University of Florida
Plant Protection Pointers
EXTENSION PLANT PATHOLOGY REPORT NO. 23
GAINESVILLE, FLORIDA, DECEMBER, 2004
(Revision No. 20)
DISEASE CONTROL PROGRAM FOR FLUE-CURED TOBACCO
by Tom Kucharek
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 organisms that lack true roots, leaves, stems or chlorophyll. Instead, the fungi
that infect tobacco in Florida have hyphae (microscopic threads) of various types which can grow in
the soil or in a host plant. These threads absorb food from the plant or organic material in the soil.
Fungi can not 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 microscopic spores.
Some spores 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 or structures that survive in the soil for
many years. The fungus that causes blue mold sometimes produces a thick walled resting spore
oosporee). As you might expect, longer intervals, in years, between tobacco crops (transplant bed or
field) on the same land are beneficial for 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, natural plant openings or
wounds (mechanical damage or nematode damage).
BACTERIA are microscopic one-celled organisms that increase by division of individual
cells. Bacterial cells 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 of tobacco occur in Florida -- angular leaf spot, Granville wilt and hollow stalk (barn rot).
VIRUSES are particles composed of a nucleic acid core and a protein coat. For tomato spotted
wilt virus (TSWV), the coat protein is a glycoprotein embedded within a lipid (fatty) envelope. No
cellular structure is present. Viruses can be spread mechanically by mans' hands, tools, or insects.
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, tomato spotted wilt virus,
tobacco etch virus, potato virus Y, and cucumber mosaic virus are the common viruses found in
tobacco in Florida. The latter three viruses are spread primarily by aphids. Tobacco mosaic virus
is spread commonly by contact. In 1987, TSWV was found first in tobacco in Florida in Alachua.
By 1988, TSWV was found in most tobacco fields in northern Florida. In 1992, the severity of
TSWV in one field in Florida reached 40%. TSWV is spread by thrips. The incidence of cucumber
mosaic virus (CMW) is increasing in tobacco in north Florida. In recent years, CMV caused
considerable damage in tobacco in a few fields in Alachua County. In 1998, 1999, and 2000, CMV
was highly destructive in Hamilton County in several fields of tobacco. Mechanical spread from
plant sap can occur with all five viruses, but such mechanical spread is of practical importance for
tobacco mosaic virus only in Florida.
A SUMMARY OF TOBACCO DISEASE CAUSING ORGANISMS AND THEIR HABITATS
IS PRESENTED IN TABLE 2.
With 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.
SEQUENTIAL DISEASE CONTROL PROGRAM FOR TOBACCO
STEP 1. CHOICE OF SITE FOR TRANSPLANT BED.
Rotate the site of the transplant bed each year. Locate transplant beds in open areas where leaf
wetness is minimized. Crop rotation plus proper fumigation aids in producing healthier plants.
The transplant beds should be isolated from tomato and pepper transplant beds and isolated from
potato fields as several diseases occur on tobacco and these solanaceous crops. Also, transplant
beds should not be near greenhouse production areas.
STEP 2. TRANSPLANT BED PREPARATION FOR FUMIGATION AND AVOIDANCE OF
RE-CONTAMINATION AFTER FUMIGATION.
The soil should be well tilled and loose with no clumps, clods or plant trash to improve vapor
movement of the fumigant. If small areas or clods within the bed are not fumigated, sources of
inocula for root or stem-rotting fungi, such as Pythium spp., Rhizoctonia solani, Fusarium sp. and
Phytophthora nicotianae(the cause of black shank) are left to re-contaminate the fumigated areas
thereby offsetting desired benefits from fumigation. Similarly, the fumigated transplant bed can be
re-contaminated from outside the bed area. Such disease-causing organisms can increase at a faster
rate in fumigated soil compared to non fumigated soil. Certain preventative measures must be used
to avoid re-contamination.
1. Do not step or move equipment in fumigated area after fumigation before or after
2. The transplant bed should be slightly raised from the surrounding soil so that
drainage is away from the plants. A drainage ditch around the bed would serve a
similar purpose. Soil or water wash from outside the fumigated areas will carry
fungal inocula (spores and mycelia).
3. Avoid handling plants unnecessarily.
4. Workers should not handle plants unless they have thoroughly washed their hands and
forearms. Dipping hands in milk reduces spread of tobacco mosaic virus.
5. Avoid re-contamination of the fumigated area with soil on tools, equipment, etc.
6. Design transplant beds so wheels of the mower do not run over plants.
7. Mow plants when they are dry.
8. Do not irrigate with pond water.
STEP 3. SOIL FUMIGATION OF TRANSPLANT BED.
Numerous soil fumigants are available. Choose a multipurpose soil fumigant such as methyl
bromide + chloropicrin or SMDC (e.g Vapam). Multipurpose materials are effective against
soilborne fungi as well as nematodes and weeds. The fumigated area must be covered with a plastic
cover for at least one week with methyl bromide/chloropicrin (2-3 days for methyl bromide alone)
after which an aeration period of at least one week is necessary for methyl bromide alone and at
least two weeks if methyl bromide/chloropicrin mixtures or SMDC products are used. If the soil
remains extremely wet, a three week aeration period may be required, particularly in soils that are
cold. Use rates as listed on the label.
STEP 4. AVOID RE-CONTAMINATION OF FUMIGATED TRANSPLANT BED. See step 2.
STEP 5. PRODUCTION OF DISEASE-FREE TRANSPLANTS IN FLOAT SYSTEMS IN
You should inspect plants before setting or purchasing. Plants with
diseases should not be used.
Disinfestation of greenhouses and handling areas for trays and soil mixes is imperative. Before
handling plants, trays, and soil, wash you hands and forearms with soap and water. Disinfest
benches and handling areas with a 10% solution of household bleach (approx. 5.25%). Before
filling float systems with water sweep or vacuum all soil and old plant material in tanks and
walkways. Then, disinfest tanks and walkways with a solution of bleach. When filling tanks, use
well water and not pond water. Disinfest hoses, particularly the outlet ends, before filling tanks.
The plastic liner should not be placed in contact with anything except the disinfested tank prior to
installation. Remove footwear when walking on liner during installation. Do not allow tools,
vacuum seeder head, soil mixes and disinfested transplant trays to come into contact with soil or
non-disinfested surfaces after disinfestation. After use, wash trays to remove soil and plant matter.
Do this away from the greenhouse. Trays should be sanitized by soaking them in a bleach solution,
placing in a steam bath (180* F for at least 1 hour), or exposing them to methyl bromide 98% (1.5
lb/1000 cu.ft.). Treatments with steam or methyl bromide are more effective than the bleach
treatment. Disposable plastic inserts for Styrofoam trays are available. The use of inserts minimizes
penetration of the Styrofoam by roots and thus, there is less chance that propagules of pathogens
(virons of tobacco mosaic virus, oospores of Pythium spp., etc.) will remain within the Styrofoam
where sanitizing agents can not penetrate. For further information on production of disease-free
transplants see Plant Protection Pointer No.25.
STEP 6. USE RESISTANT VARIETIES.
Varieties of tobacco are available that are resistant to one or several diseases including black
shank, brown spot, Fusarium wilt, Granville wilt, and tobacco mosaic virus. The degree of
resistance to a disease may not be the same in each variety labeled as resistant. For example, both
NC 2326 and Speight G-58 were sometimes labeled 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 Extension agent about your variety choice for the coming
year. In addition, see Table 1 for a list of agronomically acceptable varieties that possess resistance
to some of the major plant diseases of tobacco in Florida.
Table 1. Some variety f
resistance to some
____ __ I__________I__ _Disease _____ ___________
Southern Potato Virus
Brown Root Knot Y & Tobacco Cucumber
Variety Black Shank Spot Nematode Etch Virus Mosaic Virus
Speight G 168# H MT R S S
Speight G 172 H MT R S S
Speight G 70 H-M MT-S R S S
K 394 H-M S S S S
K399 H-M MT R S S
K326 L T R S S
K149 M MT R S S
K346 M-H (RACES 1 &2) T R S S
NC 102 H ? R MR MR or MT
NC 291 H ? R MR S
NC 297 H ? R S S
NC 55 L ? R** R* S
NC 71 (hybrid) *** H ? R S S
NC 72 (hybrid) H ? R S S
Races 1 & 3
*** Moderate to High Resistance to Granville Wilt in North Carolina but may be susceptible to
cucumber mosaic virus.
# Some resistance to Granville wilt is present when compared to other varieties.
Should Tobacco Mosaic Virus become a continuous problem use a resistant variety such as NC
102, Coker 176, NC 297, NC 628, RGH 4, NC 97, or NC 100 and see Step 12.
STEP 7. BLUE MOLD AND TARGET SPOT CONTROL IN THE TRANSPLANT BED.
Blue mold, caused by the fungus Peronospora tabacina, can cause total destruction of plants in
the transplant bed. The use of plastic bed covers rather than cheese cloth seems to have minimized
the use of spraying fungicides for blue mold control by increasing the temperatures. However, as
soon as the covers are removed for dissipating heat, blue mold can become a serious problem in the
transplant bed. Control blue mold by rotating the bed site each year, by using plastic covers and by
spraying with a fungicide. See Table 2 for available fungicides. Also, avoid excessive use of
nitrogen and irrigation. For target spot control, see step 20.
STEP 8. USE HEALTHY TRANSPLANTS.
Grow your own transplants. Avoid using transplants that are produced outdoors in south
Florida. A higher incidence of virus infection and root rot organisms of tobacco transplants can be
expected with plants produced in south Florida in open transplant beds. Greenhouse-produced
transplants from transplant houses have been good plants without viral diseases. If you run short
of plants, attain plants produced in north Florida. Transplants from south Georgia have a higher
risk of being exposed to tomato spotted wilt virus.
STEP 9. PLOW DOWN TRANSPLANT BED SITES AS SOON AS SUCCESSFUL
TRANSPLANTING IS COMPLETE.
Unattended tobacco plants in the transplant bed will become diseased thus providing inocula
for nearby fields and providing a more suitable condition for survival of organisms causing disease
on transplants produced next year. Waiting to plow down an old bed site allows wind, rain,
tractors 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
STEP 10. USE CROP ROTATION AND PLANT IN WELL DRAINED FIELDS.
Crop rotation provides time for resting spores of black shank and other pathogens to die.
Probably land once infested with the fungus that causes black shank will never be totally free of
spores causing black shank but longer intervals, such as five years between tobacco crops, have
reduced black shank. Couple crop rotation with the use of resistant varieties, nematode control,
healthy transplants, and chemical control.
Fields where water stands or puddles are conducive to development of granville wilt and black
STEP 11. AVOID HIGH SOIL pH.
A soil pH above 6.0 is advantageous for development of black shank.
STEP 12. CONTROL NEMATODES.
Nematodes cause injury to roots thereby providing fungi with an entrance point into the plant.
Black shank can be more severe in 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 when nematode damage is present.
STEP 13. SANITARY PRACTICES DURING FIELD PRODUCTION TO AVOID TOBACCO
A. Workers should not smoke or chew in tobacco fields or while working in transplant
B. Workers should dip their hands in milk solution for 30 sec. before setting or handling
plants, especially if they smoke or chew after touching infected plants.
C. Use the most effective chemical control program for suckers so that hand-suckering is
minimized. Hand-suckering can spread tobacco mosaic virus.
D. Use new transplant trays each year if greenhouse production systems are used for
transplant production. An alternative is to use disposable, plastic inserts in Styrofoam
STEP 14. AVOID USE OF POND WATER FOR IRRIGATION.
Pond water can carry plant pathogens such as Phytophthora nicotianae, the fungus that causes
black shank and Pythium spp., some of the fungi that cause root rot.
STEP 15. CONTROL WEEDS.
Weed control in the field will reduce the length of time tobacco leaves remain wet by allowing
for more aeration. This will aid in reducing blue mold and brown spot. Also, eliminate dayflower
(Commellina spp.) in and around fields. Dayflower is a major source of inoculum for cucumber
STEP 16. AVOID EXCESSIVE OVERHEAD IRRIGATION.
Excessive irrigation has increased blue mold and black shank. Blue mold has been reduced
with use of drip irrigation compared to overhead irrigation. Also, when during transplanting when
transplant beds are watered daily to facilitate the pulling of transplants, it is likely that blue mold
could become a problem even though the ambient weather is dry.
STEP 17. CHEMICAL CONTROL OF BLACK SHANK AND BLUE MOLD IN THE FIELD
(Also, see Table 3).
Black shank can be controlled with Ridomil Gold EC when used in the soil in a preplant,
incorporated manner coupled with an application at early cultivation and another application at
layby. See Table 3 for rates and timing. Ridomil can be incorporated into the soil when field beds
are made or applied immediately before transplanting. Tank mixing Ridomil with herbicides is
legal and results in no known loss in effect for either the herbicide or Ridomil. However, applying
Ridomil too early, in relation to transplanting, increases the risk of Ridomil being leached from the
top layers of soil because Ridomil is highly water soluble.
Ridomil in the soil will translocate up the root and shoot system of the plant thereby
conferring control for both blue mold (sensitive strains) and black shank. Ridomil sprayed on the
foliage of the plant will not control black shank. Remember, infection from the black shank fungus
predominates in the root and then the fungus grows into the stem. Placement of Ridomil as a layby
application should be on the soil below and to the outside of the drip-line of the leaves is best.
Ridomil moves upward in the plant, not downward in any effective amount. Use drop nozzles to
place Ridomil to the soil. Excessive irrigation can offset the hot weather suppression of Blue Mold
and further leach Ridomil out of the root zone.
Telone C17 can be used as a pre-plant fumigant for suppression of blackshank, nematodes, and
possibly other soilborne pathogens. See Table 3 for application rates. Telone II does not directly
reduce fungal diseases.
Isolates of Peronospora tabacina (blue mold pathogen) that are resistant (insensitive) to Ridomil
exist commonly in Florida. Use Dithane DF or Carbamate (Table 3) for control of Blue mold.
STEP 18. AVOID DAMAGING ROOTS OF TOBACCO WHEN CULTIVATING.
Root damage predisposes plants to infection by the fungus that causes black shank and other
STEP 19. BROWN SPOT CONTROL.
Partial control of brown spot can be achieved with resistant varieties (see Step 6). In addition,
the use of sucker control agents with maleic hydrazide (MH) has been 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.
STEP 20. TARGET SPOT CONTROL.
Rhizoctonia-induced target spot is extremely difficult to control without fungicide sprays.
Keep leaves as dry as possible for as long as possible.
STEP 21. 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. Harvesting dryer leaves reduces fuel bill costs during curing. 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.
STEP 22. HARVEST LOWER LEAVES EARLY.
Harvesting lower leaves early increases aeration within the canopy which aids significantly in
reducing blue mold.
STEP 23. USE OF RIPENING AGENT FOR SALVAGE OF DISEASE 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.
STEP 24. CUT, DISK AND PLOW DOWN OLD TOBACCO STALKS AND ROOTS
IMMEDIATELY AFTER HARVEST. DO NOT ALLOW TOBACCO SUCKERS TO SURVIVE.
Non-decomposed stalk and root residues remaining above or in the ground provide an excellent
site for propagation of disease organisms for the following year. Tobacco mosaic virus, potato virus
Y, brown spot, blue mold black shank, and root knot nematodes are reduced by this practice. In
addition several insects and weeds can be reduced by this practice as well. Destroy suckers as soon
as they appear.
TABLE 2. DISEASES OF TOBACCO IN FLORIDA.
ORGANISM DISEASES TYPE OF CARRIED VIA SURVIVES5 WITH SURVIVES IN MOVED BY WIND
ORGANISM SEED SOIL WEEDS OR RAIN
Pythium spp. Seedling blights, root rot Fungus 1 ++ ++ +
Rhizoctonia solani Seedling blights, stem rot, leaf Fungus ++ ++ +
blight (target spot)
Phytophthora Black shank Fungus ++ +
Peronospora tabacina Blue mold Fungus ++ +2 ++
Fusarium oxysporum Fusarium Wilt Fungus ++ -
Cercospora nicotianae Frogeye leaf spot Fungus + ++
Alternaria tenuis Brown spot Fungus + + ++
Macrophomina Charcoal rot Fungus ++ + +
Psuedomonas syringae Angular leaf spot Bacterium + + + ++
Ralstonia Granville wilt Bacterium ++ + +
Erwinia carotovora Hollow stalk & barn rot Bacterium + + +
Tobacco mosaic virus Tobacco mosaic Virus + ++ 6
Potato virus Y Potato virus Y (vein banding) Virus -- ++ 3
Tobacco etch Tobacco etch virus Virus -- ++ 3
Tomato spotted wilt Tomato spotted wilt virus Virus + ++ 4
Cucumber mosaic Cucumber mosaic virus Virus -- ++7 3
++ = Occurs commonly. + = occurs. = Does not occur.
2 In Texas on wild tobacco (N. Repanda)
Aphids are vectors.
4Spread by thrips
5 Includes organic debris
Spread by hands or otherwise mechanically
7Commelina spp. (Dayflower) are major sources of CMV
8 pv tobaci was formerly pv angulata
Table 3. CHEMICAL USAGE FOR PLANT DISEASE CONTROL IN TOBACCO.
CHEMICAL RATE/ACRE DISEASREMARKS
Ridomil Gold EC 1/2-1 pt Blue mold For use in the field only. Use when forming field beds or just
(sensitive strains before transplanting. Not for use for transplant production
only; in Florida, indoors or outdoors. For prolonged control in the field, use
resistant strains 1/2 pt/A directed to the soil at layby or last cultivation
have provided that no more than 1 pt/A of Ridomil Gold was
predominated in applied prior to transplanting.
Ridomil Gold EC 1-3 pts. Black Shank1 For prolonged control, use 1 pt. when forming field beds or
just prior to transplanting + 1 pt at first cultivation + 1 pt at
layby or last cultivation.
Dithane 75 DF 0.6-1.8 lb, greenhouse or Blue mold, For use in the plant bed or field. Use every 5-7 days but not
float bed Brown spot after first button or within 21days of harvest, whichever is
1.5-2.4 lb, plant bed earlier. In the field, use drop nozzles so that spray covers the
1.0-2.0 lb, field planting lower and mid-stalk leaves.
Actigard 50 WG /2 oz Blue mold2 For use in the field only. Do not use more than 1
TSWV3 oz/acre/season. Read the label about tank mixing
Carbamate 76 WDG 0.44-0.88 lb, plant bed Blue mold Use for the plant bed only. Use twice weekly
Chloropicrin 7 gal. (in-the-row) Granville wilt Apply 2-3 weeks prior to transplanting
Telone C-17 8.5-10.5 gal (in-the-row) Black Shank1 Apply 1-2 weeks prior to transplanting. Do not plant if odor
of Telone C-17 is present in the fumigation zone.
Nitrogen Excessive fertilization with nitrogen promotes blue mold.
1 Black shank control is best achieved with the use of a combination of crop rotation, resistant varieties, and proper chemicals.
2 Actigard 50 WG induces resistance within the plant and must be applied prior to the appearance of disease. For blue mold, apply
before 1% disease is present.
3 Only partial suppression of TSWV can be expected.