Pea cyst nematode
The cysts can be found embedded in the roots of host plants. These are cream coloured, lemon shaped and 0.5mm in diameter. In the soil the cysts mature to a brown colour.
Affected plants are yellow, stunted, with small leaves and tend to die early. When examined the roots have no or few nitrogen fixing nodules and the overall symptoms are consistent with nitrogen deficiency. Damaged plants can be seen in patches within the field and these grow and enlarge in subsequent years as a result of field cultivations.
In response to host plant root exudates, and in suitably warm temperatures with adequate soil moisture, larvae emerge from the overwintering cysts and move to invade the roots. When fully fed the females form cysts which can contain up to 150 eggs. These can remain in the soil for periods of up to 10 years although it is thought that the annual rate of decline is 50 % per year.
Where peas and beans are grown in short rotations it has been estimated that yield losses of up to 20 % can occur.
Yield losses can be sustained with soil populations of 5 eggs per gram of soil.