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Disease Cycle and Epidemiology

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During time periods when no living hosts are present, the fungus survives as mycelium in soil-borne host residue that was infected the previous growing season (Figure 16). It also can survive parasitically on roots of volunteer hosts, perennial hosts, or grassy weeds. The sexual stage (ascospores) is not important.

Figure 16

When roots of a susceptible plant grow near to the infested residue, the mycelium colonizes the roots and begins the infection process. "Runner hyphae" on the root surface (Figure 17) produce infections (Figure 18) at numerous locations along the roots.


Figure 17	Figure 18

Infections progress into the stele of the root (Figure 19) and move up and down the root axis resulting in root death. When numerous roots are killed (Figure 6), or the fungus reaches the crown area and rots it, the plant dies.


Figure 19	Figure 6

After plant death, the fungus survives saprophytically in the plant tissues that it colonized during its parasitic phase, completing the cycle (Figure 16).

Epidemiology
Take-all on small grains occurs worldwide. Fall-sown small grains are more severely damaged than spring-sown ones. The disease is important only where these hosts are grown in a monoculture (that is, where crop rotation is not practiced). It is usually more severe under reduced-tillage regimes, soils that are between pH 6.0 and 8.5, moist soil conditions, and soil temperatures between 5 and 15^°C (40-60^° F). Although it is usually most severe in coarse-textured soils where there is good aeration, take-all can occur in heavier soils.

Take-all also occurs on bentgrass throughout North America, western Europe, Australia, and New Zealand. It is most common when bentgrass is planted into recently cleared land or soil that has been fumigated. It may be particularly severe on recently established, sand-based golf course putting greens. It is believed that the increased severity of take-all in newly-seeded bentgrass and in sand-based greens is due to the limited populations of antagonistic bacteria. As in cereal crops, infection and colonization of roots is favored by cool, wet conditions in fall and spring, although disease symptoms may be even more apparent following hot, dry conditions in summer.

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