Pests and diseases (Soybeans)

Pests’ control

Different insects occur in soybean fields but few are normally of any economic importance, and the species that cause damage are usually not abundant enough to warrant control measures.

However, a kind of butterfly lays eggs which give caterpillars with green color and black lines in their back often hiding under soybean leaves and devour them. Caterpillars’ attacks inflicted

important damages at flowering stage and during dry season. Insect pests can be controlled with a single spray of Cypermethrin, deltamethrine or fenvalerate.

· Soybean rust disease 

Soybean diseases normally result in major yield losses in Rwanda.

The most devastating disease remains the asian soybean rust caused by Phakopsora pachyrhizi, which is also the most important foliar disease in the country. Severe infection leads to premature defoliation and yield losses up to 80%. The disease is of great economic importance when rainfall and humidity are high. Plant resistant varieties use is the best option to control disease. The unique disadvantage of using rust resistant varieties is that it is increasing inoculums levels in the field. Foliar fungicide spraying such as Benlate allows controlling disease when weather is highly humid and with heavy rains thus favorable for disease development. It is also recommended to proceed to crop rotations with maize in order to prevent the increase of inoculums levels in a field.

 · Bacterial diseases 

The main bacterial disease is caused by Pseudomonas syringae pv. glycinea . Even the disease is not severe; it appears sometimes for non-resistant varieties. Symptoms appear as specks to large, irregular spots with raised light-colored pustules in the elevated centers of the spots on the lower surface. The elevated pustules sometimes have cracks in them. Later lesions join together and the dead areas tear away to give a ragged appearance to the leaves. The best option to control the disease is the use plant resistant varieties.

· Virus diseases

Soybean is also susceptible to viruses; however it is urgent to investigate more to know exactly which viruses are prevailing in Rwanda. Most of the virus infection results in foliar symptoms such as mosaic and mottling, thickening/brittling of older leaves, puckering, leaf distortion, severe reduction in leaf size, and stunting of plants. The cultivation of virus disease-resistant

varieties is the most convenient, economical, and effective approach for controlling soybean virus diseases. As many viruses involved virus disease are seed transmitted in soybean, strategies to control the diseases include the use of free seed-borne diseases, uprooting and destruction of diseased plants, eradication of weeds and voluntary plants, seed treatment with systemic insecticides and spraying foliar insecticide to reduce insect vector activity during preflowering stage which is the most vulnerable period to virus infections of the plant.