Phytophthora causes damping-off of seedlings, root rot, and rot of lower stems. Oospores are formed in infected roots and eventually are released into the soil following breakdown of the rotted roots. Under these conditions the oospores germinate to produce sporangia which release the infective motile zoospore stage. The life cycle of P. megasperma f. sp. The fungus is spread within and between fields by irrigation water carrying zoospores.
Oospores also can spread in soil attached to machinery or hooves of animals.
Phytophthora root rot is most severe in fields with poor internal drainage or in fields where soils become saturated by excessive irrigation or rainfall. Flood irrigated fields that stay wet for up to 10 days are more likely to develop phytophthora root rot than are sprinkler irrigated stands. However, severe root rot damage has occurred in the Nebraska Sandhills on sprinkler irrigated fields that were continuously irrigated in spring, resulting in saturated sandy soils. Many older plants that have severe root rot die during the winter months. This results in severe stand thinning by spring.