ABSTRACT
This chapter is concerned with the pathological anatomy of root diseases caused by Phytophthora species, with particular emphasis on fruit tree diseases. The genus Phytophthora contains some of the most destructive fungal pathogens in agriculture and consists of more than 45 species. Where hosts have co-evolved with fungal pathogens, susceptibility to disease is the exception rather than the rule. Plants are continually exposed to attack by a vast range of air- and soilborne fungi, viruses, insects, nematodes, and bacteria. As is commonly noted in reviews of host defense mechanisms, the majority of plants remain healthy and grow vigorously most of the time. Once infection hyphae have gained entry into the outer layers of host tissue, whether inter- or intracellulary, host-specific Phytophthora spp. characteristically form haustoria, which penetrate the cell walls into the cytoplasm, but not the vacuole.
