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Disease Cycle and Epidemiology

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Black shank is a polycyclic disease. The number of cycles of infection per growing season and the extent of spread within a field are related to environmental conditions and the level and type of resistance in the cultivar planted. Chlamydospores in the soil and infested crop debris serve as the primary inoculum that initiates epidemics. Chlamydospores germinate in warm moist soil to produce one or several germ tubes that either directly infect the plant or produce a sporangium (Figure 30, 31).

Figure 30 Figure 31

Saturated soil stimulates the release of motile zoospores from sporangia, and these are the primary infective propagules. Zoospores swim through saturated soil pores or move in surface water over greater distances (Figure 32). Active movement is directed toward nutrient gradients that occur around the root tips and wounds on the host plant (Figures 10, 11). Once the zoospore contacts the root surface, it encysts (produces a cell wall), loses the flagella, then germinates to form a germ tube that directly penetrates the host epidermis (Figure 27). The pathogen continues to colonize the roots and stem via hyphae (Figure 33). Sporangia serve as secondary inoculum, and can form within 24 hours of inoculation under moderate temperature and moist soil conditions (Figure 34). Root and stem colonization results in typical root rot and black shank symptoms (Figures 3, 13, 14). New chlamydospores form on and in the root as disease progresses (Figure 35) and can either germinate to initiate new infections or serve as a survival structure until the next tobacco crop is planted.

Figure 32	Figure 33
Figure 34	Figure 35

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