PHYTOPHTHORA 



235 



the ground, they can, in the presence of moisture, produce zoo- 

 spores, as in Cystopus. The first signs of disease are discoloured 

 spots exhibiting a dark central region surrounded by successive 

 zones of greyish and pale green tissue, which rapidly become 

 brown or even blackish ; closer inspection discloses the white 



FIG. 124. Asexual reproduction in various Oomycetes. A, Small part 

 of epidermis of Potato-leaf, infected with Blight (Phytophthora infes- 

 tans), showing branched hyphae bearing gonidia (g.) emerging from 

 the stomata. B, Seedling of Cress which is " damping off," due to 

 an attack of Pythium debaryanum ; the point at which the hypocotyl 

 is giving way is indicated by an arrow. C, Hypha with sporangia 

 of the same. D, Young, and E older, sporangia of Saprolegnia, 

 showing numerous zoospores (Sp.). (A after Strasburger ; B after 

 Miyake ; C after Hesse ; D and E after Thuret.) 



tufts of hyphae bearing the gonidia, especially on the lower surface 

 of the infected leaf. The hyphae of the parasite gradually spread 

 to the underground parts, thus infecting the tubers, so that 

 early removal of diseased shoots is advisable. 



An attack of Pythium results in a rapid softening of the 



