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Pathogen Biology

The fungus persists from season to season in crop debris. The mycelium infects roots of living hosts (Figures 6 & 8) and as the root dies, the fungus saprophytically colonizes the dying tissue (Figure 11). Therefore, the fungus is almost exclusively soil-borne. Infested, soil-borne fragments can be transported by wind, water, animals, and farm or turf machinery. The fungus is a relatively poor saprophyte and does not compete well with the native soil microflora. It is also sensitive to inactivation by heat when soils are heated by solar energy. Thus, it is most prolific when host roots are plentiful and when there are relatively short saprophytic periods (weeks or a few months) between the availability of susceptible roots.


Figure 6	Figure 8	Figure 11

Gaeumannomyces graminis is the name given to the sexual or "perfect" stage of this Ascomycete fungus. The fungus produces dark, flask-shaped fruiting bodies called perithecia (Figure 12). Each perithecium (Figure 13) contains many asci (sacs). Each ascus (Figure 14, left) contains eight long, thin spores called ascospores (Figure 14, right). The sexual stage of the fungus is not important to the development of the disease.


Figure 12	Figure 13	Figure 14

Strains or varieties of the Gaeumannomyces graminis fungus are associated with different grass hosts. For example, the variety tritici (G. graminis var. tritici) causes root rot on wheat, the variety avenae affects roots of oats and bentgrasses, and the variety graminis causes a sheath rot of rice and is associated with a root decline of bermudagrass, centipedegrass, and St. Augustinegrass in the southeastern United States.

Biological control
An interesting phenomenon called "take-all decline," associated with wheat monoculture, has been described in many parts of the world. When this happens, the disease declines to an insignificant level after several years of being severe. Root-colonizing bacteria, that are antagonistic to the take-all fungus, build up in the soil to make it suppressive to take-all. An important aspect of suppression is the production of antibiotics by the bacteria that inhibit the growth of the fungus on the root. Figure 15 shows the inhibition (clear zone) of the take-all fungus by three colonies of antagonistic bacteria. Take-all decline is one of the classic examples of "biological control" of a plant disease and has been extensively studied.

Figure 15

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