From the road the field looked like this, yellow patches surrounded by healthy plants.
In the patches the plants looked like this.
Digging up plants and washing roots exposes the culprit.
Plants infected with soybean cyst are typically stunted, yellowed and dying with decaying outer leaf margins. The nematode invades the soybean root and produces a nodule that looks something like a rhizobia nitrogen fixing nodule, but is smaller and white in colour. This nodule is the reproductive stage of the female nematode and is actually the egg sac. When the female dies the egg sac or "cyst" breaks away. The eggs inside the sac can survive for several years and emerge to infect a subsequent soybean crop.
Soybean cyst is estimated to be the number one yield limiting pest to soybean production in North America. I have randomly sampled fields in our neighbourhood for cyst nematode over the last 3 years and found either no cyst present or extremely low levels of cyst. This field has not been previously sampled.
Cyst spreads easily from field to field in the dirt that travels on machinery.
Cyst can be managed, but it requires discipline. Fields need to be rotated to non-cyst crop hosts like corn and wheat. The longer the rotation away from soybeans the better.
There are resistant varieties to cyst, but we have two problems.
1. There are not many choices of cyst resistance in varieties for our maturity. Pioneer has one, 90M80, a 2700 heat unit RR variety. Growers in the fuller season areas of Ontario have more options.
2. There are only three sources of resistance for soybean breeders to work with. Cyst nematode populations are composed of many races. The population can quickly shift races in the field to overcome genetic resistance.
No joy indeed.