Managing the Major Diseases Transplant Diseases General information on the successful production of good tobacco transplants is found in Chapter 4, "Transplant Production in the Float System." The following section addresses only certain disease problems that may occur in plant beds in North Carolina. Diseases in Greenhouses. The most common diseases in greenhouses are caused by Rhizoctonia, Sclerotinia (collar rot), Pythium, and bacterial soft rot (Erwinia). Rhizoctonia causes most of the damping-off observed before clipping begins, and Sclerotinia causes the most after clipping. Damping-off caused by Pythium is preceded by extensive yellowing of the plants. TMV is rare, but devastating where it occurs. Growers must be diligent in their sanitation practices. This is especially true if TMV was observed by layby in fields. Because mowers can spread mosaic virus and bacteria, wash and sanitize blades and the underside of the deck with 50 percent household bleach before each clipping of each greenhouse. Furthermore, be sure clipping debris is thoroughly removed (usually by vacuum) by the mower. Clipping too much of the plant in one pass or allowing mower bags to fill too full causes more debris to fall back into the trays. Leaf debris in the trays or on the plants is usually the starting point for collar rot and bacterial soft rot. Used float trays are a documented source of Rhizoctonia. Before using them, thoroughly wash the trays and allow them to dry. Then fumigate the trays with methyl bromide at 3 pounds per 1,000 cubic feet. Do not fumigate inside a greenhouse. Trays may be stacked, criss-crossed up to 5 feet high, tarped and sealed on concrete or on a tarp, then fumigated. See the label for space fumigation. Allow at least 48 hours of aeration before filling with media. Growers who know greenhouse transplants were a source of mosaic should dispose of the trays and purchase new ones. Do not depend on dipping trays in any sanitation product, including bleach, to kill pathogens satisfactorily. Steaming trays at 160℉ to 175℉ for 30 minutes is an excellent sanitation practice. Other precautions should be taken to prevent soilborne pathogens from being introduced into the greenhouse. Most importantly, never dump plants or used media within 100 yards of a greenhouse. Once diseased plants have been dumped, they may serve as a collar rot source for up to five years. Walkways and entryways should be of gravel, asphalt, concrete, or other material that can be easily washed. Boots worn outside the structure should not be worn inside unless they have been sanitized with a 10 percent bleach solution. Use special care in preventing field soil from contaminating water beds in float systems. Also, do not recycle pond water among beds because it can be a source of disease inoculum. Excessive and sloppy watering, poor drainage, plant injury, overcrowding, and excessive humidity most often lead to disease problems in greenhouses. Use only media produced for tobacco transplants. Do not introduce tobacco products into the greenhouse. Do not allow weeds, especially horsenettle, to grow in the greenhouse. Environment plays a large role in disease development. Greenhouses should be fully ventilated when temperatures are not cold enough to damage the plants. Furthermore, fans placed just above the plant canopy to circulate air around the structure, polytubes, or other power ventilators that remove humidity from the greenhouse help reduce leaf moisture and subsequent disease. Pythium is most damaging at pH levels above 6.1 and at float water temperatures above 68℉. To keep water temperatures cool as long as possible, do not fill the bays with water until it is time to float the trays. Closing greenhouses during July or August to allow temperatures to reach 140℉ for eight hours per day for seven days helps kill pathogens. Heat-sensitive items should be removed, and adequate moisture should be maintained in the house. Tobacco should not be grown for any reason during a three-month period between October and February to ensure that blue mold, especially a Ridomil-resistant strain, does not overwinter. Spray Dithane DF weekly after plants reach the size of a quarter to help prevent blue mold. Field Diseases The following sections present general information about some of the most common or recently discovered diseases. Diseases are listed in alphabetical order. A condensed disease-management field guide begins at the end of this chapter. Black Shank Black shank is caused by a soil-inhabiting fungus (Phytophthora parasitica var. nicotianae) that belongs to a group of the most destructive fungi that attack plants. These fungi thrive in high-moisture areas. The black shank fungus produces three types of spores, including a swimming spore that infects tobacco roots and sometimes infects stalk stems at leaf scars (where leaves fall off). Some leaf infection can be observed after rains that splash soil onto the leaves. The symptoms of black shank are well-known to tobacco growers. Once infection occurs, death usually follows quickly. The disease is characterized by rapid wilting of all the leaves and a black lesion that extends from ground level up the stalk. In highly resistant varieties, the symptoms on the stalks are usually confined to near-ground level. When stalks are split, the pith often appears blackened and separated into discrete discs. Discing can occur because of other factors; likewise, not all plants suffering from this disease exhibit this symptom. Rotation, varietal resistance, and chemicals are usually integrated into a management program (Table 9-5). NC 71, NC 72, NC 297, Speight 168, and other highly resistant varieties are highly resistant to Race 0, but their resistance to Race 1 may be much lower. Continuous planting of these varieties crop after crop can result in a shifting from Race 0 population to a Race 1 population. If you planted one of these varieties in the last tobacco crop and observed some black shank, plant one of the older resistant varieties such as K 346 or K 394. Because some fields may already have some Race 1, use other control practices in conjunction with these resistant varieties. Blue Mold Blue Mold is caused by an airborne fungus (Peronospora tabacina), and it caused widespread losses in North Carolina during 1979 and 1980. During those years, the disease occurred in fields as well as in plant beds. The fungus is also spread by shipping infected seedlings. Its occurrence was sporadic until 1995, when it became widespread again. Ridomil-insensitive strains were first identified in North Carolina flue-cured tobacco in 1995. The disease was very widespread in 2001, most seriously affecting some counties in the western coastal plain. To prevent these problems this year, all greenhouses should be treated with Dithane Rainshield (0.5 lb/100 gal spray) every week after plants are the size of a quarter. The foliar infection is characterized by the development of round, yellow spots with gray or bluish-gray mold on the undersides of the leaves. These spots rapidly multiply in a favorable environment and coalesce to kill entire leaves. Old spots are tan to white. When systemic, the fungus penetrates the plant, interfering with normal plant growth and resulting in stunting, distortion, and eventual death. Either type of infection can cause severe losses under certain environmental conditions (usually high moisture and cool temperatures). Because air currents disperse this fungus, crop rotation and stalk and root destruction do not affect this disease in North Carolina. The fungus does not overwinter in North Carolina, so we do not know if future infestations will be sensitive to Ridomil Gold or Ultra Flourish. It is likely that some blue mold will be sensitive, and Ridomil Gold application will be of some benefit. Acrobat MZ, foliar-applied protectant fungicides, or Actigard are needed for Ridomil-insensitive blue mold. Forecasting Blue Mold (by C. E. Main). Blue mold causes sudden, widespread, and fast-moving epidemics that usually spread from south to north. The disease is spread by airborne spores blowing from infected fields and plant beds. During cool, wet, and cloudy weather, the disease can double in an infected field every four days. Blue mold is not known to survive through the winter north of Florida. Initial outbreaks in the United States originate from airborne spores from winter tobacco crops in Cuba, Mexico, or Latin America. Wild tobacco plants (Nicotiana species) growing as weeds in the southwestern United States can also serve as a source of airborne inoculum. The North American Plant Disease Forecast Center at NC State issues forecasts every Monday, Wednesday, and Friday, and more often if necessary, from March through August. The forecasts are based upon daily occurrence reports from blue mold cooperators in tobacco-producing states in the United States, Mexico, and Canada. Meteorological surface wind models are used to generate reports of favorable weather conditions and of regional weather, as well as the outlook for new outbreaks (high, medium, or low risk). Once spores arrive and infect the leaves, yellow lesions appear 7 to 10 days later during the latent period. Blue mold forecast maps of spore trajectories show the source of spores, the pathway the spores will follow in the wind, and the risk of infection, all based upon true forecasts for the next 48 hours. This provides growers with two days' warning should they decide to apply protectant fungicides, which must be applied before the spores germinate on the leaves. The forecasts, plus additional information on the disease and control recommendations, are available on the World Wide Web at the following address: http://www.ces.ncsu.edu/depts/pp/bluemold. You can also check blue mold forecasts by telephone, toll-free, at 1-800-662-7301 (code 2913 for flue-cured). Your North American Plant Disease Forecast Center welcomes the participation of all growers, Extension agents, and industry in making this forecast system work. Your suggestions are always welcome. Contact C. E. Main at [email protected]. Brown Spot Brown spot is caused by an airborne fungus (Alternaria spp.). It may be considered an "opportunistic" disease-causing agent. It does not usually become a problem in varieties tolerant to this disease if good cultural practices are followed. However, during periods of extended rainfall late in the harvest season, it can become destructive. Brown spot is a disease of senescent (old) tissue. Fusarium Wilt Fusarium wilt, although not destructive in all parts of the state, is significant in certain areas. It is caused by a fungus that lives in the soil (Fusarium oxysporum f. sp. nicotianae) and is well adapted for survival there. It can live well on decaying organic matter in the soil and can form spores that are very resistant to adverse conditions. Fusarium wilt is not as aggressive as some other diseases, such as Granville wilt or black shank, but it might also be considered an "opportunistic" disease. If tobacco plants are stressed in certain ways, such as by root wounding or nematode infection, significant fusarium wilt may develop. Although crop rotation and stalk and root destruction are beneficial to some extent, these practices do not drastically reduce fusarium wilt development because of the fungus' ability to live on organic matter and form resistant spores. Granville Wilt Granville wilt appears first as a wilting on one side of the plant. As the disease progresses, the entire plant wilts and dies. When plants survive they are usually stunted, and their leaves may be twisted and distorted. The stalk usually becomes dark, especially at the ground level. At this stage, Granville wilt may be easily confused with other diseases such as black shank. A diagnostic characteristic of Granville wilt is streaks that extend up the stalk just beneath the outer bark. Granville wilt is caused by a tiny bacterium (Pseudomonas solanacearum or Ralstonia solanacearum) that inhabits the soil. These bacteria are microscopic and can move microscopic distances. Infection occurs through wounds or openings in the root system. Hence, cultivation and nematode damage can increase the incidence of this disease. Also, roots may "wound themselves" as they grow through the soil. Therefore, Granville wilt bacteria usually have no difficulty locating a suitable entry point into the plant. It is important to remember that Granville wilt bacteria are soil inhabitors. In fact, anything that moves soil containing the bacteria will spread them from place to place. This can happen in many ways: by moving soil on machinery and other equipment, by water washing soil from one part of the field to another, by moving transplants with infested soil around the roots, and by any other means by which infested soil is moved. Relatively high soil temperatures and adequate-to-high moisture levels in the soil favor Granville wilt bacteria. In fact, wet seasons greatly increase infection by these organisms. Infection may not be noticed immediately because wilting symptoms may not appear until plants undergo a moisture stress. Thus, it is not unusual to observe symptoms of Granville wilt several weeks after infection actually occurs. The Granville wilt bacteria also can infect tomatoes, white potatoes, pepper, eggplant, and peanuts. Ragweed, common to most of North Carolina, can be infected, too, and should be controlled. See Table 9-6 for management recommendations. Cultural 1. Rotate with fescue, small grain, or soybeans. Control weeds. 2. Use varieties with high levels of resistance. 3. Destroy stalks and roots immediately after harvest. 4. Avoid root wounding. 5. Manage nematodes. 6. Fumigate in the fall or spring with one of the following treatments. Fumigants - Allow three weeks from application to transplanting. Chemical Rate (gal/A) Method Relative Control Ratinga Chloropicrin 5-6 Broadcast Very Good Terr-O-Gas 67 6-9 Row Very Good Chloropicrin 3 Row Good Telone C-17 10.5 Row Good Telone C-17 13-15 Broadcast Good a Actual control varies depending on other control practices and environmental conditions. Hollow Stalk (Soft Rot) Hollow stalk or soft rot (caused by Erwinia spp.) usually appears first near topping and suckering time. It may begin at any stem wound and is often seen in the pith at the break made by topping. Soon after infection, a rapid browning of the pith develops, followed by general soft rot and collapse of the tissue. Top leaves often wilt, and the infection spreads downward; the leaves droop and hang down or fall off, leaving the stalk bare. Diseased areas may appear as black bands or stripes that may girdle the stalk. Another phase of the disease appears as a soft decay at the junction where leaf petioles are attached to the stalk. Causal bacteria are usually present in soil and on plant surfaces. They may also be present on workers' hands as they top, sucker, or harvest the crop. These bacteria are often unimportant unless there is frequent rainfall and high humidity. These conditions favor their infection and subsequent development. The use of some contact sucker control agents may lead to an increase in hollow stalk, especially if leaf axil tissue is damaged. Remember that if affected leaves are harvested when wet and carried to the barn, they often develop barn rot during curing. Infection is most likely if ventilation is inadequate. Target Spot Target spot (Rhizoctonia) has been prevalent in North Carolina since 1984, especially in plant beds and greenhouses. In 1995, it caused the greatest losses of any disease since 1959. The fungus that causes target spot lives in many North Carolina soils. Saturated soils and leaf moisture favor sporulation of the fungus and germination of the spores into the tobacco leaves. Removing the lower leaves and ensuring adequate nitrogen are the only management tactics currently available. Target spot symptoms are quite similar to those of brown spot. With target spot, the centers of the lesions rapidly become very thin and papery and shatter if only slight pressure is applied. The concentric rings that characterize brown spot lesions may look similar to those caused by target spot. Because target spot lesions are so fragile, the necrotic areas usually drop from the leaf, leaving a ragged appearance. Target spot may occur on leaves at any plant position and, where conditions favor the problem, may cause considerable destruction. Target spot, like brown spot, is favored by frequent rainfall and high humidity. Tobacco Mosaic Virus Tobacco mosaic is caused by a virus and is the most contagious tobacco disease that growers encounter in North Carolina. The virus is a large, complex chemical molecule that, like all other viruses, requires living tissue to multiply. Once a tobacco mosaic particle is placed inside the plant, it becomes a part of that plant and will persist until the plant dies. The tobacco mosaic virus is spread in the sap of diseased plants. Anything that moves sap or juice from a diseased to a healthy plant will move the virus. That includes machinery used during cultivation and the hands or clothing of workers. It is not spread through air currents or by other carriers associated with most other diseases. Mosaic is not as sensitive to weather conditions as most other tobacco diseases. However, it is easier for plants to become infected when there is moisture on them and when they are succulent and growing rapidly. Damage is most severe when infected plants suffer during hot, dry conditions. The symptoms of tobacco mosaic are well known to most producers. The most common symptom is leaf mottling, which is alternating areas of light and dark green tissue. This symptom is especially noted in the top of the plant or in younger tissue. During periods of high temperatures and high light intensity, affected portions of leaves may die, resulting in "mosaic burn." Because of the virus-unique nature, control of tobacco mosaic must be approached differently from that of other diseases. No chemicals are labeled for mosaic control, although the milk-dip treatment is beneficial as workers perform tasks within the crop. New resistant varieties are very valuable control tools. Also, you should rotate fields, clean equipment, and discard seedling trays (if TMV was at least 20 percent by layby in any field). In addition, you should wash greenhouse clippers, transplanters, tractor bottoms and tool bars, and any other equipment that came in direct contact with the foliage and sanitize them with a 25 to 50 percent bleach solution. Tomato Spotted Wilt Virus The symptoms of tomato spotted wilt virus (TSWV) on young stems are variable but usually include darkened areas (which may appear wrinkled or sunken), decay, or brittleness. These symptoms are sometimes confused with soreshin. Leaves often have 1/8- to 1/4-inch-diameter spots, especially evident on dead tissue. Most frequently, dead tissue outlines sections of major leaf veins, which remain green longer than interveinal areas. Symptoms usually appear first in the bud or on young leaves, which are often distorted. Diagnostic laboratory tests often are needed to confirm the disease on young plants. Symptoms may occur at any time during plant growth, and those that develop on knee-high or larger tobacco are usually diagnostic; therefore, larger plants do not usually require laboratory tests. On larger tobacco, symptoms may be systemic, (usually affecting one side of the plant first) or localized (affecting just one or two leaves). Systemically infected plants are usually distorted, especially in the bud. Leaf symptoms resemble those in young plants. Brown to black parallel streaks often run from infected leaves down the outside of the stalk. Although no adequate management practices are available, vigorous efforts are being made in several states to develop strategies and tactics for controlling TSWV on tobacco. Annual broadleaf weeds in and around field borders are currently thought to be reservoirs for the virus-infected thrips. From these efforts we have learned that rouging (removing) infected plants, spraying for thrips, and rotating have little, if any, value. Planting date is an important but inconsistent factor in TSWV loss. Early planted tobacco seems most likely to get the virus, but in some years the latest planted is most damaged. Tobacco planted closest to the "average" planting date is usually the least affected. In Georgia, Admire sprayed on seedlings in the greenhouse showed some benefit in reducing TSWV in one test in one year. Weather Fleck Weather fleck is not an infectious disease but causes dark, metallic-like, sunken leaf spots (flecks) that gradually fade to white with age. Symptoms are most obvious on older leaves of young plants or on middle-aged leaves of older plants. Spots are often more common near leaf tips. Damage can be severe enough to blight bottom leaves. Weather fleck is an injury caused by the common air pollutant ozone. Ozone is heavy oxygen (O3) and is produced by internal combustion engines and by certain manufacturing processes. During periods of cloudy, overcast, or rainy weather, the concentrations of ozone that would normally escape into the stratosphere are held closer to ground level. Most importantly, it is during these conditions that leaf pores (stomata) remain open the longest and the leaves absorb the most ozone. Some varieties are much less sensitive to weather fleck than others, and growers who experience chronic difficulty should select a variety that is more tolerant. Some Tips on Planning Disease Management No one practice can be expected to provide protection from every disease, much less from the many different diseases that might attack tobacco during a growing season. Tobacco growers urgently need to assess the disease problems within each of their fields and plan management strategies well before the crop year. A "tobacco disease map" of each field is of great benefit. To develop such a map, sketch the field and mark areas of disease infestation. Update the map each time tobacco is in the field, noting any change in location and in level of infestation. Over time, growers who do this can plan control practices that should benefit them immensely as they develop production plans from season to season. For black shank and Granville wilt, the average percentage of plants diseased within a field gives a good indication of the level of that disease in the field.