Fig. 2.34Characteristic features of Oomycota. (a) Stages in the infection cycle of Phytophthora infestans(potato blight).
Strains of opposite mating types in the host tissues produce antheridia and oogonia. Fertilization leads to the produc-
tion of thick-walled resting spores (oospores). After a dormant period, the oospores germinate by a hyphal outgrowth,
which produces lemon-shaped, wind-dispersed sporangia. These initiate infection either by germinating as a hyphal
outgrowth (in warm conditions) or by cytoplasmic cleavage to release motile biflagellate zoospores in cool, moist con-
ditions. Infected leaves produce sporangiophores through the stomatal openings, leading to spread of infection to other
plants. (b) Sexual reproduction by Saprolegnia: several oospores are formed in each oogonium. (c) Asexual reproduction
in Saprolegnia: the sporangium develops at the tip of a hypha, then the protoplasm cleaves to release several primary
zoospores (each with two anterior flagella). These encyst rapidly and the cysts release secondary zoospores (laterally
biflagellate) for dispersal to new sites. (d) Asexual reproduction in Pythium. At maturity the sporangium discharges its
contents into a thin, membraneous vesicle. The laterally biflagellate zoospores are cleaved within this vesicle which then
breaks down to release the zoospores.FB4eC02 04/20/2005 02:52PM Page 42