The in-crop, symptoms of CCN or eelworm (Heterodera avenae) are plant stunting and yellowing, which often gives the crop a ‘patchy’ appearance, (Figure 3.1). The presence of CCN can be established by inspecting the primary roots and looking for abnormal branching and knotting, (Figure 3.2). Symptoms can be confirmed at flowering time by the presence of small white ‘cysts’ (1-2 mm in diameter) that are attached to the roots, (Figure 3.3).
Figure 3.1 Severe crop yellowing and stunting caused by CCN
Figure 3.2 Abnormal knotted wheat root system caused by CCN
Figure 3.3 CCN infected root system with white females
CCN is likely to be a problem when:
- susceptible cereals are grown too frequently
- susceptible self-sown cereals and wild oats are not controlled in pastures or in break crops. This is important because one wild oat or susceptible cereal per square metre is enough to maintain high nematode numbers in the soil.
A PreDicta B soil test is recommended prior to sowing to identify paddocks at risk of CCN.
CCN survives between susceptible cereal crops as eggs inside protective brown cysts that form on the roots of host plants, (Figure 3.4). In the autumn, nematodes hatch from eggs in response to moisture and low temperatures (<15 °C) (Figure 3.5). Nematodes hatch over a period of several weeks, with the peak hatch occurring about six weeks after the autumn break, (Figure 3.6). In a further eight weeks these nematodes will form viable eggs. Therefore, to prevent CCN multiplying, it is necessary to control host plants within 10 weeks of crop germination.
Each year approximately 85 percent of nematodes hatch from cysts after the autumn break, while the remaining 15 percent stay dormant until the following season. This is why it takes at least two years with break crops to control CCN. However, under dry (drought) conditions up to 50 percent of nematodes remain dormant and an extra year of break crop is advisable.
Figure 3.4 Brown CCN cysts carry the nematode over the summer
Figure 3.5 CCN eggs hatching from brown cyst
Figure 3.6 Disease cycle of the cereal cyst nematode. Illustration by Kylie Fowler
CCN has a narrow host range that is limited to cereals and some grass weeds. Susceptible cereals (some varieties are resistant) and wild oats are the most important hosts. Ryegrass, brome and barley grass are poor hosts.
Plan ahead and make sure there is at least a two year disease break following susceptible cereals or paddocks infested with wild oats. Timing of host removal is critical when establishing a disease break. In calculating the critical date to remove host species from break crops consideration should be given to the time taken for host plants to die after herbicide application. Nematodes will continue to feed until the plant is dead.
Host plants, particularly wild oats and susceptible self sown cereals, must be controlled before the nematodes have completed the development of eggs. This is approximately 10 weeks after the autumn break.
The use of resistant cereals and non-host crops, or chemical fallow in rotations as part of a two year break, is an effective method to control CCN.
- grass free pulse and oilseed crops or legume pasture
- resistant cereals - refer to the current Victorian Cereal Disease Guide (AG 1160) for a list of CCN resistant cereal varieties
- chemical fallow prepared early in the season before nematodes have produced viable eggs.