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Disease Cycle and Epidemiology

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The disease cycle

Fusarium graminearum overwinters on infested crop residues (corn stalks, wheat straw, and other host plants) (Figure 12). On infested residues, the fungus produces asexual spores (macroconidia) which are dispersed to plants and other plant debris by rain-splash or wind. When conditions are warm, humid, and wet, the sexual state of the fungus (Gibberella zeae) develops on the infested plant debris. Bluish-black perithecia form on the surface of these residues, and forcibly discharge sexual spores (ascospores) into the air (Figure 13). The ascospores are picked up by turbulent wind currents and may travel great distances in the air.


Figure 12	Figure 13

Infection occurs when the ascospores (and also macroconidia) land on susceptible wheat heads. Extruded anthers during wheat anthesis (flowering) are thought to be the site of primary infection (Figure 14). If the anthers are infected just after their emergence, the fungus will colonize and kill the florets and kernels will not develop. Florets that are infected later will produce diseased kernels that are shriveled and wilted, or "tombstone" in appearance. Kernels that are colonized by the pathogen during late kernel development may not appear to be affected, but may still be contaminated with the mycotoxin DON (see Pathogen Biology for more information about mycotoxins).


Figure 14	Figure 15

Infected kernels may be used as seed for a subsequent wheat crop. These infected seeds, if left untreated, may give rise to blighted seedlings (Figure 15). The degree to which this occurs in the field depends on the percentage of infected seed and soil conditions affecting the growth and development of the seedlings.

The aerobiology of G. zeae

Viable propagules (ascospores and macroconidia) of Gibberella zeae exist in the air before, during, and after wheat flowering. Generally, the majority of spores dispersed from crop residues travel only short distances, but given appropriate weather conditions and wind, spores may spread over long distances.

Using remote- controlled aircraft (Figure 16) and boats equipped with spore-sampling devices (Figure 17) researchers are studying the longe-range aerial dispersal of the pathogen. Spores of G. zeae have been collected hundreds of feet in the air over agricultural fields, forests, and lakes.


Figure 16	Figure 17

If viable spores were transported long distances (miles, rather than meters) it would suggest that new, more virulent forms of the fungus could spread rapidly through previously unexposed wheat production regions. These novel strains of the pathogen, moving over long distances, may have the ability to overcome current levels of resistance in existing cultivars of wheat and barley.

There is considerable debate on the relative importance of local vs. long distance spread of inoculum on disease development. In years with highly favorable weather conditions for pathogen spread and infection, local management of overwintered inoculum (e.g., tillage, spraying of debris, etc.) may have negligible impact on disease levels unless performed over extensive production areas.

The environment and FHB

FHB infection is favored by extended periods of high moisture or relative humidity (>90%) and moderately warm temperatures (between 15 to 30°C/ 59 to 86°F). These conditions present before, during, and after flowering favor inoculum production, floret infection, and colonization of developing grains.

FHB is well suited for disease forecasting because of its narrow time periods of pathogen sporulation, spore dispersal, and host infection that contribute to epidemic development. Efforts are underway to develop accurate disease-forecasting models for FHB. These models incorporate factors such as temperature, humidity, rainfall, inoculum production, and plant development as predictors for the severity of FHB.