Wednesday, September 19, 2012
Soybean Stress Disease
This is one of the many soybean samples that we have started to receive late in the season. As always, we evaluate the plants for disease. What do we know about this field? This is the second time in a row that it has been planted to soybeans and it was previously pasture ground before that. There has been no soil test done to evaluate fertility. Most of the field is having problems, which leads me to believe that the main problem is not a disease.
There were several plants that appeared to have insect injury on the stems and pods. It is difficult to know what type of insect fed on the plants without seeing the actual insect; however we suspect that it may have been bean leaf beetles.
The stems of all the plants were split and a few of the plants consisted of a dark pith. This area was cultured to determine what was causing this darkening of the inner stem. Sometimes we isolate various pathogens, but in this case, we isolated a fusarium sp. and this points to Fusarium blight or wilt. The following is information about this disease was taken from the Compendium of Soybean Diseases: Symptoms of this disease will appear midseason during hot weather, particularly on plants growing in sandy soils. This disease has not been reported in seedlings. The most characteristic symptoms is a black or brown of vascular system in roots and stems, which is evident when stems are split. Leaves of affected plants may become chlorotic, wither, and eventually drop. Flaccid leaves and wilting of stem tips are most common on young plants. Pods of infected plants are often poorly developed, but root rot is minor. If available, cultivars with resistance to Fusarium and soybean cyst nematodes should be grown. High quality seeds should be planted in warm, well-drained soil, and cultivation practices that prevent or reduce soil compaction and promote favorable soil moisture should be used. Good soil fertility should be maintained. Crop rotation with non-hosts may reduce inoculum levels.
The roots of the soybeans were examined and found to be somewhat infected with Rhizoctonia sp. The Compendium of Soybean diseases states that as the season progresses, root symptoms can be found on the plants at the advanced vegetative to reproductive growth stages. These plants may have been infected as seedlings and have lesions that are enlarging to girdle the stems (as seen on this sample). Infected plants can be stunted, yellow, and have poor root systems because lateral roots often decay, leaving only the taproot and secondary roots. Wilt symptoms associated with root rot occur throughout the early reproductive stages, when the disease occurs with warm, dry weather, herbicide injury, soybean cyst nematode damage, and other stresses. Diseased plants first appear in areas where water and nutrients are poor, such as on hillsides, and may be scattered in lightly infested fields or in circular, discolored areas in heavily infested fields. Disease patches are sometime elongated, following the direction of tillage. There is no resistance available for R. solani in soybeans. Good cultural practices promote seedling health during early stages. Other stresses, such as herbicide injury, poor soil, insect damage, and feeding by SCN should be reduced.
Can we blame this field's problems on disease? The answer is no. I am sure some of the blame can be put on the hot and dry weather, but further investigation will need to be done to find out if other stresses could be causing these soybeans to be more vulnerable to these diseases.