteria, Xanthomonas campestris pv. vesicatoria and Xanthomonas vesicatoria (some literature will also mention Xanthomonas axonopodis pv. vesicatoria). A number of races occur for each of these pathogens, with some occurring more commonly on pepper and others on tomato. Both bacteria are gram-negative rods, have a single polar flagellum used for mobility, and are found only in association with plants or plant materials. The BLS pathogens are seedborne, both within the seed and on the seed surface. BLS may also survive on plant debris in the soil for 1-2 years, therefore a 2-year rotation out of pepper and tomato is essential.

Seed can be treated with hot water (122°F for 25 minutes) or with Clorox® (EPA Reg. No. 5813-1; label available from Clorox at 800-446-4686). Hot water is more effective for controlling bacteria on and within seed, but hot water can adversely affect germination if not properly performed (see ref. 3). Treating the seed yourself nullifies the seed company's liability and voids their guarantees. Mix 1 quart of Clorox® bleach (calcium hypochlorite) with 4 quarts of water to treat up to 1 pound of seed in a cheesecloth bag, add ½ tsp. of surfactant (dishwashing detergent), and submerge in the solution with agitation for 40 minutes, rinse under running tap water for 5 min, and dry seed thoroughly. Treated seed should be dusted with Thiram 75W [dithiocarbamate] (1 tsp. per pound of seed), and planted soon after treatment.
Some varieties currently have resistance to all three races of BLS (BLSR1, 2, 3) that commonly occur in our area. These include Boynton Bell, Aristotle, Commandant, Enterprise, Revolution, X3R Camelot, and X3R Wizard. King Arthur is resistant to race 2 and Admiral is resistant to races 1 and 2. Resistance to races 1 and 3 are most important for the Northeast.

Use of disease-free seed and a 2-year rotation in the field should solve most of the BLS problems, but some persistent cases may require chemical treatments. Streptomycin (Agri-Mycin 17, Agri-Strep) sprays (1 lb per 100 gallons or 1 ¼ tsp per gallon) may be applied to transplants prior to transplanting. In the field, applying fixed copper (1 lb active ingredient per acre) plus maneb (1 ½ lb 80WP per acre) has been shown to reduce the spread of BLS.

Bacterial Soft Rot (BSR)

Bacterial soft rot is caused primarily by Erwinia carotovora subsp. carotovora. The bacterium is commonly associated with plants, soils and surface water, and thus is a common contaminant. BSR is primarily a post-harvest problem except when fruit are injured in the field by insect feeding. The European corn borer larvae tunnel under the calyx (cap), and their entry holes are marked by sawdust-like frass. Insecticide treatments should coincide with peaks in adult activity as determined by pheromone or light traps. Registered insecticides include cyfluthrin (Baythroid 2), esfenvalerate (Asana XL), permethrin (Ambush), and spinosad (SpinTor 2SC). Hot pepper varieties are most resistant to larval feeding, while green bell peppers are most susceptible.

Post-harvest wash water can spread the bacterium from contaminated to healthy fruit, therefore most peppers are packed dry to minimize BSR. If wash water is used, maintaining 25 ppm chlorine in the wash water (1 TBS of Clorox®, 5.25% sodium hypochlorite, per 8 gallons of water). Make sure that the wash water is not cooler than the fruit temperature, or bacteria will move into the fruit or stem end.

Oomycetes
Pythium Damping Off (also caused by Phytophthora spp., and Rhizoctonia solani)

Growing media can be a source of various soil-borne fungi, so care must be exercised in selection of the appropriate media and attention paid to characteristics that will allow the growing media to remain moist but not continually wet. Addition of soil amendments that contribute to suppression of soil-borne pathogens can be considered. SoilGard 12G, containing the naturally occurring fungus Gliocladium virens is known to be antagonistic to fungi such as Pythium and Rhizoctonia, two of the more common fungi responsible for damping off.

Pythium Root Rot

Pythium root rot generally occurs after peppers are transplanted in polyethylene mulch/drip irrigation culture. Cultural practices that contribute to Pythium root rot are planting in low areas of the field, overwatering in an attempt to reduce wilting, and planting into beds with fresh plant material (cover crop, weeds, etc.) before microbial breakdown of the plant material has occurred. The infected roots of infected plants will appear brown rather than white, and the cortical tissue of the main affected roots can easily be removed from the central steele with a finger nail. After removing transplants from the greenhouse and prior to transplanting, plants can be drenched with the systemic fungicide mefenoxam (Group 4 fungicide) (Ridomil Gold 4E or Ultra Flourish 2E). Apply Ridomil 4E at 0.75 fl.oz. /2,000 ft2/100 gallons of water or Ultra Flourish 2E at 1.5 fl.oz. /200ft2/100 gallons of water.

Phytophthora Crown Rot and Aerial Blight

Phytophthora blight can be one of the most serious diseases affecting pepper as well as eggplants, tomatoes, and the entire cucurbit family. Because it affects such a wide range of vegetables, growers are challenged to develop adequate rotational strategies. Consequently, control must depend upon cultural, chemical and selection of resistant varieties when available. Phytophthora blight is caused by the soil borne oomycete Phytophthora capsici. The disease can be divided into two distinct phases, a crown rot phase and an aerial blight phase.

In the crown rot phase of the disease, a black girdling lesion occurs at the soil line. In some plants the lower tissue of the wilted plants must be removed to expose the girdling lesion in the cortical tissue beneath the epidermis. Most cases of the crown rot phase occur in July and August in the lower areas of the field and from there the disease can spread to adjoining areas of the field. Phytophthora is considered a weather event disease, meaning that heavy rainfall (in excess of 2 inches) leading to saturated soils is critical for infections to occur. Generally soil temperatures are > 65°F and air temperatures are in the range of 75-85°F.

The aerial phase of Phytophthora blight occurs later in the season as the spores produced on the lesions of plants infected in the crown rot phase are spread by heavy, wind driven rains. These typically occur following a tropical storm or hurricane, another major weather event. Infection occurs at the axil of a branch and stem with a 2-3 inch black, girdling lesion developing on the stem. All of the leaves on the branch above the lesion will wilt and eventually the entire plant dies.

Cultural control measures aim to mitigate the affects of the weather events mentioned above. Avoid planting in low-lying areas of the field that are prone to standing water following rain events. Raised and dome shaped beds without depressions in the top will allow for speedy movement of moisture away from the crown region of the plants. Provide drainage at the end of the field to allow excess water to flow out of the fields. When crown rot infected plants occur in the field, remove infected plants to avoid production of spores leading to the aerial phase of the disease.

Chemical control measures may be necessary to augment the cultural practices mentioned above. This is especially true in fields with a history of Phytophthora blight and that are likely to experience saturated soils following heavy rains. The fungicide mefenoxam (Group 4 fungicide) (Ridomil Gold 4E, Ultra Flourish 2E) can be applied as a banded spray over the row shortly after transplanting or it can be injected through the drip irrigation system to protect against the crown rot phase of the disease. Mefenoxam needs to be reapplied twice at 30-day intervals after the transplant application. Two weeks after the last application of mefenoxam, begin foliar applications of a fixed copper fungicide with a spreader sticker to provide protection against the foliar phase of the disease. Tanos (a mixture of famoxadone [Group 11] and cymoxanil [Group 27]) is also labeled for peppers. For best results tank mix Tanos with a copper fungicide, and for resistance management do not make more than one application of this mixture before alternating with a fungicide with a different mode of action.

Resistant varieties are being developed to reduce the incidence of Phytophthora blight in pepper. Resistance genes are required for both the crown rot and aerial phases of the disease, and these must be bred into commercially acceptable varieties. The varieties 'Emerald Isle' and 'Reinger' possess resistance to the crown rot phase of Phytophthora, but do not possess sufficient horticultural characteristics to be commercially acceptable. The variety 'Paladin' has excellent resistance to the crown rot phase of Phytophthora but does not provide sufficient resistance toward the aerial phase. The variety 'Aristotle' provides only tolerance to the crown rot phase and like 'Paladin' has insufficient resistance for the aerial phase. Both 'Paladin' and 'Aristotle' do have excellent horticultural characteristics similar to the variety 'Camelot'. One occasional flaw in both 'Paladin' and 'Aristotle', and possibly related to Phytophthora resistance, is the development of a 'silvering' pattern on the fruit. 'Paladin' also develops fine shoulder cracks when allowed to mature to the red stage, and is therefore not recommended for the red fruit market. Additional Phytophthora tolerant hybrids include 'Conquest' and 'Revolution'.

Fungi
White Mold

White mold is caused by the soil borne fungus Sclerotinia sclerotiorum. Many vegetable crops are susceptible to this fungus, although corn and grasses are not. Leading susceptible crops include tomato, cabbage, lettuce, carrot, celery, snap bean, several cucurbits, and of course pepper. The pathogen produces hard, black sclerotia, like small, flattened and elongated raisins which serve as the overwintering means for the fungus. These sclerotia, which can survive in the soil for years, may be produced inside of the stems or on the surface of affected areas. Sclerotia germinate at an optimal temperature of 52°F; Sclerotinia is a low-temperature fungus, able to cause infection from 32-82°F. The fungus also requires abundant moisture for a week or longer for infection to occur. Sclerotia germinate to produce slender stalks that end in an apothecia (cup-shaped structure in which asci and ascospores are produced) or they may germinate by mycelium in some Sclerotia species. Although ascospores are short lived, they are blown within a field, landing on senescent or injured susceptible tissue and penetrate directly. In pepper, infections occur on stems or in the axil of branches.

Pepper growers in western NY lost 5% of their pepper crop due to white mold infections during the cool and wet growing conditions for summer 2003. Rotation out of pepper and not using other susceptible crops in rotational scheduling will be critical for next season and into the foreseeable future. Mycoparasites are known to destroy existing sclerotia and inhibit the development of new sclerotia. The commercial product Contans WG (Coniothyrium minitans, EPA Reg. No. 7244-1, and OMRI listed) has shown great promise in significantly reducing sclerotial populations. The product needs to be applied to the soil prior to planting (1-4 lb/A), and once applied, incorporated into the top 2 inches. If incorporation will be greater than 2 inches, then the application rate should be increased to 2-6 lb/A.

Anthracnose

Anthracnose, also known as ripe fruit disease, is potentially caused by three species of the fungus Colletotrichum: C. coccodes, C. capsici, and C. gloeosporioides. Although most commonly seen on maturing hot and sweet peppers, under appropriate conditions infections can occur on immature fruit, stems, and even leaves. Infections appear as sunken lesions on the fruit. The lesions may turn black with the formation of setae and sclerotia, or the center of the lesion may develop pustules (acervuli) that contain a salmon-colored spore mass. Colletotrichum typically produces microsclerotia that allows the fungus to overwinter in the soil. Microsclerotia can survive for many years, but even a 2 or 3-year rotation out of susceptible crops (mainly solanaceous) can significantly reduce inoculum.

For late maturing red peppers the following fungicides are registered: maneb (Group M3), 7DTH; Quadris and Cabrio (both Group 11 fungicides), 0DTH.

Viruses
Cucumber Mosaic Virus (cucumovirus, aphid transmitted, not seed transmitted in pepper, many weed hosts)

Cucumber mosaic virus (CMV) is the most common virus infecting peppers in the Northeast. The virus can infect more than 800 plant species worldwide. CMV is readily transmitted from perennial weeds by aphids in a nonpersistent method. It is often the earliest virus transmitted in the spring. Important weed hosts include common milkweed (Perennial), common chickweed (Winter Annual, but can become perennialized in cool moist areas, also CMV is seedborne in this species), marsh yellow cress (A, Biennial, short-lived P), and yellow rocket (Win A, Bie) and more (3, a more complete list is provided). As aphid populations develop on peppers during the spring and summer, extensive spread may occur. Pepper plants on the edge of fields and rows are frequently the first plants to be infected.

Destroy important weeds before the crop is established in the field. Intercropping with corn or other nonsusceptible tall barrier crops have been used keep virus from invading the crop. Rouging infected plants especially from the ends of rows before secondary spread occurs may be helpful. Because of the nonpersistent manner of transmission, control of aphids to prevent spread within the crop is not an option. Inheritance of resistance to CMV is very complex, so it is doubtful there are of any truly CMV-resistant peppers.

Tobacco Mosaic Virus (tobamovirus, mechanical transmission, seed transmitted, solanaceous weed hosts)

TMV is worldwide in distribution and can readily be transmitted by physical contact. No insect vectors are known. TMV is one of the most stable plant viruses, capable of surviving on dried plant debris and roots of tomato and probably pepper for many years. It is known to be seedborne in pepper and tomato. Although the natural host range of TMV is wide, it is primarily a problem for solanaceous crops (pepper and tomato).

Sanitation is important for the control of TMV. This is particularly true in greenhouse settings where the virus has been diagnosed previously. Dispose of all plant material including roots. Sanitize all flats and bench surfaces with a strong disinfectant prior to establishing a new crop and make sure the greenhouse and surrounding areas are free of weeds that may harbor the virus. Some key perennial weed species include marsh yellowcress (Rorippa islandica), broadleaf plantain (Plantago major), horsenettle (Solanum carolinense), and smooth (Physalis subglabrata) and clammy groundcherry (P. heterophylla), to name a few (3). Because TMV is seedborne in pepper and other solanaceous crops, make sure to purchase disease-free seed from a reputable seed company. If seed is of questionable quality, the seed should be soaked for 30 minutes in a 10% solution of household bleach or for 15 minutes in a 10% solution of trisodium phosphate (Na3P04), often used to soften dried paint brushes. Either of these treatments will remove most virus from the surface, unless the virus is in the seed endosperm. Recently released varieties have moderate to high tolerance to some strains of TMV.

Tomato Spotted Wilt Virus (tospovirus, thrips transmission, not seed transmitted, many weed hosts)

Tomato spotted wilt virus (TSWV) causes brown spotting or dark ringspots on foliage and fruit, and stunting and distortion of the young growth of pepper plants. TSWV is transmitted by at least 8 species of thrips, with the tobacco thrips (Frankliniella fusca) and western flower thrips (F. occidentalis) considered to be the most important vectors. Thrips acquire TSWV by feeding on infected plants only as larvae. After a latent period of 3-7 days, they are then able to transmit the virus to uninfected plants for the remainder of their lives. TSWV has a host range in excess of 600 plant species, but many of these plants do not support thrips reproduction and are considered 'dead ends' for virus spread.

A recent survey of the role of weed hosts for TSWV and the tobacco thrips in North Caroline concluded that key weeds included mouseear (P) and common chickweed (Win A, but can become perennialized in cool, moist areas), spiny sowthistle (A), dandelion (P), blackseed plantain (P), and a buttercup species (A) (3). Sanitation around greenhouses is essential as well as growing vegetable transplants in a greenhouse separate from ornamentals that commonly serve as reservoirs. There is no cure for infected plants, which should be removed from the greenhouse or the field as soon as they are detected. SpinTor (spinosad) has been one of the most effective controls for thrips on labeled crops (such as tomatoes and peppers) and applications on peppers for European corn borer will also provide incidental control of thrips present.

References
1. Compendium of Pepper Diseases. 2003. Ed. K. Pernezny, P. D. Roberts, J. F. Murphy, and N. P. Goldberg. APS Press, St. Paul, MN. 63pp.
2. Northeast Pepper Integrated Pest Management (IPM) Manual. 2001. Ed. T. Jude Boucher and Richard A. Ashley. University of Connecticut, Cooperative Extension System.136pp.
3. Vegetable MD online web site: http://vegetablemdonline.ppath.cornell.edu/ for selected fact sheets, news articles (ie. Managing Bacterial Leaf Spot in Pepper), and images.
4. Fungicide Resistance Action Committee site for Fungicide Groups: http://www.frac.info/publications/FRACCODE_sept2002.pdf

Sumber: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/PepDisease_Con.htm

Penyakit Tanaman

PLANT DISEASES CAUSED BY VIRUSES

Plant viruses consist of a nucleoprotein that multiplies only in the living cells of a host. The presence of viruses in host cells often results in disease.

400 or more viruses are known to attack plants (2000 viruses are described for plants, animals, bacteria, etc.). viruses are generally specific, what infects a plant does not cause disease in an animal, and vice versa.

The first record of a disease that was later found to be caused by a plant virus was on tulips in the 17th century in the Netherlands.

First experimental demonstration of the infectious nature of viral disease was recorded by Lawrence, who described the transmission of a disease of jasmine by grafting.

Adolf Mayer (1886) described a disease of tobacco called mosaikkranheit (tobacco mosaic). Disease could be transmitted to healthy plants with sap from diseased plants.

Dmitrii Iwanowski (1892) demonstrated that the agent in tobacco mosaic was filterable. He demonstrated that the causal agent of tobacco mosaic could pass through a filter that retains bacteria.

1898 Martinus Beijerinck - demonstrated that the causal agent was not a microorganism but a contagium vivum fluidum (contagious living fluid). He was the first to use the term virus, which is the Latin word for poison. He concluded that this was not a toxin, because repeated inoculations of diluted infected sap yielded similar amounts of disease as it was passed from one plant to another. If it had been a toxin, it would eventually be diluted away.

Loefler and Frosch (1898) described the first filterable infectious agent in animals - the foot-and-mouth disease virus and Walter Reed (1900) - described the first human virus, yellow fever virus.

In 1929, F. O. Holmes provided a tool by which the virus could be measured by showing that the amount of virus present in a plant sample preparation is proportional to the number of local lesions produced on appropriate host plant leaves rubbed with the contaminated sap.

1935 W. M. Stanley isolated and purified some tiny white crystals from leaves of mosaic-infected tobacco plants. He treated healthy plants with TMV, which had been precipitated out of infected tobacco juice with the help of ammonium sulfate and a technique he had developed. The healthy plants contracted tobacco mosaic disease. Due to the high protein content of the purified virus particles, he concluded that the virus was an autocatalytic protein that could multiply within living cells. Although his conclusions were later proved incorrect, Stanley's work merited him receiving the Nobel Prize. He won the Nobel Prize in chemistry in 1946 for this work.

1937 - Bawden and Pirie demonstrated that virus consists of protein and nucleic acid (RNA).
1939 - Kausche - saw virus particles for the first time with the electron microscope.
1955-1960's Much was learned by various workers, regarding the infectivity of viral (TMV) RNA and the structure and arrangement of viral (TMV) coat protein.

1971 - T. O. Diener discovered viroids, which only consist of nucleic acids. Smaller than viruses, caused potato spindle tuber disease (250-400 bases long of single-stranded circular molecule of infectious RNA). About a dozen other viroids that cause disease in a variety of plants have been isolated. No viroids have ever been found in animals.

1980- Cauliflower mosaic virus, whose genome is a circular double-stranded DNA chromosome, was the first plant virus for which the exact sequence of all its 8,000 base pairs was determined.

In 1982, the complete sequence of the bases in the single-stranded tobacco mosaic virus RNA was determined, as were those of smaller viral RNA and of viroids.

1986 - Use of transgenic plants to obtain resistance against viruses (TMV).

VIRUS DISEASES OF PLANTS ARE USUALLY DESCRIPTIVE OF THE TYPE OF SYMPTOMS THAT THESE CAUSE IN THE HOST

For example, the symptoms of specific plant diseases form the basis for the following disease names: tobacco mosaic, turnip crinkle, barley yellow dwarf, ring spot of watermelon, cucumber mosaic, spotted wilt of tomato.

Some viruses have a broader host range than the name of disease or virus may imply. For example, tobacco mosaic virus (TMV) infects tomato, eggplant, peppers, in addition to tobacco.

PROPERTIES AND MORPHOLOGY OF PLANT VIRUSES

noncellular, ultramicroscopic particles, that multiply only in living cells. very, very small! (size measured in nanometers).

most plant viruses consist of protein shells surrounded by a core of positive-stranded nucleic acid (normally ssRNA - nucleotides (guanine, uracil, cytosine, adenine) + 5 carbon sugar called ribose + a phosphate group), but sometimes these viruses contain dsRNA or dsDNA (2 strands of nucleotides with thymine substituted for uracil and deoxyribose instead of ribose).

5-40% of virus is nucleic acid 60-95% is protein

Protein coats or shells can be different shapes, but are normally rod, filamentous, isometric, quasi-isometric/bacilliform or variants of these structures. See Figure 14-25 on page 501 in handout. For example, Tobacco Mosaic and Barley Stripe Mosaic viruses are rods, while broad bean wilt and maize chlorotic dwarf viruses are isometric or more spherical in shape.

VIRUS GENOME

Minimum number of genes in a plant RNA virus could be two: a coat protein and an RNA replicase gene (as is the case with RNA phages). Evidence indicates there are usually 3-5 gene products.

Plant positive-stranded RNA viruses frequently possess divided genomes (refer to Figure 14-4 in handout). In addition, viral genomes are separately encapsulated. Viral genomes consisting of two or three different nucleic acid components, all required for infection are called bipartite, tripartite, or multipartite viruses. More than a single species of genomic RNA. Refer to pages 271-273 in textbook.

Multipartite viruses are potentially at an evolutionary disadvantage. Infectivity dilution curve for Alfalfa mosaic virus (requiring B, M, Tb particles for infectivity) is steeper than for tobacco necrosis virus (single particle). Partition of genome could potentially hinder transmission or infection by a virus.

SATELLITE VIRUSES AND RNAs

Kasinis in 1962, described the first satellite viruses. These viruses are serologically unrelated to their helpers and the two genomes exhibit little if any sequence similarity. Satellite viruses are dependent for its replication on the presence of a second, independently replicating virus.
Satellite RNAs have no coat protein of their own and are encapsulated with the help of other viral RNAs.

TRANSMISSION

Mechanical transmission through sap by plants touching one another, through root grafts, and manhandling.

Vegetative propagation and grafting.
Seed, pollen, mites, nematodes, dodder, fungi (carried by zoospores and mycelium) and insects (aphids, leafhoppers, scale insects, thrips, grasshoppers, beetles, whiteflies). For example, cucumber mosaic virus and barley yellow dwarf virus moved by aphids.

DETECTION OF PLANT VIRUSES

Due to the inability to observe plant viruses visually by observing them directly through the light microscope, virologists must resort to the following methods of detecting their presence and in diagnoses.
1. Ability to transmit disease via plant sap by rubbing plant, grafting, dodder or insect transmission.
2. Indexing - indicator plants - sensitive to specific virus and will react a certain way if exposed..
3. Visual inspection with EM.
4. By eliminating possibility that symptoms are not due to other sources (e.g., herbicide, nutritional deficiencies.
5. Serological Tests (ELISA - enzyme-linked immuno sorbent assay). Refer to Figure 14-24; page 499 in the handout.

Indirect (virus + Ab virus + Enzyme conjugated Ab) and direct (double-antibody sandwich technique) (Ab virus + virus + Enzyme-conjugated Ab).
1. Virus or Ab virus added to well and these become attached to walls.

2. Antibody or virus added to well and these attach to their counterpart (i.e., antigen to antibody).

3. Second antibody with enzyme conjugate attaches to first antibody/virus complex.

4. Substrate is catalyzed by enzyme and this causes a color change. ELISA tests are extremely sensitive (small amounts of antisera are needed) results are quantitative, large samples can be run at same time (96 well plates), results can be gathered in a few hours instead of days. ELISAs along with serial dilutions of plant sap and applications of this to the leaves of susceptible hosts (by counting the number of lesions) can be used to quantify the amount of virus present.

MANAGEMENT

Milk inactivates many viruses - use milk to wash tools/hands. "Milk does a plant body good!" Soap and water work well too!

Removing diseased plants, killing and removing potential virus vectors (primarily weeds and insects).

disease-resistant cultivars.
disease or virus free seed, roots or tubers.
cross protection (inoculation with a less-virulent strain of a virus protects the plant from a more virulent strain later when exposed to it).

heat (some viruses are killed at temperatures that will not kill host). For example, dormant propagative organs dipped in hot water (35 C) for few minutes or hours, or by growing plants in greenhouse at 35-40 C for several days, weeks or months may inactivate virus.

TOBACCO MOSAIC

Caused by Tobacco Mosaic Virus (TMV) worldwide distribution primarily infects tobacco and tomato, but more than 350 species are susceptible.
tobacco leaves become mottled with light and dark green areas; leave become distorted, puckering or blistering, especially areas of new growth.
stunting of plant growth. in tomato, mottling of leaves occurs and leaflets become long and pointed.

TMV is a rod-shaped particle which are 300 nm long by 15-18 nm in diameter. It possesses ssRNA and a protein coat. difficult to inactivate, and can survive for 5 years in dead, dried tissues and many months in living plant tissues. many strains, that vary in virulence from severe to mild symptoms. virus is spread from plant to plant through injuries caused by crop worker, contaminated equipment and chewing insects.

virus overwinters in dead plant tissues and debris, on contaminated equipment, in contaminated soil, greenhouse containers, bedding, tools, and in living hosts, including weeds like horsenettle, Solanum carolinense, and other crop plants (tomato, pepper, and eggplant).

Management of Tobacco Mosaic Disease
use virus-free seed (tomato seed can by treated with acid or bleach)
transplant in noninfested soil
fumigatation with methyl bromide or heated.
no chewing of tobacco or smoking around seedbeds or in greenhouses.
to eliminate spreading of virus wash hand with soap and water or milk.
spraying plants with milk (whole or skim) seems to help reduce
infections. crop rotation with nonhost crops (corn, rice, other cereal grains).
resistant cultivars

Tobacco Mosaic Virus: The Prototype Plant Virus

The stability of the TMV virus particle accounts for its having been the first virus to be identified, purified to homogeneity, and then biochemically and biologically characterized.

Small coat protein subunits (capsomeres) aggregate to form a helical protein coat or capsid (see Fig 2.10 and 2.11 on pages 46 and 47). The virus particle contains an axial channel that is 4 nm wide and the viral RNA lies within a groove in the surrounding protein helix. The nucleic acid core is not in the axial channel, but passes about halfway between the interior channel and the exterior surface of the rod. The overall particle is rod-shaped, narrow, and rigid. The pitch of the helix is 2.3 nm, and each turn contains 16 1/3 coat protein molecules. A full-length virion contains 130 helical turns.
TMV particle is resistant to nucleases and proteolytic enzymes. TMV particles will fall apart in both alkaline and acid solutions. Denaturation is often reversible, as long as temperature and pH are not too extreme. Removal of the denaturant allows the native structure of the viral protein to re-form and near its isoelectric point (pH 4 to 6), the TMV coat protein aggregates to form rod-shaped particles that look exactly like TMV virions.

When virus is subjected to neutral pH with either detergents (e.g., SDS) or 6 M urea or by extraction with phenol then RNA can be extracted in an intact form. When isolated TMV RNA are added to native TMV protein, these form stable "reconstituted" virus, which is more stable (stable from pH 3 to 9) then protein alone (unstable below pH of 4 and above pH 6).

Protein and RNA are more infectious than naked RNA alone (nearly 1000 times the amount of naked RNA is required to cause infection).
Proof that the viral RNA was the sole determinant of tobacco mosaic disease was obtained by a mixed reconstitution of RNA from Holmes ribgrass mosaic virus (RMV) with the protein subunits from TMV. Reconstituted virus caused localized lesions on plants instead of a systemic infection and formed new RMV virus (RMV RNA + protein coat containing histidine and methionine - not found in TMV).

Assembly of Helical Viruses
Aggregates of 33 protein molecules form the double disk. This combines with viral RNA. Attachment of the nucleic acid to the protein aggregate begins at the origin of assembly site (OAS) about 800 nucleotides from the 3' terminus of TMV common strain RNA. Rod growth toward the 5' terminus of the viral RNA is rapid, involving addition of double disks; encapsidation of the 3' terminus proceeds more slowly, through the addition of A protein monomers or small aggregates. Refer to Figure 6.6 in the textbook or to Figure 2.14 on page 50 in handout #2). Cotranslation disassembly - the protein coat is displaced at the 5' end by ribosomes in host cell.

TMV RNA 3' TERMINUS
3' end of TMV RNA ends with the sequence -C-C-C-A and can be charged with an amino acid (histidine). This region is non-coding and be folded into a tRNA-like structure preceded by a series of four pseudoknots. Why? Four possibilities exist.
Donating an amino acid during some stage of protein synthesis.
Facilitating translation by disrupting base pairing between the 3' and 5' - terminal regions of the viral RNA
Acting as a recognition site for the viral replicase to initiate negative-strand synthesis
A molecular fossil from the original RNA world where tRNA-like structures tagged RNAs for replication and prevented the uncontrolled loss of nucleotides from third 3' terminus. Subgenomic mRNAs and translational read-through in TMV replication.

Five open reading frames or ORFs are found in the genome of TMV. Subgenomic mRNAs and translational read-through are two strategies employed by TMV to regulate gene expression.Plant positive-sense RNA viruses have developed several other mechanisms to facilitate and/or regulate the expression of individual genes. 5 strategies of regulating gene expression.

CUCUMBER MOSAIC

wide host rang, including banana, bean, celery, crucifers, cucumbers, gladiolus, lilies, melons petunias, spinach, squash, tomatoes, and zinnias.

symptoms resemble those of tobacco mosaic and it is difficult to distinguish between the two diseases
Cucumber mosaic virus (CMV) is a polyhedral virus - particles are 30 nm diameter. transmitted mechanically by rubbing and by aphids. overwinters in many weeds and crop plants. Control and Management
resistant cultivars of cucumber, muskmelon, spinach, and tobacco.
reduce aphid populations by eliminate weeds and spraying with aphicides.
erect barriers between cucumbers and inoculum source (i.e. row of sunflowers).
when working with plants wash hands with milk.
aluminum strips between rows reflect UV light, which acts a repellent to aphids.

BARLEY YELLOW DWARF
Barley Yellow dwarf disease infects barley, oats, rye, and wheat.
losses on oats as high as 50 %; 30% on what and barley.
yellowing and dwarfing of leaves, stunting of plants, reduced root system, reduced grain production. leaves may turn red or bronze in color = "red leaf"
BYDV is a polyhedral virus 25 nm in diameter.
not mechanically transmissible.
not seed borne.
does not over winter in plant debris or soil.
survives only in living plant tissue (crop or wild grasses) and bodies of aphids (different species are involved, including the oat-bird cherry aphid and the English grain aphid). Symptoms appear 3-6 weeks after infection by feeding aphids. Management of Barley Yellow Dwarf Disease
avoid planting small grains near large grassy areas - act as source of virus.
insecticides are not that useful.
do not plant oats or barley near end of the normal seeding period.
Subviral Pathogens and Other Virus-like Infectious Agents
Among subviral pathogens, only viroids and prions replicate independently; the prion particle, lacks a genomic nucleic acid.
Satellite viruses and satellite RNAs contain conventional nucleic acid genomes, but their replication is dependent on the presence of a helper virus.
Satellite viruses and RNAs do not exhibit substantial sequence homology with their helper viruses. Satellite Viruses
Satellite viruses were first observed in plants. Their replication is dependent on the presence of a helper virus that provides the replicase, but the satellite virus is not required for helper virus replication. There is little or no sequence similarity between the genomes of satellite viruses and those of their helper viruses. Satellite viruses generally produce their own capsid protein.
Example: The helper virus for satellite tobacco necrosis virus (STNV - a 18 nm particle), is tobacco necrosis virus (TNV - a small 30 nm icosahedral virus). STNV contains a monocistronic mRNA for the synthesis of its 22kDa coat protein. Note that satellite viruses are generally smaller than helper viruses.
STNV is an obligatory parasite of its helper, dependent for its replication on the presence of TNV in the cells it enters.
Specificity of this dependence is illustrated by (1) the inability of plant viruses other than TNV to act as helper and (2) variation in the ability of certain TNV strains to support the replication of different strains of STNV. The presence of STNV greatly suppresses the replication of TNV.
Satellite RNAs

Cucumber mosaic virus (an epiphytotic of lethal necrotic disease among tomato plants in France in 1972). This was unusual because of the severity and atypical nature of the symptoms associated with infections of tomato plants (See description of cucumber mosaic in prior text). Enhanced response of tomatoes to this disease were associated with host used for virus propagation; virus grown in tobacco or tomato yielded an enhanced necrotic response, those propagated in cucurbit hosts yielded a reduce necrotic response. RNA extractions of necrotic tomato plants revealed the presence of variable amounts of a small RNA species in addition to the expected three genomic and one subgenomic RNAs. This 5th RNA species was designated CARNA 5 (i.e., CMV-associated RNA 5 or CMV satRNA), a satellite RNA.

When CARNA 5 is present with CMV, symptoms of the disease are enhanced in tomato plants, while in tabasco pepper the presence of CARNA 5 tends to attenuate the disease symptoms caused by CMV. Other factors including sequence changes in the satellite RNA, use of a different helper virus strain, and changes in environmental conditions appear to greatly enhance or suppress symptoms caused by the helper virus.

Genome Structure of Satellite RNAs
Satellite RNAs have been found associated with members of five different plant virus groups. With the exception of TCV (turnip crinkle virus) RNA C, satellite RNAs exhibit only limited sequence homology within the genomes of their respective helper viruses. Satellite RNAs appear to from two size classes - large that are similar in size to the genomes of satellite viruses and much smaller.

VIROIDS
Viroids are the smallest known agents of infectious disease - small (246-375) nucleotides), highly structured, single-stranded RNA molecules lacking both a protein capsid and detectable messenger RNA activity.

The first viroid disease to be studied was potato spindle tuber disease. Disease was first recognized and described by Schulz and Fosom in 1923, but it wasn't until Diener demonstrated in 1971, that the fundamental differences between the structure and properties of its causative agent, potato spindle tuber viroid (PSTVd). Vd for viroid.
Viroids exist in vivo as nonencapsidated, low-molecular-weight RNAs;
Infected tissues do not contain virus-like particles;
Only a single species of low-molecular-weight RNA is required for infectivity;
Viroids do not code for any proteins;

Despite their small size, viroids are replicated autonomously in susceptible cells and no helper virus is required.
Single-stranded viroid RNAs are resistance to digestion by ribonucleases and possess a high degree of thermal stability (not easily denatured). The genome of viroids, such as PSTVd are organized into a series of short double helices and small internal loops which form the basis for five domains. Conserved central domain (highly conserved and site where cleavage and ligation to form circular progeny occur), pathogenicity domain (modulate symptom expression), variable domain (greatest sequence variability among otherwise closely related viroids), and two terminal domains (replication and evolution).

GENETIC ENGINEERING OF PLANTS: THE CONTROL & USE OF PLANT VIRUSES
Transformation - introduction of a new gene into a plant via some mechanism (i.e., Agrobacterium-mediated gene transfer, bombardment, electroporation, plant virus-mediated gene transfer). Transgenic plants are produce by:
Addition of new genes (to express a new gene product).
Suppression of a gene or a gene product (in the plant - e.g., against ethylene gas production to prevent senescence of fruits or against a potential plant virus - e.g., TMV). Potential benefits of transgenic plants
Genetic engineering can produce plants that are:
able to synthesize oils, starches, and plastics
able to synthesize enzymes for food processing and other uses
more nutritious foods (e.g., plants with a higher protein content, and wider profile of essential amino acids - methionine-rich beans or lysine-rich corn)
able to fix nitrogen for growth
freeze resistant
pest resistant
herbicide resistant
disease resistant. In general, transformed plants that express a viral coat protein are resistant to infection by both that virus and related viruses. Viral coat protein may interfere with viral uncoating of the invading pathogen. Coat protein-mediated resistance now available shows a lot of promise in inferring genetic resistance to various viral pathogens, including TMV. Other promising strategies being investigated include the expression of antisense viral RNAs (which interfere with viral replication) and viral satellite RNAs, and noncoat viral genes (producing high resistance to specific strains).

Potential problems
Allergies to transformed plant products.
Accidental movement of novel genes into wild relatives of cultivated plants.
Consumer resistance to using genetically-modified plant products, especially food.
Ethical and moral considerations. (e.g., releasing transgenic marijuana plants that are poisonous, exploitation of genetic resources for personal gain). Strategies for introducing novel genes into plants

1. Agrobacterium-mediated gene transfer
Agrobacterium tumefaciens is the causal agent for crown gall in wide range of host plants. It enters through wounds and injuries and causes a localized region of uncontrolled cell division (a tumor or gall) on the plant. The bacterial cell contains a Ti or Tumor-inducing plasmid. The Ti plasmid contains genes that call for the production of opines (C and N source for the bacterium) and regulate cytokinin and auxin production in plants (causing hyperplasia - excessive cell division = tumor).
Ti genes removed by using restriction enzymes
Target gene/gene for antibiotic resistance are introduced
Modified Ti plasmid introduced back into A. tumefaciens
Bacteria allowed to infect desired host (usually a tissue culture of the host)
Bacteria incorporates its DNA into host
Antibiotics added to growing callus to (a) eliminate the bacterium and (b) to select against untransformed cells

Callus in tissue culture used to regenerate whole plant 2. Bombardment
DNA-coated particles shot into plant tissue using a microprojectile gun (gun powder charge).
Electrical-acceleration of DNA-coated particles.
Final Product: Population of transformed/untransformed cells. Transformed cells are selected and through tissue culture work a new plant is generated. 3. Electroporation
Protoplasts (cell wall removed with enzymes) or partially-digested apical meristems using enzymes.
Sudden electrical discharge opens up membrane and DNA is allowed to enter.
Final Product: Population of transformed/untransformed cells. Transformed cells are selected and through tissue culture work a new plant is generated. 4. Plant virus-mediated transformation
Tobacco could be infected and the product harvested from altered plants.
‘Geneware' and ‘Pharming'.
The coat protein genes of brome mosaic and tobacco mosaic viruses were replaced by bacterial chloramphenicol acetyltransferase. Such viruses are unable to systemically invade the host plant, and have only limited potential as expression vectors. In the case of TMV, this problem is solved by adding the foreign gene to the viral genome rather than substituting for one of the normal viral genes. Trial runs indicate that expression levels achieved are much lower than those of TMV coat protein. Insertion of such genes at different locations in the viral genome or a satellite-like virus associated with helper viruses may increase these yields.
Mycoviruses and the Biological Control of Chestnut Blight
Mycoviruses are probably widespread in fungi in nature, despite the fact that relatively few have been isolated and characterized, and still fewer have been experimentally transmitted to test fungi and demonstrated to be infectious. A survey of 50 isolates of Rhizoctonia solani from the field, 49 were found to have RNA resembling that of mycoviruses.
Typical mycovirus particles are isometric in shape and have a diameter between 25-48 nanometers. Some mycoviruses are rod-shaped or have some other form. This nucleic acid core usually consists of dsRNA and infrequently contains dsDNA. Some mycoviruses appear to be multipartite. Strains of Penicillium chrysogenum, from which the antibiotic penicillin is commercially produced appears to be infected by a mycovirus. Mycoviruses appear to be transmitted by means of cytoplasm and through spores. Mycoviruses appear to be retained in the cytoplasm of a fungal strain indefinitely.
Many mycoviruses do not cause symptoms in their hosts, despite the fact that cells may harbor large numbers of virus particles. Viral infection in the edible mushroom Agaricus brunnescens causes a degeneration of the mycelium and development of malformed basidiocarps, resulting in a reduction of yield.

Chestnut Blight and Hypovirulence
Introduced in New York City in 1904. Destroyed practically all American chestnut trees throughout natural range in eastern third of the U. S. From Canada to nearly the Gulf of Mexico.
50% of overall value of eastern hardwood timber stands destroyed.
Causal Agent is Cryphonectria parasitica.
Fungus found in North America, Europe and Asia.
Fungus penetrates bark and stems though wounds and grows into inner bark and cambium.
Canker (swollen or sunken) forms and bark becomes covered by pimple-like pycnidia (asexual spores called conidia are contained in an ooze and spread by birds, insects and splashing rain drops) and perithecia (ascospores - shot directly into air).
Cankers eventually girdle the tree killing it. Doesn't kill the root system so shoots continue to come up years later only to become infect and die.
Elimination of American Chestnut (along with habitat destruction and hunting) contributed to the extinction of the passenger pigeon.
In Europe, several strains were found that have reduced virulence (hypovirulence)
Hypovirulent strains contain dsRNA enclosed in a membraneous vesicle. Infected tree can often throw-off an infection and cankers can heal in time if fungus is hypovirulent.
To transmit dsRNA to virulent strains is accomplished by bringing a hypovirulent strain in contact with a virulent strain - hyphal anastomosis or fusion transfers the "virus".
Problem with widespread application of this biocontrol technique is that somatic or vegetative incompatibility often prevents movement of contagious agent from one fungal colony to another.
Only members of the same VCG or anastomosis group can transfer the dsRNA virus.
Sandra Anagnostakis - Connecticut Ag. Expt. Station has spearheaded much of this work.
Michael Milgroom, Cornell Univ. "Population biology of chestnut blight fungus".
Recent efforts to circumvent the problems of vegetative incompatibility and infer hypovirulence to other species of fungi have sought to incorporate the viral genome directly into the genome of the fungus. "European Hypovirulence and construction of transgenic fungal strains for biological control of chestnut blight". D. L. Nuss of the Univ. Of Maryland. This way the viral genome would spread throughout the population through sexual recombination and vegetative compatibility.

This page was assembled by Martin J. Huss, who can be reached at mhuss@astate.edu.Last revised on: October 31, 2002. SUmber dari : http://www.clt.astate.edu/dgilmore/Virology/plant_viruses.htm

Penyakit Tobacco Mosaic Virus

Tomato-Tobacco Mosaic Virus Disease
F. L. Pfleger and R. J. ZeyenPlant Pathology

The plant disease caused by tobacco mosaic virus is found worldwide. The virus is known to infect more than 150 types of herbaceous, dicotyledonous plants including many vegetables, flowers, and weeds. Infection by tobacco mosaic virus causes serious losses on several crops including tomatoes, peppers, and many ornamentals. Tobacco mosaic virus is one of the most common causes of virus diseases of plants in Minnesota.

Many viruses produce mosaic-like symptoms on plants. Mosaic-like symptoms are characterized by intermingled patches of normal and light green or yellowish colors on the leaves of infected plants. Tobacco mosaic damages the leaves, flowers, and fruit and causes stunting of the plant. The virus almost never kills plants but lowers the quality and quantity of the crop, particularly when the plants are infected while young.

Virus-infected plants often are confused with plants affected by herbicide or air pollution damage, mineral deficiencies, and other plant diseases. Positive identification of tobacco mosaic virus in infected plants often requires the services of a plant pathologist and the use of an electron microscope. Although it may take a plant pathologist to diagnose tobacco mosaic virus in many ornamental plants, the majority of tomato plants showing mosaic symptoms usually are infected by tobacco mosaic virus.

Common Plant Hosts

In Minnesota, common plant hosts for the mosaic virus are tomato, pepper, petunia, snapdragon, delphinium, and marigold. Tobacco mosaic virus also has been reported to a lesser extent in muskmelon, cucumber, squash, spinach, celosia, impatiens, ground cherry, phlox, zinnia, certain types of ivy, plantain, night shade, and jimson weed. Although tobacco mosaic virus may infect many other types of plants, it generally is restricted to plants that are grown in seedbeds and transplanted or plants that are handled frequently.

Symptoms

In tomatoes, the foliage shows mosaic (mottled) areas with alternating yellowish and dark green areas. Leaves are sometimes fern-like in appearance and sharply pointed. Infections of young plants reduce fruit set and occasionally cause blemishes and distortions of the fruit. The dark green areas of the mottle often appear thicker and somewhat elevated giving the leaves a blister-like appearance. Symptoms on other plant hosts include various degrees of chlorosis, curling, mottling mosaic, dwarfing, distortion, and blistering of the leaves. Many times the entire plant is dwarfed and flowers are discolored. Symptoms can be influenced by temperature, light conditions, nutritional factors, and water stress.

Disease Cycle

Viruses differ from fungi and bacteria in that they do not produce spores or other structures capable of penetrating plant parts. Since viruses have no active methods of entering plant cells, they must rely upon mechanically caused wounds, vegetative propagation of plants, grafting, seed, pollen, and being carried on the mouth parts of chewing insects. Tobacco mosaic virus is most commonly introduced into plants through small wounds caused by handling and by insects chewing on plant parts.

The most common sources of virus inoculum for tobacco mosaic virus are the debris of infected plants that remains in the soil and certain infected tobacco products that contaminate workers hands. Cigars, cigarettes, and pipe tobaccos can be infected with tobacco mosaic virus. Handling these smoking materials contaminates the hands, and subsequent handling of plants results in a transmission of the virus. Therefore, do not smoke while handling or transplanting plants.

Once the virus enters the host, it begins to multiply by inducing host cells to form more virus. Viruses do not cause disease by consuming or killing cells but rather by taking over the metabolic cell processes, resulting in abnormal cell functioning. Abnormal metabolic functions of infected cells are expressed as mosaic and other symptoms as previously described. Infected plants serve as reservoirs for the virus and the virus can be transmitted easily (either mechanically or by insects) to healthy plants.

Control

Unlike fungicidal chemicals used to control fungal diseases, to date there are no efficient chemical treatments that protect plant parts from virus infection. Additionally, there are no known chemical treatments used under field conditions that eliminate viral infections from plant tissues once they do occur. Practically speaking, plants infected by viruses remain so. Thus, control of tobacco mosaic virus is primarily focused on reducing and eliminating sources of the virus and limiting the spread by insects. Tobacco mosaic virus is the most persistent plant virus known. It has been known to survive up to 50 years in dried plant parts. Therefore, sanitation is the single most important practice in controlling tobacco mosaic virus.

Control for Seedling Growers and Gardeners

The most common method of transferring the virus from plant to plant is on contaminated hands and tools. Workers who transplant seedlings should refrain from smoking during transplanting and wash their hands frequently and thoroughly with soap and water. Tools used in transplanting can be placed in boiling water for 5 minutes and then washed with a strong soap or detergent solution. Dipping tools in household bleach is not effective for virus decontamination. Any seedlings that appear to have mosaic symptoms or are stunted and distorted should be removed and destroyed. After removing diseased plants, never handle healthy plants without washing hands and decontaminating tools used to remove diseased plants.

Persons purchasing small tomato plants for transplanting should beware of any plants showing mottling, dwarfing, or stunting. Avoid the purchase of any affected plant. Gardeners are advised to follow the same procedures recommended for greenhouse workers when handling tomato transplants. Other control methods for home gardeners include roguing (removal of diseased plants), destruction of diseased and infected plants, and control of weeds and chewing insects. When roguing and destroying mature diseased plants from the home garden, be sure to wash hands and decontaminate any tools used in the process before contacting healthy plants.

Control for Commercial Producers

Commercial greenhouse producers of tomatoes should follow control practices for seedling production as stated above. It is essential for commercial growers to constantly inspect and rogue diseased production plants while the plants are in the seedling stage. An experienced individual, who is familiar with the tobacco mosaic virus symptoms, should do the initial inspection.

Roguing of young production plants is recommended and should take place before workers are allowed to prune or tie up production plants. When removing diseased plants, also remove one plant on either side of the diseased one. The reason for this is that it is almost impossible to remove a diseased plant and not contaminate the healthy adjacent plants. Never attempt to transplant a healthy tomato into the soil from which a diseased plant was removed. Roots from diseased plants will remain in the soil and provide the virus inoculum for the new transplant.

As a matter of routine, soils from which production plants have been removed, following harvest, should be steam sterilized before the introduction of new seedlings. Steam sterilization can be accomplished by steam or air-steam mixtures. In the preparation of soil for steam sterilization, sift it to remove clumps and large pieces of organic matter. The total soil mixture will have to be heated to a temperature of 200° F for 40 minutes. Since high temperatures are required, steam sterilization must be done in an enclosed system. Temperatures within the steam sterilization system should be monitored by high temperature thermometers to make sure the desired temperature has been reached. Steam sterilization of soil also will eliminate fungi, insects, nematodes, and weeds from the soil. Steam sterilization also is recommended for gravel mixtures used in hydroponic operations following the same procedure described above.

Grow individual production plants in separate containers so that the soil or growing media can be removed when roguing infected production plants. Remember that the soil harbors old root tissues that may serve as inoculum when new roots are introduced. Growing production plants in separate containers is also useful for the control of root diseases caused by fungi and bacteria.

Sumber dipetik dari : http://www.extension.umn.edu/distribution/horticulture/DG1168.html