Rhizoctonia Web Blight

From BASF: The Chemical Company Website

By - Paul Pilon

Although Rhizoctonia  is a soil borne disease, contrary to what many growers believe, it rarely causes root rot infections. Rhizoctonia often infects plants during production and causes number of different diseases including aerial blight, leaf spots, cutting rot, stem rot, bulb rot, and dampening off. Aerial blight or web blight caused by Rhizoctonia is particularly problematic for growers during late spring and summer production when the environmental factors are favorable and the crop canopy is dense.

Web blight occurs very quickly and causes plant stems and leaves to collapse. When observing the symptoms closely, you can see the fine web-like strands of the fungus (mycelium). The dead leaves remain attached to the stems, dangling, suspended by threadlike hyphae from the pathogen.  Rhizoctonia web blight is often confused with Botrytis blight; the key identifier between them is the presence of the powdery, gray mold (fuzzy gray or brown spore masses) that occurs with Botrytis blight that does not occur with Rhizoctonia web blight.

The environmental conditions favorable for web blight are warm growing conditions (above 80° F), high humidity, and wet plant and soil surfaces.  It most commonly occurs when crops are spaced closely together and there is a dense plant canopy (poor air circulation).  Overhead irrigation or rainfall promotes disease development when the plant canopies are dense.  In many instances, infected starting materials and reusing flats and containers that have not been properly sanitized are the sources of infections. 

The occurrence of web blight can be reduced using cultural strategies such as avoiding plant stress (extreme wet/dry cycles and excessive levels of soluble salts in the root zone), providing adequate spacing between crops, and irrigating at a time of the day when the foliage and media surfaces will dry quickly.

Biological controls (Gliocladium, Streptomyces griseoviridis, and Trichoderma harzianum) can be effective when they are used preventatively.  Fungicides containing the active ingredients chlorothalonil, fludioxonil, iprodione, kresoxim-methyl, PCNB, pyraclostrobin, pyraclostrobin + boscalid, and thiophanate-methyl are effective at controlling Rhizoctonia web blight.

The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned. ALWAYS READ PRODUCT LABELS AND USE THEM AS DIRECTED ON THE LABEL.