POTATO BLIGHT (PHYTOPHTHORA INFESTANS) 357 



scattered about like spores. When the conidia germinate, 

 instead of producing hyphae they produce zoospores, which, after 

 swimming about for a few minutes, lose their cilia and begin to 

 produce new hyphae. If favorably located, the new hyphae find 

 entrance to a leaf through its stomata and start the disease anew. 

 The oogonia and antheridia resemble those of Saprolegnia, but 

 are produced on short hyphae 

 within the tissues of the host. 

 The oospore has a heavy wall 

 and is not liberated until the 

 tissues of the host surrounding 

 it decay. The oospores are well 

 fitted to endure winter condi- 

 tions, and as the dead leaves 

 are scattered, the oospores con- 

 tained are also scattered, and 

 when freed it is probable that 

 they often start the disease the 

 following year. 



Potato Blight 1 (Phytophthora 

 infestans). This Fungus, com- 

 monly called the Late Blight of 

 the Potato, is a near relative of 

 the Grape Mildew. It attacks 

 the leaves, stems, and tubers 

 of the Irish Potato and is very 

 rapid and destructive in its 

 work. Figure 309 shows the 

 leaves of a Potato plant affected 

 with this disease. Like the 



Grape Mildew, after the mycelium is well established in the 

 host, conidiophores are produced (Fig. 310). The conidia may 

 grow directly into hyphae or produce zoospores (Fig. 311). 



1 Late Blight and Rot of Potatoes. Circular 19, Cornell University Agr. 

 Exp. Sta. 



Investigations of the Potato Fungus, Phytophthora Infestans. Bulletin 168, 

 Vermont Agri. Exp. Sta. 



Germination and Infection with the Fungus of the Late Blight of Potato 

 Research Bulletin 37, Wisconsin Agr. Exp. Sta., 1915. 



Studies of the Genus Phytophthora. Vol. 8, No. 7, pp. 233-276, Jour 

 Agr. Research, U. S. Dept. Agr., 1917. 



FIG. 308. Reproduction in the 

 Downy Mildew of the Grape, a, 

 conidiophores bearing conidiospores 

 on the ends of their branches; b, 

 conidiospores; c, oospore; z, zo- 

 ospore. Much enlarged. From 

 Farmer's Bulletin 284, U. S. Dept. 

 of Agriculture. 



