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Disease Cycle and Epidemiology

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Disease Cycle

Sclerotia (ergots) may be left on the soil at the end of the season or sown with the seed when cereal or grass crops are planted. The sclerotium (or ergot) is the survival or overwintering structure of C. purpurea. A period of 4 to 8 weeks of near freezing weather is required for germination of the sclerotium. Sclerotia germinate in the spring, just prior to flowering in the cereals and grasses. Ascospores are ejected into the air and are disseminated by air currents. Only those ascopores that land on a host stigma or ovary can cause infection.

The stigma of a grass flower is large and featherlike; this feature helps trap windborne pollen. This same feature traps the airborne ascospores. Ascospores, which are the primary (initial) inoculum, germinate and infect the ovary within 24 hr.

Within five days, conidia form on the surface of the infected ovary. They are exuded in a clear-to-tan , sticky, sugary honeydew (Figure 7). Conidia are the secondary inoculum and are dispersed to other flowers by physical contact, splashing rain, and insects. The honeydew attracts insects to the wind-pollinated flowers. Insects contaminated with conidia may visit healthy flowers where new infections are initiated. Conidia from ergot-infected wild grasses, particularly in fence rows, can be the primary inoculum in cereal and grass seed production fields.

Figure 7

Over time, the infected ovary develops into a hard, dark colored sclerotium (or ergot) (Figures 1,3).

Figure 1

Figure 3

Epidemiology

Claviceps purpurea is common in temperate climates in which the cold period required for sclerotial germination is met. In warmer climates, such as the southeastern U.S., sclerotia are colonized by other fungi and do not survive well.

Rainfall or high soil moisture is required for stroma formation and ascospore production. In the cereal grains and many of the grasses, resistance to infection develops after fertilization. Thus, conditions that delay or interfere with pollination, such as cool, wet weather, can increase the period of susceptibility.

Conidia are an important means of secondary spread. Any transfer of honeydew from infected to healthy flowers can lead to infection. Secondary spread can occur through any means that moves conidia to healthy flowers, including rainsplash, insects, head to head contact, and moving equipment.

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