Disease Cycle

In the absence of the oospore stage, Phytophthora infestans survives between potato crops as mycelium in infected tubers.

If infected tubers are left behind at harvest or dumped 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 , freshly cut seed tuber surfaces are especially susceptible to infections from airborne spores in contaminated storage facilities.

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).