2008 Year in Review

From the Fields

Sudden death syndrome (SDS), a destructive disease associated with yield loss, has been detected in some northern Illinois soybean fields.

The disease is caused by a soilborne Fusarium fungus that infects the soybean root system early in the season (as early as 2 to 3 weeks after planting), especially when soils are cool and wet. The disease produces a toxin resulting in chlorosis (yellowing) and necrosis (death) of the interveinal tissue of leaves. These foliar symptoms typically appear in late July and early August, usually visible after pods begin filling.

The disease appears to be favored by high yield environment, early planting, early maturity, compacted soil, poor drainage, and cool, wet conditions during the summer. There appears to be an interaction between SDS and soybean cyst nematode.

The name comes from the fact that even though the disease may need a few weeks to develop, the time from leaf defoliation to plant death is short. SDS can affect the entire field, but normally affects only scattered areas within a field.

What are the symptoms of SDS infected plants? Initial symptoms are usually scattered chlorotic spots that occur between the veins on leaves and the leaves may be cupped or curled. These spots enlarge between veins to become brown lesions surrounded by chlorotic areas. The process continues until only the mid-vein and major lateral veins remain green. Leaves often drop from the top of the plant, leaving the petiole (leaf stem) attached to the main stem. Severe foliar symptoms give affected field areas a brownish cast, whereas healthy plants remain green. Pod drop may also occur.

The effect on yield depends upon the growth stage at the time of initial symptom development and the speed and severity of foliar symptoms.

It is important to split the stem lengthwise and check the color of the pith to get a definitive diagnosis. Plants with SDS will have white pith with no discoloration or decay. There may be a slight gray-brown discoloration of the vascular system just inside the outer “bark” of the stem, but the pith remains white.

In addition to the above ground symptoms, SDS affected plants will have rotted roots and diseased plants may be easily pulled from the ground. This is due to the root rot phase of the disease initiated earlier in the growing season. Other symptoms include necrosis of the plant crown. Research has shown that it is possible to have infection of the root system, without visible foliar symptoms.

How does one prevent or manage the disease? No in-season control of SDS exists. But, management begins with proper identification.

Yield loss can be minimized by planting varieties with relatively high levels of tolerance or resistance. Many seed companies provide resistance ratings for their varieties. Variety information is also shown at this University of Illinois web site VIPSIf SDS is identified, one should also take soil samples to determine the presence of soybean cyst nematode (SCN) and if it is detected, plant a variety that is also resistant to SCN.

To help reduce risk of SDS, select varieties that mature at different times. Delay planting (but not past the suggested time for your area) or extend planting so that all soybean are not at the same growth stage at the same time. Plant fields with a history of SDS last. Efforts to minimize soil compaction and improve drainage may help reduce disease severity.

Additional information is available at University of Illinois Extension offices.

by  Editor, theCity1.com
September 2, 2008