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Posted 23 July 2007. PMN Crop News.

What's Causing Big Yellow Patches in Soybean Fields?

University of Illinois at Urbana-Champaign.

Urbana, Illinois (July 6, 2007)--Many soybean producers have been noticing large yellow areas in their soybean fields. Excessive heat and dry weather can be a catalyst to really pinpoint areas where the soybeans are stressed and while we are wet now, it was very dry for a long time. Well, what is going on in those areas? Suzanne Bissonnette, University of Illinois Extension Integrated Pest Management Educator, says one cause might be soybean cyst nematode (SCN).


Most producers know that when SCN numbers are excessively high, plants are stunted, yellow, and yield poorly. However, in our area, yellowing symptoms from cyst root damage typically do not show up. SCN can reduce yields 15 to 30% without causing any stunting or yellowing. In the yellowed areas we are seeing right now though, if you find cysts on the roots, one thing you can know for certain is that SCN has been in that field causing losses for many, many, many seasons already.

Gently dig up the roots from the periphery of the yellowed areas and examine for the female cyst nematode. Right about now she should be a glistening white to cream color about the size of a grain of sand. If you are over 40 this may require a 10X hand lens. If you don't find cysts on the roots of the affected areas, it doesn't necessarily mean you don't have a cyst problem. They could have been knocked off the roots when being dug, or they may have already dropped off the root on their own.

Compaction from ponding, other root diseases, and nutrient imbalances can also be responsible for the yellow patches we are now seeing as well. But, the best long-term management decision is to test the field for SCN. The action threshold for SCN, which is the number of nematodes that indicates some action should be taken has been lowered, it is now 'one,' not three like before. If you actually find cysts on soybean roots, you are already over the threshold for the field. In this case, I still recommend testing so that you have a baseline number for population monitoring.

Should testing indicate you are over the threshold, your management decision is easy. Select and plant a resistant variety the next time you plant that field to soybean. Your long-term management decisions have been simplified too. Rather than having the goal of working the population down to below the threshold, we now recommend continued selection of cyst resistant varieties, rotating sources of resistance.

For specific information on yield and disease resistance for soybean varieties tested in Illinois check out the 'Varietal Information Program for Soybean' website at If you don't want to type that long address into your browser do a quick Google search with the keywords "VIPS home soybean."