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Identifying and Managing Southern Blight by Paul Pilon

August 03, 2010

Southern blight (Sclerotium rolfsii) can be a very destructive disease that infects several annuals, perennials, and woody ornamentals. Southern blight is more common in southern locations, but is also commonly observed in northern locations. These pathogens attack crops very quickly, often catching growers by surprise. Sclerotium is relatively difficult to control compared to other plant pathogens making it even more problematic for growers.

Sudden wilting of otherwise healthy appearing plants is usually the first symptom of Southern blight. Initially, this wilting only occurs during the heat of the day and the plants often appear to recovering following irrigation or during the cooler evenings.  However, as the disease progresses, the plant eventually does not recover and permanently collapses.  

Sclerotium initially attacks host plants at the soil line and rapidly moves up the stem as it destroys plant tissues with oxalic acid and pectolytic enzymes it produces as the pathogen spreads. Besides wilting, other symptoms on some plant species are expressed as chlorotic leaves, leaf abscission, or reduced growth and vigor which often results in death.

While scouting, observe the base of the plant; infected plants usually have water-soaked, soft, or rotted plant tissues near the soil line.  A white to cream colored, fan-like fluffy fungal growth (mycelium) that forms on the lower stem, leaves, and soil near the base of the plant is often present. A tattle tale indication of Sclerotium infections is the presence of small clusters of light to dark brown mustard seed sized ball-like masses (with or without mycelium) along the soil surface and on the lower plant stems. These ball-like structures are called sclerotia and contain the pathogen that may attack other plants when the opportunity arises.

Southern blight is considered a warm season disease that can develop over a broad range of temperatures (46 to 104° F) but most commonly occurs when the day temperatures are unseasonably warm (81  to 95° F) and the conditions are moist (hot & humid). Sclerotium commonly appears during the summer months following extended periods of rainfall. During periods of warm, humid temperatures and frequent rainfall, growers should scout their crops more frequently- looking for wilted plants and other symptoms of Southern blight.

Prevention and good sanitation practices are the primary methods of reducing the occurrence of Southern blight. Sclerotium is most commonly spread from location to location from infected tissue; be sure to obtain starting materials from a clean and reputable supplier. Never store potting mixes directly on the ground and do not reuse growing medium or pots previously infected with this disease. Produce plants at an adequate spacing to improve the air movement and encourage quicker drying of the foliage and surface of the growing mix.

The sclerotia are very resistant to environmental extremes and can lay dormant in the soil for several years.  Growers should not produce susceptible crops in areas previously infected with Sclerotium for a minimum of five years. 

If symptoms are detected and a Sclerotium infection is identified, carefully remove the infected plants immediately to reduce the sclerotia from being produced and disseminated throughout the crop. When removing infected plants from the production site, avoid dropping infected plant parts onto or near adjacent plants in the production area. 

Once plants are infected, there are no good curative measures growers can implement to control Sclerotium rolfsii.  Fungicides containing flutolanil, metribuzin, pentachloronitrobenzene (PCNB), pyraclostrobin + boscalid, and triadimefon are effective when used preventatively or can be used to treat surrounding areas where infected plants have been removed to reduce the occurrence of future infections.

Paul Pilon
Perennial Solutions Consulting
paul@perennial-solutions.com

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