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.
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