\m 33 14 40 2 1 'Disease Control Program for Flue-Cured Tobacco' \m 34 14 95 2 2 'Introduction' \m 91 38 111 3 2 'Sequential Disease Control Program for Tobacco' \m 92 39 38 3 3 'Choice of Site for Transplant Bed' \m 105 43 46 3 3 'Transplant Bed Preparation for Fumigation' \m 139 56 95 4 3 'Soil Fumigation of Transplant Bed' \m 152 62 64 4 3 'Avoid Recontamination of Fumigated Transplant Bed' \m 155 63 64 4 3 'Use Resistant Varieties' \m 194 81 63 5 3 'Blue Mold Control in the Transplant Bed' \m 205 86 51 5 3 'Use Healthy Transplants' \m 212 88 75 5 3 'Plow Down Plant Bed Sites After Transplanting' \m 226 93 95 5 3 'Use Crop Rotation' \m 236 97 65 5 3 'Control Nematodes' \m 245 100 72 5 3 'Sanitary Practices During Field Production to Avoid TMV' \m 260 106 12 6 3 'Chemical Control of Black Shank and Blue Mold in the Field' \m 299 124 79 6 3 'Brown Spot Control' \m 310 128 87 7 3 'Harvest While Plant are Dry' \m 323 134 69 7 3 'Use of Ripening Agent for Salvage of Diseased Crop' \m 330 137 18 7 3 'Cut, Disk and Plow Down Stalks and Roots After Harvest' \m 342 141 23 8 2 'Habitats of Organisms Causing Plant Diseases of Tobacco' \m 398 157 21 9 2 'Fungicides Registered for Blue Mold Control in Transplant Beds' \m 436 172 48 10 2 'Chemicals for Black Shank and Blue Mold Control in the Field' \t'November 1986' \a \a Dr. Tom Kucharek, Extension Plant Pathologist \a Plant Protection Pointer No. 23 \a DISEASE CONTROL PROGRAM FOR FLUE-CURED TOBACCO Dr. Tom Kucharek, Extension Plant Pathologist \h 1 'Disease Control Program for Flue-Cured Tobacco' \h 2 'Introduction' Plant Protection Pointer No. 23 DISEASE CONTROL PROGRAM FOR FLUE-CURED TOBACCO Introduction Tobacco diseases reduce yields and monetary returns. They do so by retarding desirable plant development in the transplant bed and in the field. Tobacco diseases in Florida are caused by fungi, bacteria and viruses. FUNGI are plants that lack true roots, leaves, stems or chlorophyll. Instead, they have hyphae (microscopic threads) of various types which cannot grow in the soil or in a host plant. These threads absorb food from the plant or organic material in the soil. Fungi cannot make certain necessary food products, so they must live on food products manufactured by other organisms. Fungi reproduce by various methods. They produce different types of spores (microscopic seeds). Some spores are air-borne and thus can be spread by air currents. Fungi that cause diseases such as blue mold, brown spot and frogeye leaf spot produce such spores. Many fungi-causing seedling blights, root rots, or wilts (black shank, Fusarium wilt) produce "heavy duty" spores that survive in the soil for many years. The fungus that causes blue mold also produces a thick-walled resting spore (oospore). As you might expect, longer intervals (in years) between tobacco plantings (transplant bed or field) on the same land are beneficial in disease control as many of these resting structures die over a period of time if a suitable host plant is not present. When pathogenic fungi come into contact with a tobacco plant, they grow on it and eventually penetrate the plant either directly through plant tissue, through natural plant openings or through wounds (mechanical damage or nematode damage). BACTERIA are microscopic one-celled plants which increase by division. Bacteria enter plants through wounds or natural plant openings. Bacterial diseases are favored by moist conditions from dew, rain or overhead irrigation. Currently three bacterial diseases are seen on Florida tobacco -- angular leafspot, Granville wilt and hollow stalk (barn rot). VIRUSES are particles composed of a nucleic acid core and a protein coat. No cellular structure is present. Viruses can be spread mechanically by mans' hands, tools, or aphids. Viruses reproduce in living cells only. Viruses are so small they can be visualized only with the aid of an electron microscope. At the present time, tobacco mosaic virus, tobacco etch virus and potato virus Y are the common viruses found in Florida tobacco. A summary of tobacco disease-causing organisms and their habitats is presented in Table 1. \bWith this basic information at hand, it becomes understandable that no one control measure will be totally effective in controlling all tobacco diseases. Instead, a coordinated sequential program of several methods should be used. Even if this approach is used to the best of your ability, you will not achieve total elimination of disease. \h 2 'Sequential Disease Control Program for Tobacco' \h 3 'Choice of Site for Transplant Bed' Sequential Disease Control Program for Tobacco (Transplant Bed) 1. Choice of Site for Transplant Bed Rotate the site of the bed each year. This practice plus proper fumigation is added insurance for disease-free plants. The bed should be isolated from tomato and pepper transplant beds and isolated from potato fields as several diseases (viruses, Granville wilt) occur on tobacco and these solanaceous crops. \h 3 'Transplant Bed Preparation for Fumigation' 2. Transplant Bed Preparation for Fumigation and Avoidance of Recontamination after Fumigation The soil should be well-tilled and loose with no clumps, clods or plant trash as they inhibit vapor movement of the fumigant. If small areas or clods within the bed are not fumigated, a source of inocula for root or stem-rotting fungi, such as Pythium sp., Rhizoctonia solani, Fusarium sp. and Phytophthora parasitica (the cause of black shank) is left to recontaminate the fumigated areas thereby offsetting desired benefits from fumigation. Similarly, the fumigated transplant bed can be recontaminated from outside the bed area. Such disease-causing organisms will increase at a faster rate in fumigated soil compared to non-fumigated soil. Certain preventative measures must be used to avoid recontamination. a. Do not step or move equipment in fumigated area after fumigation (before or after planting). b. The bed should be slightly raised from the surrounding soil so that drainage is away from the bed. A drainage ditch around the bed would serve a similar purpose. Soil or water wash from outside the fumigated areas will carry fungus inocula (spores and mycelium). c. Avoid handling plants unnecessarily. d. Workers should not handle plants unless they have thoroughly washed their hands and forearms. Dipping hands in milk or a phosphate detergent (Trisodium Phosphate, Spic N' Span) reduces spread of tobacco mosaic virus. e. Avoid recontamination of fumigated area with soil on tools, equipment, etc. \b\v \h 3 'Soil Fumigation of Transplant Bed' 3. Soil Fumigation of Transplant Bed Numerous soil fumigants are available. Choose a multipurpose soil fumigant such as Brom-O-Gas if hand-maneuvered applicators are to be used, or methyl bromide + chloropicrin (MB 98-2, MBC 2-1 or Terr-O-Gas 67) if a tractor-drawn chisel applicator is to be used. Multipurpose materials are effective against soilborne fungi as well as nematodes and weeds. The fumigated area must be covered with a plastic cover for two to three days after which an aeration period of two to three days is necessary for Brom-O-Gas, two weeks if Terr-O-Gas 67 or Asgrow 2-1 are used, and 7-10 days after Asgrow 98-2 is used. Use rates as listed on the label. \h 3 'Avoid Recontamination of Fumigated Transplant Bed' 4. Avoid Recontamination of Fumigated Transplant Bed (see no. 2). \h 3 'Use Resistant Varieties' 5. Use Resistant Varieties. Tobacco varieties are available that are resistant to one or several diseases including black shank, brown spot, Fusarium wilt and Granville wilt. The degree of resistance to a disease may not be the same in each variety labelled as resistant. For example, both NC 2326 and Speight G-58 are labelled as moderately resistant to black shank but NC 2326 is more susceptible to black shank than Speight G-58. In addition, the variety should possess resistance to root knot nematode and still be saleable. As changes occur from year to year for this complex issue, consult with your county agent about your variety choice for the coming year. L = low resistance, M = moderate resistance, H = high resistance, R = resistant, T = tolerant, MT = moderate tolerance, S = susceptible, VS = very susceptible ------------------------------------------------------------------------------- Disease ----------------------------------------------------------- Variety Black Shank Brown Spot Southern Root Knot Nematode ------------------------------------------------------------------------------- Speight G28 H T R Speight G70 H MT R Speight G80 H MT R NC 2512 L MT R NC79 M MT R K326 M T R NC2326 M MT S Speight G58 M MT R Coker 206 H S R McNair 944 H VS S K399 H MT R K394 H S S ------------------------------------------------------------------------------- Should Tobbaco Mosaic Virus become a continuous problem, use a resistant variety such as Coker 176 and NC 628 and see step 11, below. \b\v \h 3 'Blue Mold Control in the Transplant Bed' 6. Blue Mold Control in the Transplant Bed Blue mold, caused by the fungus Peronospora tabacina, can cause total destruction of plants in the bed. The use of plastic bed covers rather than cheese cloth seems to have minimized the use of dusting or spraying of fungicides for blue mold control. Plastic covers increase the temperatures, often above 86 F (30 C). Control blue mold by rotating the bed site each year, using plastic covers and by spraying with a fungicide. See Table 2 for available fungicides. Also, avoid excessive use of nitrogen and irrigation. \h 3 'Use Healthy Transplants' 7. Use Healthy Transplants Grow your own transplants. Avoid using transplants from south Florida. A higher incidence of virus infection and root rot organisms of tobacco transplants can be expected with plants produced in south Florida. \h 3 'Plow Down Plant Bed Sites After Transplanting' 8. Plow Down Plant Bed Sites as Soon as Successful Transplanting is Complete Unattended tobacco plants in the bed will become diseased thus providing inocula for nearby fields and providing a more suitable condition for survival of organisms causing disease on next year's transplants. Waiting to plow down an old bed site allows the tractor and implements a greater chance to spread disease-infested soil around the farm area. Plowing down the old bed site early is a sanitary practice that is basic to the production of quality tobacco. In the Field \h 3 'Use Crop Rotation' 9. Use Crop Rotation Crop rotation allows time for resting spores of black shank and other diseases to die. Probably land once infested with black shank will never be totally free of spores causing black shank but longer intervals, such as five years, between tobacco crops have been shown to effectively reduce black shank. Couple this control with the use of resistant varieties, nematode control, healthy transplants, and chemical control. \h 3 'Control Nematodes' 10. Control Nematodes Nematodes cause injury to roots thereby providing fungi with an entrance point into the plant. Black shank can be more severe on a black shank-resistant variety if nematode damage occurs. Also, fungus root rots will be more severe if nematodes are not controlled. Brown spot has been shown to be more severe with nematode damage. \h 3 'Sanitary Practices During Field Production to Avoid TMV' 11. Sanitary Practices During Field Production to Avoid Tobacco Mosaic Virus a. Workers should not smoke or chew in tobacco fields or while working in transplant beds. b. Workers should dip their hands in milk or a phosphate detergent (Trisodium Phosphate, Spic N' Span, etc.) solution for 30 seconds before setting or handling plants, especially if they smoke or chew, or after touching infected plants. \b c. Use the most effective chemical control program for suckers so that hand suckering is minimized. Hand suckering can spread tobacco mosaic virus. \h 3 'Chemical Control of Black Shank and Blue Mold in the Field' 12. Chemical Control of Black Shank and Blue Mold in the Field (also see Table 2) Black shank can be controlled with Vorlex. This material will also reduce nematodes. Multipurpose fumigants need to be injected 6 to 8 inches deep in the soil, covered and compacted immediately. Black shank can also be controlled with Ridomil 2E used in a preplant- incorporated manner. Use up to 1 1/2 gallons of Ridomil per acre for black shank, but do not use the higher rate unless black shank has been a severe problem in the field to be treated or you plant a susceptible variety. Ridomil can be soil-incorporated when field beds are made. Tank mixing Ridomil with herbicides is legal and results in no known loss in effect of either the herbicide or Ridomil. Ridomil is expensive and high rates are needed for black shank control; so do not use Ridomil for black shank control unless you need it. Ridomil 2E is highly effective for blue mold control in the field when used at no more than one quart per acre preplant incorporated. Two quarts per acre can be used for season-long control, but in Florida reliable control should be achieved with 1 quart per acre because the diminished residue often coincides with hot weather that inhibits blue mold development. Excessive irrigation can offset the hot weather benefit and further leach Ridomil out of the root zone. A layby application of Ridomil 2E at 1 quart per acre to the soil can be used to initiate or prolong blue mold and black shank control. Do not make this application if more than 2 quarts per acre of Ridomil 2E were applied at transplanting. Ridomil 2E in the soil will translocate up the root and shoot system of the plant thereby confering control for both blue mold and black shank. Ridomil 2E sprayed on the foliage of the plant will control blue mold but not black shank. Remember, black shank infections predominate in the root and then the fungus grows into the stem. Placement of the layby Ridomil application on the soil below and to the outside of the drip-line of the leaves is best. Also, use of typical sucker control spray booms (3 nozzles over the top) will not control blue mold on the lower leaves. Ridomil moves upward in the plant, not downward in any effective amount. \h 3 'Brown Spot Control' 13. Brown Spot Control Partial control of brown spot can be achieved with resistant varieties (see step 5 above). In addition, the use of sucker control agents with maleic hydrazide (MH) is reported to reduce brown spot by altering the chemistry of the plant. Plants under stress from drought, nematodes, black shank and other disorders, are reported to be more susceptible to brown spot. Earlier plantings usually sustain less damage from brown spot than later plantings. \b\v \h 3 'Harvest While Plant are Dry' 14. Harvest While Plants are Dry Harvesting while plants are wet is conducive for spreading hollow stalk, a highly contagious bacterial disease. The bacterium will be transmitted from infected plants to healthy plants more readily if plants are wet. Also, harvested leaves taken to the barn are more apt to be contaminated thereby increasing the barn rot (jelly rot) phase of this disease. Workers should be taught to recognize plants with hollow stalk and avoid touching those plants while harvesting. The barn rot phase of this disease can be reduced by using wider spacings between leaves along with maximum recommended heat. Harvesting dryer leaves reduces fuel bill costs during curing. \h 3 'Use of Ripening Agent for Salvage of Diseased Crop' 15. Use of Ripening Agent for Salvage of Diseased Crop Use a ripening agent to hasten harvest if foliar diseases are beyond control. When using a ripening agent, use a fine mist spray with a complete nozzle arrangement to obtain thorough coverage for even ripening. \h 3 'Cut, Disk and Plow Down Stalks and Roots After Harvest' 16. Cut, Disk and Plow Down Old Tobacco Stalks and Roots Immediately After Harvest Stalk residues remaining above-ground and undercomposed roots provide an excellent site for propagation of disease organisms for the following year. Tobacco Mosaic Virus, Potato Virus Y, Brown Spot, Blue Mold and Root Knot Nematodes are reduced by this practice. In addition, several insects and weeds can be reduced by this practice as well. \v \v \b\v \h 2 'Habitats of Organisms Causing Plant Diseases of Tobacco' TABLE 1: HABITATS OF ORGANISMS CAUSING PLANT DISEASES OF TOBACCO ++ = Occurs commonly; + = occurs; - = Does not occur. * Includes organic debris. ------------------------------------------------------------------------------- Carried Survives Survives Moved by Organism Diseases in seed in soil* on weeds wind or rain ------------------------------------------------------------------------------- Pythium spp. Seedling blights, - ++ ++ +- (fungus) root rot Rhizoctonia Seedling blights, - ++ ++ +- solani (fungus) root rot Phytophthora Black shank - ++ - + parasitica var. nicotianae (fungus) Peronospora Blue mold - ++ + (Texas) ++ tabacina (fungus) Fusarium Fusarium wilt - ++ - - oxysporum var. nicotianae (fungus) Cercospora Frogeye leaf spot - + - ++ nicotiana (fungus) Alternaria Brown spot - + + ++ tenuis (fungus) Pseudomonas Angular leaf spot + + + ++ angulata (bacterium) Pseudomonas Granville wilt - ++ + + solanacearum (Bacterial wilt) (bacterium) Erwinia Hollow stalk & - + + + carotovora barn rot (bacterium) Virus Tobacco Mosaic - + ++ - Virus Potato Virus Y - - ++ - (Vein Banding) (aphids are vectors) Virus Tobacco Etch - - ++ - (aphids are vectors) ------------------------------------------------------------------------------- \v \v \b\v \h 2 'Fungicides Registered for Blue Mold Control in Transplant Beds' TABLE 2: FUNGICIDES REGISTERED FOR USE IN TOBACCO TRANSPLANT BEDS FOR CONTROL OF BLUE MOLD ------------------------------------------------------------------------------- Formu- Fungicide lation Use Pattern Rate ------------------------------------------------------------------------------- Ferbam (Carbamate) WP Apply 2 times/week 2-3 lbs/100 gal water until transplanting using 3 gal/100 sq yd Maneb (Dithane WP Apply 2 times/week 1-2 tsp/4 gal water M-22 Special) until transplanting using 2-5 gal/100 sq yd Maneb (Dithane FZ, FL Apply 2 times/week 0.4-3.2 qts/100 gal Manex) until transplanting using 2.5-5.0 gal water/100 sq yd Streptomycin SP Apply at 5-7 day inter- 1/2 - 1 lb/100 gal/acre (Agristrep, vals in bed as soon as 2 lb/acre for curative Agrimycin 17) possible after rains. spray Apply as needed. For curative action in the field apply at 5-7 day intervals. Ridomil 2E EC Apply broadcast before 1 qt/acre (1 fl oz or or at time of seeding 2 TBSP/150 sq yd) to soil surface followed by 1/2 inch of water. Manzate 200 FL Apply as necessary. 0.4 qt/100 gal water. Flowable Use 3-5 gal of spray mix/100 sq yd ------------------------------------------------------------------------------- \v \v \b\v \h 2 'Chemicals for Black Shank and Blue Mold Control in the Field' TABLE 3: CHEMICALS FOR BLACK SHANK AND BLUE MOLD CONTROL IN THE FIELD Chemical control of Black Shank is expensive and should be used only in problem fields only where susceptible or moderately resistant varieties are used. A combination of a resistant variety plus a treatment with a multipurpose fumigant or Ridomil 2E may result in higher yields for a field known to have a severe Black Shank problem. ------------------------------------------------------------------------------- Diseases Chemical Rate/Acre Controlled Remarks ------------------------------------------------------------------------------- Ridomil 2E 1 qt Blue mold Broadcast preplant incorporated Ridomil 2E 1-6 qt Black shank Broadcast preplant incorporated Ridomil 2E 1 qt Black shank ON SOIL at last cultivation if & blue mold more than 2 qt/acre had not been used previously. Apply below and beyond drip-line. Vorlex 25-40 gal Black shank Broadcast with chisels 6-8" Pythium root apart and 6-8" deep. Seal by rot packing soil at surface. (Row treatment of this material results in proportionally lower rates per acre being used compared to broadcast treatments. Row treatments cannot be expected to result in effective season-long control especially if a susceptible variety such as NC 2512 or Hicks is used. The row treatment is effective in reducing Black Shank and Root Knot inocula in the treated zone only. Roots of tobacco will grow out of the treated zone and then infection can occur. Symptoms of Black Shank and Root Knot will be delayed in time of occurrence with row treatments.) Agri-Mycin 17 100-200 ppm Blue mold Spray every 5-7 days. Agristrep 100-200 ppm Blue mold Spray every 5-7 days. Dithane FZ 1.8-3.2 qt Blue mold Spray as necessary. Manzate 200 FL 0.8-1.3 qt Blue mold Spray as necessary. Manex 1.4-2.5 qt Blue mold Spray as necessary. Nitrogen ----- Blue mold Higher rates of nitrogen result Fertilizer in increased susceptibility of tobacco to blue mold. -------------------------------------------------------------------------------