Farmer's Weekly (South Africa)
Dealing with sudden death syndrome in soya bean
This disease causes leaf drop, which in turn inhibits the process of pod filling. However, many questions about the disease remain unanswered, and it is difficult to control.
Sudden death syndrome (SDS) is a relatively new disease of soya bean in South Africa.
To date, it has been found in Mpumalanga and the Bergville/ Winterton area of KwaZulu-Natal (KZN), according to Archana Nunkumar, a scientist at the KZN Department of Agriculture and Rural Development.
SDS is caused by the soil-borne fungus Fusarium virguliforme. Initial infection occurs on the root and crowns of young soya bean plants at the seedling stage. SDS produces toxins that cause leaf drop, reducing the ability of the plant to provide nutrients to promote pod fill. In the US, losses of up to 80% in individual lands have been recorded, notes Nunkumar.
signs of the disease
Symptoms occur after flowering during mid pod-fill. Most dramatic are the leaf symptoms, which occur soon after infection. They consist of circular to irregularly shaped pale green to chlorotic spots. The spots may enlarge and become necrotic, or they may join together. Eventually, part or all of the chlorotic tissue becomes necrotic, with green tissue remaining only near the major leaf veins.
The crowns of SDS-infected plants are usually discoloured, while the roots are discoloured and decayed. Occasionally, blue-green spores of the fungus are seen on the tap root.
Development
As far as is known, SDS is not transmitted by seed. As the primary inoculum occurs in the soil, any disturbance of the soil or infected plant debris can transfer the fungus as well.
The exact time of infection is unknown, although research has shown that the fungus can be isolated from plant roots as early as 40 days after planting, with symptoms appearing only later.
SDS appears to develop in cool, wet weather and compacted soil. A temperature below 15°C is ideal for infection. SDS is a difficult disease to manage, and there are currently no fungicides or resistant varieties. However, you can follow these guidelines to lessen the impact of the disease: • Select soya bean varieties that mature at different times. • Extend planting time so that all the beans are not simultaneously at the same growth stage. However, do not plant outside the optimal planting time for your area. • Learn to identify SDS. Conditions such as brown stem rot, stem canker, charcoal rot and chemical burn, produce similar leaf spots. • Where feasible, improve drainage and reduce soil compaction. This helps aeration and improves root growth. • Crop rotation is of limited value. SDS has been recorded where soya bean has been continually cropped, where it has been rotated with one or two seasons of maize, and where it has not been grown for years. However, continual soya bean cropping is not recommended as it increases inoculum build-up.
• Source: Nunkumar, A. 2015. ‘Sudden death syndrome of soybean’. KZN Department of Agriculture & Rural Development. Retrieved from www.kzndard.gov.za/images/ Documents/RESOURCE_CENTRE/ GUIDELINE_DOCUMENTS/ Research_and_Technology_ Bulletin/2015.04%20 Sudden Death Syndrome of Soybean.wpd.pdf.
Select soya bean varieties that mature at different times