Rhizoctonia Diseases in Tobacco Greenhouses
TB07 - Tobacco Disease Note 7
W. A. Gutierrez, H. D. Shew, and T. A. Melton
The production
of healthy tobacco transplants is the first step toward the production
of high quality tobacco in the field. In North Carolina around 95% of
the transplants are produced in greenhouses using the float tray system.
One of the most common diseases of tobacco seedlings in greenhouses is
damping off caused by Rhizoctonia solani. Estimated losses from
this pathogen in the production of tobacco seedlings in North Carolina
in 1999 were about 0.8%. In addition to damping off, some strains of R.
solani cause target spot, a leaf disease initiated by the basidiospores
of Thanatephorus cucumeris (name of the sexual stage of R. solani).
Basidiospores of Thanatephorus are produced in hymenia, which are
formed on the soil surface, on infected stems, and on leaves during period
of high relative humidity, prolonged leaf wetness, and moderate temperature.
  Figure 1 (top) Target spot; (bottom) Hymenium on the lower stem at the soil surface. |
Symptoms
Rhizoctonia solani causes two types of diseases on tobacco seedlings in greenhouses: target spot caused by R. solani strain AG3 in most cases, and stem rot, sore shin, or damping-off caused by R. solani strain AG4.
Target
spot. This disease is usually observed as small foci when the
canopy is already formed (high humidity and high temperature). Symptoms
on leaves begin as small, round, water-soaked spots about 2-3 mm in diameter
(figure 1). Under favorable conditions these lesions enlarge rapidly,
becoming light green, almost transparent, with irregular margins and chlorotic
halos. In infested areas, lower leaves turn brown and stick to the surface
of the tray and the presence of brown spider-like webs (mycelium) may
be observed attached to leaves and stems. When periods of high relative
humidity, prolonged leaf wetness, and moderate temperature are present,
cream color hyphae (hymenium) are formed on the soil surface, on infected
stems, and on leaves (figure 1). Then production of spores start which
are wind dispersed all over the greenhouse. When conditions are
not favorable for basidiospore production (low moisture), leaf spot isolates
may cause damping-off and sore shin of tobacco seedlings. This strain,
in a few cases, kills the plant.
Damping-off. This disease is usually
observed at early stages of seedling growth (first 3 weeks).
The first symptom is a small water soaked lesion on the stem close to
the soil line that rapidly becomes brown and sunken. If higher temperatures
and humidity persist, lesions become very constricted and the stems break-off.
The lesions continue to grow throughout the stem and leaves causing them
to turn brown and die (figure 2).
Primary
Sources of Inoculum
 Figure 2. Damping off symptoms.  Figure 3. Rhizoctonia in crevices of a tray, serving as a source of inoculum. |
The primary
sources of inoculum for Rhizoctonia diseases are infested trays.
Resting structures (sclerotia) of R. solani are formed in trays
where the disease developed the previous season. These structures
are formed on the surface and in the crevices of styrofoam trays. After
the season ends, trays are washed and stored until next season. On washed
infested trays, small specks can be noticeable on the surface. Resting
structures of R. solani on the surface of trays can be easily inactivated
with a 10% clorox solution, however those in the crevices of the styrofoam
can be difficult to render inactive. From these areas, new infective hyphae
start to grow and infect plants.
Control Recommendations
The best control for Rhizoctonia diseases is sanitation of trays. The best control (100%) was obtained with methyl bromide and steaming treatments. Current control recommendations are: Thoroughly wash previously used trays and allow the to dry, then fumigate with methyl bromide at 3 lb/1000 cubic feet; do not depend on dipping trays in any sanitation product, including bleach, to satisfactory kill fungal pathogens. Considerations to be taken when using methyl bromide: Crisscross trays up to a 5 feet high, tarp and sealed on concrete or on a tarp, then fumigate. Check that air temperature is above 65 F at the time of the fumigation. Do not fumigate inside the greenhouse. Allow at least 48 hours of aeration before using trays.
Additional
Pictures
Symptoms of damping off
- Initial infection in the greenhouse and seedling (Figure 3, 4)
- Close ups of primary infection of collar rot (Figure 5, 6)
- Older seedlings and secondary infections (Figure 7, 8)
- Close up of secondary infections on tobacco leaves (Figure 9, 10)
- Late stage of secondary infection (Figure 11, 12)
- Hyphae and mycelium of Rhizoctonia solani (Figure 18, 19, 20)
- Hymenium of Rhizoctonia solani - Thanatephorus cucumeris (Figure 22, 23)
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For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service
Recommendations
of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data. All recommendations for pesticide
use were legal at the time of publication, but the status of registration
and use patterns are subject to change by actions of state and federal
regulatory agencies.
North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress
of May 8 and June 30, 1914. Employment and program opportunities are offered
to all people regardless of race, color, national origin, sex, age, or
disability. North Carolina State University at Raleigh, North Carolina
A&T State University, U.S. Department of Agriculture, and local governments
cooperating.
Revised July 2001 by Plant Disease and Insect Clinic