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

Symptoms of target spot Primary inocolum of Rhizoctonia Pathogen structures Alkaline Water Agar (AWA) a semi-selective medium to isolate Rhizoctonia solani

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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