Rhizoctonia Blight

Rhizoctonia blight is an infection caused by the same fungi responsible for belly rot of cucumber, black scurf of potatoes,, collar rot, damping off, and root rot. Rhizoctonia solani translates from the Latin for root murder.

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

Rhizoctonia blight attacks dry beans, beets, chickpeas, escarole, peanuts, peas, potatoes, safflower, turnips, yacón, 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 as reddish-brown streaks or small sunken areas. These cankers grow and become 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. They get in through damaged roots and lower stems, but the fungi can also pierce the plant’s epidermis or enter the stoma. Then the fungi use plant cells for food and shelter, and reproduction begins.

Rhizoctonia blight management

This pathogen prefers warm, wet weather. Fungi become active in early summer. But symptoms do not appear until later in the season. It is far better to prevent Rhizoctonia blight with these tips:

• Only use certified disease-free seeds and seedlings.
• Look for resistant varieties.
• Quarantine new plants.
• Improve drainage.
• Reduce soil compaction with aeration and mulch.
• Wait until temperatures are warm for plants to grow more 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.