North Carolina Plant Disease and Insect Clinic
Plant Disease Fact Sheets

Rhizoctonia Diseases in Tobacco Greenhouses

The brown discoloration of the seedlings is due to Rhizoctonia infection.


M. Mila, W. A. Gutierrez, and H. D. Shew

Introduction

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 2008 were about 1.7%. 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.

Symptoms - Pathogen Biology

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. 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 (Figure 2). 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, hyphae (hymenium) are formed on the soil surface, on infected stems, and on leaves. Then production of spores start which are wind dispersed all over the greenhouse.  When conditions are not favorable for basidiospore production (i.e. 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 (Figure 3). If higher temperatures and humidity persist, lesions become very constricted and the stems break-off (Figure 4). The lesions continue to grow throughout the stem and leaves causing them to turn brown and die. 

Primary Sources of Inoculum

The primary sources of inoculum for Rhizoctonia diseases are infested trays. 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. 

Disease Management

Tray sanitation: The best control for Rhizoctonia diseases is sanitation of trays. The best control was obtained with methyl bromide or steam.

Fungicides: See NC Agricultural manual: http://ipm.ncsu.edu/agchem/6-toc.pdf

 

Other resources: http://ipm.ncsu.edu/Production_Guides/Flue-Cured/flue_cured.pdf

 

Creation Date: 2001

Revision Date: April 2010

Key Words: tobacco, disease, greenhouse, sclerotia, Rhizoctonia solani

Glossary Terms: basidiospores, hymenia, foci, sclerotia