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Disease Management

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Disease Cycle
In the absence of the oospore stage, Phytophthora infestans survives between potato crops as mycelium in infected tubers (Figure 9).

If infected tubers are left behind at harvest or dumped (Figure 20) at the edges of fields, sporangia may be produced on the infected tubers or new sprouts the following spring. Air currents carry sporangia to healthy potato foliage. Sometimes seed potatoes can become infected (Figure 21); freshly cut seed tuber surfaces are especially susceptible to infections from airborne spores in contaminated storage facilities.

Figure 20

Figure 21

In the presence of water and at cooler temperatures, sporangia germinate indirectly (Figure 16) by the production of zoospores (Figure 18). At warmer temperatures, the sporangia germinate directly (Figure 17) by the production of a germ tube.

Several days after infection, new sporangia are produced on sporangiophores (Figure 15) which emerge from stomata. The deciduous sporangia may be dispersed by wind or water to new parts of the same potato plant or new plants.

Sporangia may also be washed through the soil to infect tubers. If both mating types come into contact with each other, thick-walled oospores (Figure 19) may be produced to persist in soil or plant tissues. Oospores usually germinate by producing a sporangium (Figure 22).

Figure 22

Epidemiology
Temperature and moisture are the most important environmental factors affecting late blight development. Sporangia are formed on the lower leaf surfaces (Figure 4) and infected stems (Figure 5) when relative humidity is < 90%. Sporulation can occur from 3-26°C (37-79°F), but the optimum range is 18-22°C (64-72°F). Sporangia germinate directly via a germ tube at 21-26°C (70-79°F). Below 18° C (65°F), sporangia produce 6 to 8 zoospores which require water for swimming.

Each zoospore is capable of initiating an infection, which explains why disease is more severe in cool, wet conditions. Cool nights, warm days, and extended wet conditions from rain and fog can result in late blight epidemics in which entire potato fields are destroyed in less than two weeks. Infected tubers can sporulate in poorly controlled storage areas (Figure 11) where conditions are too humid. Condensation produces water droplets on the surface of infected tubers which may then cause the pathogen to sporulate and contaminate neighboring tubers, leading to destruction of the entire pile by soft rot bacteria.

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