Soybean
Stem Rot
Sclerotinia sclerotiorum
Fungus
In a Nutshell
- Spots on fruits, leaves or petioles.
- Spots covered with white, cottony mold.
- Later dark wart like structures.
- Wilting of stem and upper plant parts.
Soybean
Symptoms
Symptoms vary among host species, but there are a number of similarities. Initially, water-soaked spots with an irregular shape appear on fruits, leaves, or petioles. As they enlarge, the affected areas become covered by a white, profuse cottony mold, at later stages scattered with visible grayish or black wart-like reproductive structures called sclerotia. "Dry" lesions may develop on at the base of stems and branches, clearly delimited from the healthy tissues. During later stages, the fungus encircles the stem and the upper parts of the plant tend to wilt, turn brown and die. Hardened fungal growth form inside the stem and replace the plant tissues. This may result in death and consequent lodging of plants. Infected pods and seeds may shrivel or may be replaced by black fungal growth.
Recommendations
Organic Control
Granular formulations of spores of the fungal parasite Coniothyrium minitans or species of Trichoderma have been applied to soils to reduce the fungal load of Sclerotinia and hinder the development of the disease.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatment if available. Foliar fungicide applications are only recommended in fields with severe disease development. Treatments will vary depending on the crop in question and the developmental stage. Control of Sclerotinia diseases of cabbage, tomato and beans is difficult. However, fungicides based on iprodione or copper oxychloride (3 g/l of water) provide effective control on lettuce and peanuts. The development of resistance has been described for some of these compounds.
What caused it?
The symptoms are caused by the soil-borne fungus Sclerotinia sclerotiorum, which can survive for long periods of time on plant debris or in the soil. Most of its life cycle takes place in the soil. This explains why the symptoms begin in leaves and plant parts in contact or near the ground. When conditions are favorable, it resumes growth on organic matter and occasionally by invading plant tissues. As they colonize all plant parts, seeds may also carry the pathogen, either on the seed coat or internally. The new batch of spores produced on the plant are airborne. The humid microclimate under the canopy favors the spreading of spores onto the stems. Initial development requires several hours of leaf wetness and temperatures from 15 to 24°C. The presence of external nutrients also favors its growth. This fungus has a broad range of host plants such as bean, cabbage, carrots and canola.
Preventive Measures
- Use healthy seeds from a certified source.
- Choose resistant or more tolerant varieties, if available for the crop in question.
- Do not plant on previously infested land.
- Use a wider row distance allowing good ventilation of the crops.
- Use wires or stakes to support the plants.
- Monitor the field for signs of the disease.
- Control weeds in and around the field.
- Prune infected branches or crop parts.
- Do not over-fertilize during late stage of growth.
- Avoid excessive irrigation during later stages of plant growth.
- Do not till the land as no-till systems have a lower risk of disease development.
- Rotate crops with non-host plants such as cereals.
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