The pathogen overwinters as mycelium in infected potato tubers. These tubers may be in the ground as volunteer potatoes, in cull piles, or planted as seed. The mycelium will spread up the new sprouts, particularly in the cortical tissue. When the mycelium reaches the aerial plant parts, it will produce sporangiophores which emerge through stomata of the stems and leaves. The sporangia produced on these sporangiophores can become airborne or rain dispersed. When the sporangia land on wet potato leaves or stems, they can germinate and cause new infections. Mycelium emerging from this infection site will penetrate new tissue leading to the formation of lesions. New sporangiophores will be produced and protrude through stomata. They will appear as white fungal growth on the underside of leaves where lesions are apparent. These sporangiophores will produce more sporangia which can be spread by wind and rain. The sporangia can not only be dispersed to leaves and stems, but also to the soil or tubers at the soil surface. The sporangia can infect directly or through the formation of zoospores. The zoospores produced from the sporangia can move in water and penetrate the tubers through lenticels or wounds. At harvest time tubers can be contaminated and consequently infected by sporangia on the soil surface or plant tissue. The infection of tubers may not be apparent at harvest but it will develop in storage.

The development of late blight epidemics will depend greatly on temperature and humidity. Sporangia lose their viability in 3 to 6 hours at relative humidities below 80 percent. Sporangia germinate only when free moisture or dew is present on leaves. The fungus will sporulate most abundantly at humidities near 100 percent.

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