The much-discussed disease sudden death syndrome (SDS) is now appearing in central Illinois soybean fields. The symptoms usually appear in the flowering growth stage, which indicates we are on schedule this year for appearance of the disease. Although SDS, like most diseases, is difficult to predict because of the many environmental and other variables that influence its development, in some Illinois areas the conditions seem to have been favorable for development of SDS. This disease is often severe where yield potential is high. In addition, SDS appears to be favored by early planting, compacted soil, poor drainage, soybean cyst nematode, and heavy rainfalls throughout summer.
SDS is caused by the soilborne fungal pathogen Fusarium solani f.sp. glycines. The foliar symptoms are most obvious. Chlorotic spots develop between the veins on leaves, and the leaves may become cupped or curled. The spots typically enlarge to become brown lesions surrounded by yellow areas. The leaves often detach from the petioles as the disease progresses. Gray to light brown discolored areas develop inside the root and in the vascular tissue of the lower stem. Roots often rot, and plants may then be pulled easily from the ground. The foliar symptoms can appear very similar to brown stem rot (BSR) symptoms, but the pith remains white in plants infected with SDS while the pith becomes brown, especially at the nodes, in plants infected with BSR.
SDS is difficult to manage. Yield losses can be minimized by planting cultivars with relatively high levels of tolerance or partial resistance to SDS. Crop rotation has not shown consistent benefits. It may also be beneficial to plant SCN-resistant cultivars and plant later than normal where SDS has been a problem. Deep tillage may help reduce disease severity in compacted areas. Information on SDS tolerance/resistance for commercial varieties from Illinois trials can be found at Web sites from the University of Illinois (http://web.aces.uiuc.edu/VIPS/v2home/vips2home.cfm) and Southern Illinois University (http://www.siu.edu/~soybean/).
SDS symptoms on soybean leaves.
In addition to SDS, Septoria brown spot is present in many Illinois soybean fields. This common and usually insignificant disease is caused by the fungal pathogen Septoria glycines. The pathogen can be spread and may overwinter on infected plant debris; it also may be seed transmitted. Septoria brown spot is favored by warm, wet weather; continuous soybeans; and minimum tillage. It often first appears on young leaves early in the summer and progresses to other leaves and plant parts throughout the season.
Infections begin on lower leaves and progress upward to develop later on upper leaves. Small, dark brown spots (pinpoint to 1/5 inch in size) develop on both surfaces of leaves; the spots may grow together to create irregular brown patches. Infected leaves turn brown and yellow and may drop prematurely. Because this disease is usually minor and causes insignificant damage, no special effort is usually required for disease management. Resistant varieties are not available. Rotation with nonlegume crops is recommended, and under severe conditions, tillage and fungicides may be warranted.--Dean Malvick
Septoria brown spot symptoms on soybean leaves.