Life History of Fusarium Head Blight and Environmental Conditions for Epidemic
Fusarium graminearum is the most prevalent causal organism of Fusarium Head Blight, and deoxynivalenol (DON) is the most notable mycotoxin that accumulates as the disease progresses during the growing season. DON levels of <1.0 ppm are permitted in wheat destined for human consumption. Wheat grain with DON levels over 5 ppm may not be marketable. DON contamination of winter wheat in Ontario frequently exceeds 1.0 ppm.
All winter wheat varieties in Ontario are genetically susceptible to infection, especially when favourable environmental conditions occur during specific stages of wheat development. As a result, the severity of DON contamination is largely dependent on the weather during specific developmental stages of the pathogen and the host. Epidemics are common when weather is ideal for both inoculum production and head infection around anthesis. Fungal growth occurs with sustained moisture on plant residue at the soil surface. This growth produces sexual fruiting bodies called perithecia (Figure 3). The wheat is only susceptible after heads are emerged and the crop is in flower (Figure 4).
 
We developed a model to identify critical weather conditions which promote DON contamination of wheat grain; the model was trained using data from 399 farm fields across southern Ontario from 1996 to 2000. An array of environmental conditions were assessed across these fields; DON levels varied from 0 to over 29 µg g-1 in mature wheat grain. Analysis of this data showed that the timing and frequency of specific weather for DON accumulation occurs in three periods around heading. These periods are likely associated with inoculum production, wheat head infection, and fungal growth within the head.
The first period occurs between 4 and 7 days before wheat head emergence (Figure 5), and is likely associated with inoculum production. Inoculum (spores) are required for head infection during anthesis. Frequent rainfall and warm temperatures during this 4-day period are ideal for fungal growth on residue at the soil surface. Dry weather (i.e. dry on the soil surface) and/or cool conditions reduces the amount of inoculum.
The second critical weather period occurs between 3 and 6 days after head emergence (i.e. at anthesis) (Figure 5). The chance of infection increases with frequent rainy days (>3 mm/day), but decreases with the number of days when maximum air temperatures are at least 32EC during this 4-day period.
The third critical weather period occurs after anthesis, or between 7 and 10 days after head emergence (Figure 5). Frequent rain or a wet canopy during this period is likely associated with colonization of the Fusarium fungus in wheat heads.
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