Symptoms and Signs
There are a number of root rot pathogens that can cause similar root rot damage to bean plants.
Black root rot, caused by Thielaviopsis basicola, occurs on blackeyes as well as on other dry bean types. Brown to black necrotic tissue develops on the belowground stems and roots. Fissures often develop in necrotic cortex tissue. The fungus can be identified by the presence of dark resting spores (chlamydospores), which can be seen on diseased tissue with a 20x magnifier hand lens.
Fusarium root rot is characterized by lesions that develop on underground stems and tap roots. Over time, the brick-red lesions turn brown and longitudinal fissures occur in the cortical tissue of affected areas. If the surface of the lesion is scraped away, small red flecks can be seen in the plant tissue. In severe situations, the entire root system may be killed, although sometimes, new roots can occur above the lesion.
Rhizoctonia root rot occasionally occurs on the upper taproots of older plants as discrete, reddish brown lesions. Infections of plants older than four weeks are not common and usually only cause minor damage.
Roots with severe root rots will not be able to deliver adequate water supplies to the plant during times of high demand. Too much or too little water can cause the infected plant's leaves to turn yellow. In general, infected plants tend to be small with yellowing leaves that undergo early senescence and produce low yields.
Comments on the Disease
Fusarium root rot is most commonly encountered during mid to late season in fields with a long history of bean production. The disease causes little damage to unstressed plants, but under conditions of drought, poor nutrition, or oxygen-stressed, waterlogged soil, Fusarium solani can be one of the causes of early maturity ("cut out") and marked reduction in yield.
All of the root rot pathogens can survive for several years in the soil.
Management
Long-term crop rotation to non-susceptible crops such as grasses (monocots) may help to reduce soil inoculum. Avoiding excess irrigation or long drought stress may also help manage root rots in beans. Seed treatments for Rhizoctonia and Fusarium root rot pathogens may help manage these diseases. To prevent Thielaviopsis root rot, plant when soil is warm.
| Common name | Amount per acre** | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticide having the greatest IPM value first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| Note: When using treated seed, be sure to place it in the row below the granular rhizobia. | ||||
| SEED TREATMENTS | ||||
| A. | FLUDIOXONIL/MEFENOXAM | |||
| (Maxim XL) | See label | 48* | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylpyrroles (12) / Phenylamides (4) | ||||
| COMMENTS: Treats Rhizoctonia, Fusarium spp., and others. | ||||
| ** | Mix with sufficient water to obtain full coverage. |
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number. |
| * | REI exception: If the seed is treated with the product and the treated seed is soil-injected or soil-incorporated, the Worker Protection Standard, under certain circumstances, allows workers to enter the treated area if there will be no contact with anything that has been treated. |