Sclerotinia white mold (Sclerotinia sclerotiorum) is a fungal pathogen that affects a wide range of crops including dry beans. It infects the flowers on the dry bean plant. White mold thrives in cool, moist conditions, such as those associated with the end of the growing season here in the northeast. Now is the time when you may start to notice signs of white mold infection in your dry bean or soybean fields.
The symptoms associated with white mold infection are bleached, brown lesions, and white cotton-like mycelia (Fig 1). Symptoms can appear on the stems and pods of the dry bean plant. It does not affect the leaves directly, although while scouting your field you may notice a ‘flag leaf’ or singular wilted, yellow leaf indicating white mold infection lower down on the plant under the canopy. The pathogen survives as sclerotia (compact masses of hardened fungal mycelium; Fig 2) in the soil for several years. Sclerotia can reproduce either by the production and release of airborne spores that come into contact with the plant or by direct contact with the plant with the fungal growth in soil or neighboring plant.
Fungicide applications can be useful for preventing infection of the dry bean flowers if you have a field with a history of white mold. But the applications must be done at the time of flowering. Once white mold symptoms develop, any applications at this point can only protect the further developing flowers and will not cure the current infection. Dr. Sarah Pethybridge shared a lot of helpful information about dry bean diseases including white mold and the use of fungicides (organic and conventional) in our Beans for Lunch webinar series (click to view recording).
To reduce the risk of future white mold infection, it is important to rotate your dry bean crop with non-host crops such as corn and cereals. Crop residue management is also very important. Do not leave white mold infected residue on the soil surface. If possible, cover up the plant residue with soil to promote degradation. Other ways to reduce the risk of white mold infection is by increasing airflow to the plants. This can be done by reducing seeding rate or increasing row spacing. Additionally, reducing weed pressure can be beneficial. Many broad-leaf weed species such as lambsquarters, ragweed, pigweed, and velvet leaf, act as hosts to white mold.
