Root Rot Complex in Edible Beans
- Who are the Players?

There are many organisms that cause root rot symptoms on edible beans. In Ontario, the four main fungal pathogens are Fusarium, Pythium, Rhizoctonia and Chalara (formerly Thielaviopsis). These organisms can occur individually or in combination, as is often the case. This is referred to as "root rot complex". The amount of damage is related to the general health of crop, past history, cultivar susceptiblity and environmental conditions.

Symptoms can appear on plants at any stage of development. Early season infection results in typical pre-emergence (seed decay) and post-emergent (seedling death) "damping-off" symptoms, thereby reducing plant stands (poor emergence). Plants that survive early infection ("damping-off") or become infected later display characteristic "root rot" symptoms such as discoloured roots, stunting, wilting, etc.

What Do They Look Like?

Fusarium begins as small reddish, brown lesions (first few weeks) which as the plants ages join to form larger lesions or streaks on the tap root surface. A brown -reddish internal discolourization of the water conducting tissue can be seen by splitting the tap root, crown and lower stem. Adventitous roots may develop on plants that have a damaged tap root. These adventitious roots are formed above the damaged area often close to the soil. The plant is attempting to by-pass the damaged root system by establishing a new root system. This new root system though can be prone to dry conditions. Late infection seldom results in dead plants but rather in stunted, unthrifty looking ones.

Figure 1 - Fuarium root rot discolouration of root. Note proliferation of roots
on severely infected plants (right) - Photo courtesy of R. Hall

Figure 2 - Internal (vascular) discolourization of
dry edible bean due to Fuarium wilt.

Pythium infection has a characerstic brown, water-soaked (wet) lesion that starts at the base of the tap root. This lesion advances up the root and stem, eventually stopping 2 or 3 cm above the soil line. Seedlings are often killed resulting in stand establishment problems. Although older seedlings and mature plants may not die from Pythium infection, their roots are often pruned resulting in a stunted, poorly anchored, wilted and unhealthy looking plant.

Figure 3 - Pythium wilt in dry edible beans (Photo - R Hall)

Rhizoctonia forms reddish brown sunken lesions on the stem and tap root most frequently near the soil line. The lesion can girdle the entire stem causing stunting or death of the plant. This lesion is distinctively "brick-red" in colour, noticeable immediately after removing the plant from the soil. This is one method of distinguishing Rhizoctonia from Fusarium. The intensity of the "brick-red" colour will fade rapidly with exposure to the air.

Figure 4- Rhizoctonia cankers on hypocotyls and roots.
(Photo courtesy of Schwartz)

Black root rot results in brown to black lesions being formed on the tap and lateral roots. Under severe conditions the entire tap root may be black.

Figure 5 - Black root (Hagedorn/Abawi)

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