The most characteristic symptom of take-all (Gaeumannomyces graminis var. tritici) is a blackening of the roots, (Figure 3.7). The take-all fungus gets into the centre of the root (stele) so that when roots are snapped and observed end on, the root is black to the core. In severely infected plants the sub- crown internode may also become infected and the blackening may also progress to the stem base under the leaf sheath, (Figure 3.8).
Plants affected by take-all usually occur in patches within a crop, but may also be scattered randomly throughout. In a severe outbreak the entire crop may be affected.
Above ground, plants appear stunted and yellow- green in colour with reduced tillering. The take-all fungus blocks the water conducting tissue in the plant and restricts water uptake. This can cause infected plants to ripen prematurely, which often results in deadheads or white heads with little or no grain filled, (Figure 3.9). Low soil moisture during October and November increases the occurrence of deadheads.
Figure 3.7 Take-all infection of wheat seedling. Note the fungus is inside the roots
Figure 3.8 Severe take-all symptoms on mature plant showing blackened roots, sub-crown internode and crown
Figure 3.9 Patches of deadheads caused by take-all
Take-all can only survive between susceptible crops in the root and tiller bases of previously infected plants. Following autumn rains, the take-all fungus grows out of this material and attacks the roots of susceptible plants. In the absence of hosts, the fungus is unlikely to survive for more than one year. The build up of take-all is greater during wet springs, but its carry over is reduced following significant summer rain events (ie > 25 mm in a single event) (Figure 3.10).
Figure 3.10 Disease cycle of take-all in cereals. Illustration by Kylie Fowler
Plan ahead and have a grass free break in the year before cropping. Start fallow or remove grass weeds from break crops before the end of July (end of June in the Mallee) to prevent the fungus multiplying and being carried into the next crop. This will also give plenty of time for plant debris, in which the fungus survives, to break down.
Chemical treatments applied to seed or fertiliser are available to reduce the impact of take-all if sowing into paddocks with medium levels of take-all. However, it is best not to sow wheat into a paddock with high levels of take-all. Barley is more tolerant of take-all than wheat, and delayed sowing can reduce the impact of take-all by giving more time for inoculum break down. Little can be done for an infected crop during the season.
Take-all is likely to be a problem when:
- a cereal crop follows a wheat or barley crop or where grass weeds are not controlled in a break crop
- there is a wet spring in the previous season and susceptible crops or grasses are present
- there is no significant summer rain.
CEREAL CYST NEMATODE (CCN)
RHIZOCTONIA ROOT ROT (aka BARE PATCH)