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 (Gr. phyton: plant; phthora: destructan) contains some of the most destructive fungal pathogens in agriculture and consists of more than 45 species. 1-3 The group of fungi, commonly known as water molds, belong to the oomycetes * which reproduce asexually by motile spores and zoospores, and sexually by heterogametangial contact to produce thick-walled, nonmotile oospores. 6 , 7 Some species are homothallic while others are heterothallic. 8 , 9 Pathogenic species include:

Host-specific pathogens of one or several related hosts such as P. infestans (Mont (de Bary) on potato 10 , 11 and tomato; 12 , 13 P. megasperma Drechs. f.sp. glycinea (Kuan & Erwin) or P. megasperma Drechs. var. sojae Hildeb. on soybean, 14-16 and P. vignae Purss. on cowpeas 17 , 18

Nonspecific pathogens with a broad host range such as P. cinnamomi Rands pathogenic to over 1000 plant species throughout temperate and tropical climates, including avocados 19 and English walnuts; 20 , 21 P. cactorum (Leb. & Cohn) Schroet., pathogenic to hundreds of host species, including apple, 22 - 24 pears, 25 and peaches; 26 P. citrophthora (Sm. & Sm.) Leonian, a citrus pathogen; 27-31 P. syringae (Kleb.) Kleb., pathogenic to both stone fruits, such as apricots 32 and almonds; 33 and P. palmivora (Butl.) Butler, pathogenic to cocoa and pawpaw, among over 700 other hosts. 34 , 35