Damping-off of is a general term that describes the death of seedlings from fungal disease. There are several fungal organisms that can cause damping off, including Botrytis, Sclerotinia, Alternaria, Pythium and Rhizoctonia. The last two are the most common culprits.
At some point in his or her career, most growers have had the experience of seeing sections of seedling flats turn brown right at the soil line and topple over. This is often an indicative sign of Rhizoctonia (Rhizoctonia solani). Older, sturdier seedlings may die, but are more likely to stay upright.
While some fungi are host-specific, Rhizoctonia does not discriminate, and it can infect practically all herbaceous and woody greenhouse crops. Younger plants are most susceptible. The collapse and death of seedlings, which have brown lesions on the stems right at the soil line, is the most commonly associated sign of Rhizoctonia, but the disease can kill seedlings before they even emerge from the soil. Severely infected seed does not even germinate, or if it does, the newly sprouting roots and stems are killed before they emerge. When Rhizoctonia infects older plants, it causes a brown or black canker at the base of the stem near the soil line. If the canker grows, it eventually interferes with the uptake of water and nutrients and the plant dies. Sometimes Rhizoctonia can be seen on fruit or leaves close to the soil surface because it has spread there by splashing water. In high humidity, the fungus may grow up the stem from the soil line.
Rhizoctonia survives from year to year by producing sclerotia, which are tiny, irregularly shaped, brownish-black structures that can survive for many years in the soil and on plant matter. It can also survive as a branching, filamentous structure called the mycelium, which also has the ability to persist in soil, at the end of hoses and on tools and containers until conditions are right for infection.
Prevention And Control
Rhizoctonia does not produce spores, so it is spread mainly by water or wind moving infected soil particles. It is also easily spread by human workers who track infected soil on their feet or hands and by diseased cuttings or plants.
Unfortunately, R. solani thrives in normal greenhouse conditions. It doesn’t need high humidity or excessive moisture to thrive, although such conditions will enhance its rapid spread. It favors warmer temperatures, so it is most common in the spring and summer.
As with most greenhouse problems, prevention is key. Purchase disease-free seed from a reputable source. Workers need to strictly observe sanitation guidelines, and greenhouse floors should be kept clean and free of soil and plant debris. Infected plants should be rogued out at the first sign of disease and a protective drench should be applied to the rest of the crop. Preventative fungicidal drenches can be applied to crops that have been found to be commonly affected by Rhizoctonia in the operation in the past.
Ball, Vick. (Ed.). Ball RedBook. 16th ed. Batavia, IL: Ball Publishing, 1998.
Ceresini, Paulo. Rhizoctonia solani. North Carolina State University. http://www.cals.ncsu.edu/course/pp728/Rhizoctonia/Rhizoctonia.html
Damping-off Disease. Cornell Greenhouse Horticulture. Cornell University. http://www.greenhouse.cornell.edu/pests/pdfs/diseases/RootRots.pdf
Wick, Robert L. 1998. Damping Off of Bedding Plants and Vegetables. University of Massachusetts Amherst. http://extension.umass.edu/floriculture/fact-sheets/damping-bedding-plants-and-vegetables