Soya bean pests and diseases…some control strategies


IN this episode, we are discussing control of pests and diseases.
Like most other crops, soya bean is affected by a number of diseases and pests.
We shall outline some of the methods used to control specific pests and diseases.
Uses of specific chemicals will be outlined.
Chemicals mentioned are only examples and farmers are encouraged to consult chemical companies and experts for advice.
There are many products coming on the market and this article may not exhaust discussion on all these products.
The main strategy that has been used to control soya bean diseases is to breed varieties that are resistant or tolerant to the diseases.
Farmers are encouraged to approach seed houses that stock soya bean seeds to get more information on specific varieties and their performance characteristics.
We shall start by looking at some agro-chemicals in soya bean production.
Fungal diseases attack soya bean, sometimes resulting in poor germination.
Thiram is one fungicide that is widely used to suppress fungi and improve seed germination.
Diseases controlled include damping off and soreshin.
The usual recommended rate is 170g of thiram to a 100kg of soya bean seed.
Thiram can be safely applied to inoculated seed. The seed must be planted soon after inoculation.
Carbryl 85WP is a powdered insecticide for the control of semi-loopers; heliothis boll worm and other leaf eaters.
The application rate usually recommended is 1kg/ha.
Other insecticides are readily available on the market.
An alternative to use of insecticides is to collect dead and dying semi-looper larvae.
These are crushed into powder, mixed with water and sprayed at edges of soya bean fields. Crushed larvae contains a virus which kills the semi-loopers.
This is a form of biological control which is relatively safe for the environment as no artificial pesticides are used.
Another pesticide called dimethoate is used to control aphids.
The rate recommended is 1 litre per hectare.
Farmers must read the container label for more information on how to safely use this and other pesticides.
The next group of useful pesticides are the fungicides. There are several commercial products including, but not limited to examples like, tilt, score, shavit, punch extra, funginex and folicur. The disease controlled is soya bean rust
The application rate varies with the product; users should read the label on container.
Scouting for rust disease is important from five to six weeks after germination, just before flowering.
A preventive spray of fungicide is recommended at first flowering, whether disease has appeared or not.
We now review some important diseases of soya beans. These include rust, red leaf blotch and frog-eye leaf spot.
Rust: This is currently the worst disease affecting soya beans in Zimbabwe.
Potential yield losses of between 30-90 percent may be incurred if controlled late.
The disease was first recorded in the 1997/98 season and has spread from commercial farming areas to communal and resettlement areas. Farmers failing to control the disease have suffered severe yield losses.
Rust first appears on the lower leaves as a yellow discoloration, rapidly spreading upwards through the canopy. Infected leaves drop prematurely; seeds mature early, resulting in incompletely filled pods. It is not rust disease if top leaves are the ones first affected.
Control of rust disease
Currently soya bean breeders have released several resistant or tolerant varieties to soya bean rust. Chemical control is very effective using fungicides. Chemicals used to control soya bean rust are expensive but their cost is small compared to the potential income farmers will get from sales of the soya bean crop.
Farmers planting retained seed of older varieties must watch out for rust disease.
Currently, six fungicides listed in Table 1 are being used.
Availability and costs vary. Farmers are therefore advised to buy these in advance in order to avoid last minute rush as failure to spray results in reduced yields.
It is very important to always read the instructions on the label before handling, storing or using the fungicides.
The correct protective clothing should be worn at all times and empty containers should be disposed of safely after use.
NB: Mention of the above does not mean endorsement. The author has not specifically evaluated these products and is advising farmers to seek information from the companies who sell the products.
Appropriate time of application for fungicides
It is recommended that soya beans should be sprayed twice or thrice depending on the pressure of the disease in the season.
First spray: At flowering (50 days after planting for most varieties)
Second spray: Three weeks later (70 days after planting).
Third spray: A third spray (at 90 days after flowering) might not be economic except in areas with high disease pressure.
Farmers are strongly encouraged to spray for rust on every soya bean field whether the disease has appeared or not. Spraying after symptoms have appeared on the soya bean leaves will not prevent losses through greatly reduced yields.
Red leaf blotch
Large areas of red colouration (blotches) appear on many leaves. They reduce the area available for the plant to manufacture its food. Severe infections may cause an acute defoliation prior to flowering of very susceptible varieties. Current varieties are resistant to red leaf blotch but older varieties may be severely affected, especially in very wet seasons.
Control: The disease is controlled by rotation with other crops such as maize or planting of resistant varieties.
Frog-eye leaf spot
This is common on susceptible varieties late in the growing season following warm, humid weather. Crop residues from the susceptible varieties and wind borne spores primarily cause frog eye leaf spot disease in subsequent crop.
Therefore, farmers should avoid using home-retained seed that is infected with this disease. Chemical control is not necessary. Farmers are encouraged to plant certified seed of resistant varieties.
Calibration of knapsack sprayer must be done carefully to avoid an overdose or underdose of chemicals.
If the canopy is full, a flat fan nozzle should be used to obtain the best penetration, if the canopy is not fully closed, a hollow cone nozzle should be used.
Farmers should seek advice from their AGRITEX officers whose job it is to provide farmers with technical back-up services.


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