Anne Dorrance, Assistant Professor, Plant Pathology
Mac Riedel, Professor, Plant Pathology

Fifty-five percent of the 3,678 soil samples had soybean cyst nematodes. Thanks to the Ohio Soybean Council and the North Central Soybean Research Program for sponsoring the soil sampling program and the SCN campaign to raise awareness. We can now begin to manage these populations to limit yield losses. The next question is how many eggs or cysts do you need before you start losing beans? Fortunately, our SCN team has been running field trials to find that very answer. The results from a trial in 1999 can be found on the IPM web site. From the study, susceptible beans out yielded resistant beans by as much as 5 to 10 bu/A when SCN populations were below 200 eggs/200 cc of soil. However, when SCN populations were between approximately 2000 to 6-8,000 eggs/200 cc of soil, the resistant varieties had a 5 to 10 bu/A advantage over susceptible varieties. Above 10,000 the resistant varieties had a greater than 10 bu/a advantage.

Remember that SCN populations are quite variable across a field, and what your average numbers of eggs/200cc of soil is dependent on how well you took your soil sample (figure 1). SCN occurs in fields in pockets or hot spots. Generally, if we could maintain our three-crop rotation, we would not see dramatic increases of SCN over time like we do now in continuous soybean systems (figure 3). Figure 2 gives an example of some real data and estimated data from Ohio production fields under different cropping systems.

Our next biggest challenge will be to maintain our sources or resistance. Within every SCN field there are individuals that have an appetite (can reproduce) on the different sources of resistance. When we utilize those sources of resistance exclusively such as the PI88788, we favor those nematodes that can reproduce well on that source. Figure 4, figure 5 and figure 6 illustrate what can happen in a field where susceptible or PI88788 cyst resistant beans are planted. In order to manage SCN effectively over the long-term it is essential to rotate crops AND rotate sources of resistance to avoid any yield lossed due to SCN.