Symptoms and Signs
Pythium ultimum usually causes preemergence rot and, in some cases, damping-off of young seedlings (a weakening of seeds or seedlings before or after they germinate). Symptoms include water-soaked lesions with eventual collapse of the hypocotyl (stem of germinating seedling) at or below ground.
Rhizoctonia solani causes postemergence damping-off of seedlings that is characterized by sharp-edged oval to elliptical reddish brown lesions on the hypocotyl. Heavy infection may girdle the stem and the seedlings may die. Often the lesions heal over as the plant ages.
Thielaviopsis basicola causes a black root rot on young seedlings and older plants. The dark discoloration of roots and the presence of resting spores (chlamydospores visible with a microscope) are diagnostic of this pathogen.
Comments on the Disease
The fungi that cause seedling diseases occur commonly in soils. Most Pythium species are active during cool, wet weather. Seedling diseases caused by Pythium spp. are usually not severe unless beans are planted in cold wet soils. Warm soil temperatures favor Rhizoctonia solani. In addition, R. solani is usually more of a problem when the bean crop follows alfalfa or sugarbeets.
Management
Cultural Control
For blackeyes, plant when the average soil temperature is greater than 68°F (20°C). For limas, the soil temperature should be a bit warmer at 70°F.
To avoid Pythium and Rhizoctonia:
- plant into moist, but not overly wet, soil (from residual rain or irrigation moisture) rather than irrigating immediately after planting
- plant deep enough to put seed into moist soil but avoid planting too deep and exposing more of the hypocotyl to infection.
Crop rotation may help reduce inoculum in the soil.
Organically Acceptable Methods
Use cultural control for an organically certified crop.
Monitoring and Treatment Decisions
Seed treatments may be helpful in fields with a history of seedling diseases. Combine fungicides that are active against Pythium and Rhizoctonia solani. There are no seed treatments effective against Thielaviopsis basicola. If seedling disease appears during crop emergence, consider using treated seed in the future.
| 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. | MEFENOXAM | See label | 48* | NA |
| (Apron XL) | ||||
| MODE-OF-ACTION GROUP NAME (NUMBER1): phenylamide (4) | ||||
| COMMENTS: Controls Pythium spp. For protection against other soilborne diseases, such as Rhizoctonia spp., Apron XL should be applied in combination with other registered seed treatment fungicides. | ||||
| B. | AZOXYSTROBIN | |||
| (Dynasty) | 0.153–0.765 fl oz per 100 lb seed | 4* | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): methoxy-acrylates (11) | ||||
| COMMENTS: Controls Rhizoctonia. Label suggests combining with Pythium active seed treatment for protection against seedling diseases. | ||||
| C. | 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. Combine with Apron XL to treat Pythium spp. | ||||
| D. | THIRAM | |||
| (Thiram 42-S) | 2 fl oz per 100 lb seed | 24* | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Dithio-carbamates and relatives (M3) | ||||
| COMMENTS: Reduces losses from seed decay, damping-off, and seedling blights caused by many seedborne and soilborne organisms. | ||||
| E. | CAPTAN | |||
| (Captan 4L) | 2.6 fl oz per 100 lb seed | NA | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): phthalimides (M4) | ||||
| COMMENTS: Do not use on lima beans. Controls soilborne fungi which cause seed decay and seedling blight. | ||||
| ** | 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. |