Plantwise Factsheets for Farmers
Rotation against purple seed stain in soya
Recognize the problem
seed stain is fungal disease in soybean. The disease first causes dark purple
points on the leaves which then turn into larger purple lesions. The leaf symptoms
appear during the onset of seed set. Then, the affected leaves become dark
brown and leathery. This gives a bronzing look to the leaves. The symptoms may
be confused with leaf sunburn but in case of the disease, the change in leaf
colour may extend to the stem and pods which sunburn does not cause. In
severe infection, the top leaves fall off. The plant looks like it is mature,
but in a plant infected by purple seed stain, the lower parts are green. Infected
seeds get stained, turn purple and get a low price on the market.
seed stain is a major problem in soyabeans throughout Zambia. This disease is
seed-borne. The disease also survives on soyabean residues. Early infections
do not show damage symptoms, but contribute to later infections in the
season. The symptoms on the leaves are a result of the fungus and sunlight
working together. Crop rotation of soyabeans is an effective way of
controlling the disease. This is because the fungus does not live on other
crops, except on legumes such as beans, cowpea and peas. Chemicals are less effective
because the disease is inside the plant.
After a period of 3 years
of continuous cropping with soyabeans, always rotate with non-legume crops
like maize, sorghum, wheat and finger millet. This is independent from
whether the disease is detected in your field or not.
- In case of heavy infection
of soybeans by purple seed stain (1/4 to 1/2 of plants), do not wait 3 years
to rotate the crop. Rotate immediately the next season.
- Never rotate soyabeans
with legumes, such as beans and cowpeas as they can have the disease.
- During the rotation period
with cereals, tillage should completely bury soyabean residues to around 25
cm of soil depth.
- Apply fertilizer to the
required of amount for a particular rotated crop to avoid soil fertility
problems in soyabeans the next season.
- Basal fertilizer such
as D-compound should be applied in the rotated crop during planting as it is
rich in potassium. This is because potassium is essential for the control of
the disease in soyabeans the next season.
The recommendations in this factsheet are relevant to:
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