Cereal cyst nematode of wheat
Cereal cyst nematodes (CCN; Heterodera spp.) are some of the most economically damaging nematodes to wheat production. Most small grain cereals and some grasses are hosts. Common symptoms on wheat include stunting and poor plant growth appearing in patches across a field, reduced number of tillers, and plants appearing more yellow than surrounding healthier plants. Clusters of excessive branching (“bushy knots”) occur on roots at points of nematode invasion. White, lemon-shaped female nematodes may be seen among the bushy knots during plant heading. After plant maturation, female bodies harden into cysts and turn dark brown. Nematode cysts within bushy knots on roots is diagnostic of this disease. Cysts enclose and protect nematode eggs, allowing them to remain viable for several years. Symptoms can be observed from tillering stages through ripening.
CCN completes one life cycle annually, with juveniles hatching and invading roots in early and midspring. CCN hatching is dependent on moisture and temperature. Nematodes tend to produce the most cysts on small grains in sandy, well-drained soils and in cereal production systems lacking rotation. Greatest yield loss occurs when dry land production systems experience late-season water stress.
Resistant wheat varieties exist that prevent CCN reproduction. Because root invasion occurs before resistance is activated, some resistant varieties may not produce acceptable yields as they are intolerant of CCN invasion. The best varieties have resistance and CCN-invasion tolerance, and can produce near-normal yields. Rotation to a non-host crop for at least two years significantly reduces CCN in soil. Other effective rotations include a non-host crop plus a fallow year between susceptible wheat or barley crops. Growing winter wheat tends to give plants a greater chance of developing a strong root system, helping plants tolerate nematode invasion in spring. Reducing plant stress through proper fertilization and irrigation (when necessary) can also help prevent yield losses. Destroy volunteer grasses since they can serve as hosts to CCN.
Gallery images: M. Burrows.