These graphs are the "answers" to the worksheet handed out at the Regional Agronomy Schools. We hope you did your homework! The numbers came from fields in Ohio, where we have multiple year data but most came from Dr. Mac Riedels crop rotation study. THESE ARE GENERAL TRENDS. For the lawyers and bean counters out there, this is the trend that we have seen over and over again, there are many environmental factors, weeds, soil sampling techniques that all influence what the actual SCN numbers are in any given field.

We hope that this exercise will help everyone to understand how crop rotation works as a disease management tool.

Soybean Cyst Nematode populations increase 10-fold in newly infested fields with every susceptible soybean crop. As their numbers increase (over 5,000+) they begin to compete with each other for feeding sites and food and populations only increase 3 to 6 fold. This is very dependent on soil type and the environmental conditions that exist in a given field during the production season. Populations of cyst only decline by 1/2 for every year of a non-host crop - PROVIDED that all alternate hosts (weeds) are controlled.

For each of these charts start with a beginning population of 50 eggs/cup of soil. Chart the SCN Populations. Remember, 2,000 SCN eggs per cup of soil can result in significant yield losses even on resistant soybeans.

Field - 1. Continuous soybeans- susceptible bean

The resistance to soybean cyst nematode is measured as a proportion. The number of nematodes that can successfully reproduce on a resistant bean compared to a susceptible bean. Which means that SOME nematodes will reproduce on a resistant soybean. We have found in Ohio, when resistant beans are planted the first few times - three things may happen 1. The population will decline, 2. Stay the same. 3. Will increase. However, we have always seen when resistant beans are planted continuously, the populations always increase and at the 10-fold level.

Field 2. Continuous Soybean - Resistant Beans for two years, then the SCN populations that can reproduce on the source of resistance begin to increase at the same rate as they originally would have on susceptible beans.

Field 3. Corn - soybean rotation (only susceptible varieties were used in this field) , in any year with corn, populations drop by half.

Field 4. Corn - soybean rotation, in any year with corn, populations drop by half. In this field, during the soybean rotation, different sources of resistance were used. The first planting is with a susceptible bean, the second with corn, Resistant bean PI88788, corn, Resistant bean-Hartwig, corn, susceptible soybean

Field 5. Wheat, Corn, Soybean rotation--

Remember, even susceptible beans can produce high yields when LOW populations of SCN exist in soils. The challenge is to keep the populations low. Populations can increase dramatically in a short period of time as you should have seen on these graphs. However, the decline to low levels is a long process. SCN can be managed successfully with crop rotation. Both of the crop that is to be planted and the sources of resistance.

Bonus Field: Effects of double crop or intercropping soybeans in wheat on SCN populations