Sclerotium rolfsii is a very common soil-borne fungus infecting a wide range of vegetables, ornamental and field crops. It is most active during warm, wet weather in tropical and subtropical regions. The fungus causes rots of the lower stem, roots and crown. It can also cause rot of fruit in contact with the soil
Symptoms develop on plant parts in or near the soil. The most common symptom is a brown to black rot of the stem near the soil line. The stem becomes girdled and the plant wilts suddenly and dies. A coarse, white, cottony fungal growth, containing white, spherical resting bodies (sclerotia) covers the affected area. The sclerotia soon become light brown and resemble cabbage seed. Fruit symptoms usually develop where there has been contact with the soil. Decay may progress rapidly, eventually causing complete collapse.
The fungus can survive for years as sclerotia in the soil or in host plant debris. Sclerotia spread with soil movement, infested plant material and contaminated equipment. Infection and disease development are favoured by warm, moist conditions. Sclerotium diseases often develop on crops produced under sub-optimal growing conditions, when plant vigour and quality has been compromised by other factors.
Vegetable crops commonly affected include bean, beetroot, capsicum, carrot, cucurbits, sweetpotao, potato and tomato.
Control of sclerotium diseases is difficult when soil and weather conditions favour the fungus. Management systems that can reduce the disease severity include the following:
- Ensure plant residues have decomposed before planting.
- Deep ploughing soil to bury host debris and sclerotia is a useful measure.
- Include non-susceptible crops such as maize and small grains in rotations to reduce inoculum levels in soil.
- Drench transplants with the recommended fungicides.
Treated areas should receive direct and full sunlight. Soil solarization will significantly reduce viable sclerotia. It will also help control other soil borne diseases, plant parasitic nematodes, and some weeds. To be effective in control of southern blight, solarization must be repeated every year.
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