Rhizoctonia Blight

Rhizoctonia blight is caused by Rhizoctonia solani, the same fungi that cause belly rot of cucumber, black scurf of potatoes, collar rot, damping-off, root rot, and several turfgrass diseases. The name translates from the Latin for ‘root murder’.

Rhizoctonia blight on bean roots (Howard F. Schwartz) CC BY 3.0

Rhizoctonia blight most commonly attacks dry beans, beets, chickpeas, escarole, peanuts, peas, potatoes, safflower, turnips, yacón, as well as longleaf pines and, more recently, coastal redwoods.


Rhizoctonia blight symptoms

Stem cankers, root rot, and seedling death are the most common symptoms of Rhizoctonia blight. The hypocotyl, or first stem, is often affected. Typical cankers start out as reddish-brown streaks or sunken spots. These cankers get larger and darker over time, creating a rough texture. Eventually, these cankers can girdle the stem. All this damage to the root system ultimately causes the plant to collapse and die.

Rhizoctonia blight symptoms of beet (Howard F. Schwartz) CC BY 3.0

Rhizoctonia blight lifecycle

Like many other fungi, the one that causes Rhizoctonia blight is a soil-borne disease that can lay in wait for years. It does this by clustering a bunch of hyphae into a hard resting body called sclerotia. Mycelia may also lay dormant in the soil. When neighboring plants grow or decompose, they release chemicals that trigger the dormant fungi to awaken and attack. Entry is often gained through damaged roots and lower stems, but the fungi can pierce the plant’s epidermis or enter through the stoma. Plant cells are then used for food and shelter and reproduction begins.

Peanuts infected with Rhizoctonia blight (Lindsey D. Thiessen, Jason E. Woodward) CC BY 4.0

Rhizoctonia blight management

This pathogen prefers warm, wet weather and it normally takes hold in early summer, but you wouldn’t know your plants were infected until much later in the season. To prevent Rhizoctonia blight from occurring in your garden, use these tips:

• only use certified disease-free seeds and seedlings and resistant varieties
• improve drainage and reduce soil compaction with aeration and mulch
• wait until temperatures are warm for plants to grow quickly
• avoid overhead watering which can splash fungi onto nearby plants
• employ crop rotation

The fungicide pentachloronitrobenzene (PCNB) is somewhat effective against Rhizoctonia blight, but there is debate about its safety. It was removed from the U.S. market altogether in 2010. The EPA authorized its use in limited cases one year later. I don’t want it anywhere near my food crops (or me).


Once again, prevention is the best medicine. Keep your soil healthy. Space your plants out properly. And toss sick plants in the trash.