They are bad in nurseries. Phytophthora species are very serious root rot in irrigated agriculture - zoospores. There are several species which cause root rot of commercial crops and orchard trees. The initial above ground symptoms occur well after infection and colonization has progressed. Some Phytophthora root rots are very aggressive and can kill whole fields in a short time if condition are right.
Older trees show decline and dieback rather than death.
Many Phytophthora species cause diseases of the lower stem of many woody hosts. Rain splash and wind spreads sporangia and these diseases can be VERY serious. See colored overheads for P. infestans .