Late season soybean diseases
Sudden Death Syndrome
Symptoms of sudden death syndrome (SDS), caused by a strain of Fusarium virguliforme, may appear several weeks before flowering but are more pronounced after flowering. Foliage symptoms begin as scattered yellow blotches in the interveinal leaf tissue. These yellow blotches may increase in size and merger to affect larger areas of leaf tissue. Yellow areas may turn brown but veins remain green giving the leaves a striking appearance. Infected plants may wilt and die prematurely. Severely affected leaflets may drop off the plant leaving the petiole attached or may curl upward and remain attached to the plant. Root systems may show deterioration and discoloration of lateral roots and taproot. When split open, internal tissues of the taproot and stem may show a light gray to light brown discoloration.
Management options for SDS are somewhat limited but should include planting varieties which have performed well where SDS has been a problem, improving drainage in poorly drained fields, avoiding compaction, staggering planting dates, delaying planting until soils are warm and dry, avoiding continuous crop soybean, maintaining good crop vigor, avoiding crop stress including stress from soybean cyst nematode and harvesting fields with SDS in a timely fashion.
Soybean Vein Necrosis
Last year a relatively new virus disease of soybean, soybean vein necrosis, showed up in many soybean fields in various regions of the state. Initially, small light-green to yellow patches develop near main leaf veins. These patches then develop a mottled light green-yellow-brown pattern. The veins in these areas of the leaflet may become clear to almost translucent which is referred to as vein clearing. As the disease progresses these areas turn reddish-brown with a browning of the veins. The reddish-brown areas may have a scaly or scabby appearance. On more susceptible varieties the brown areas may expand killing larger areas of leaf tissue and giving a scorched appearance to the leaves.
The virus which causes soybean vein necrosis belongs to a group of viruses called tospoviruses which are spread by thrips. At this point the disease is still a relatively new disease and there are more questions about it than answers. It appears that the virus is spread from soybean to soybean by thrips but which species(s) of thrips is unknown. Other hosts, especially weed hosts, have not been confirmed. And there are many questions related to the disease cycle, possible yield losses and appropriate management strategies. Varieties seem to vary in their susceptibility to this virus disease and symptoms may vary with varieties.