64 PLANT DISEASES 



behind by the continued growth of the branch above it. 

 This peculiar mode of reproduction is the only constant 

 feature that distinguishes the genus Phytophthora from 

 Peronospora. The conidia give origin to a number of 

 zoospores when placed in a drop of water, or on a damp 

 surface, as that of a leaf covered with dew. The zoospores 

 move about actively for some time in the water, and 

 finally settle down and emit a slender germ-tube, which 

 enters the tissue of the leaf through a stoma, or bores 

 directly through the epidermis. 



Conidia that are washed by rain upon young tubers of 

 the potato that are exposed, produce zoospores, and these 

 latter infect the young potato, entering its tissues and 

 forming a mycelium. The mycelium of the fungus also 

 passes down diseased stems of the potato, and thus infects 

 the tubers, the mycelium either passing into a latent con- 

 dition until the following season, when it renews its 

 activity and grows along with the stems springing from 

 the tuber ; or it continues to grow after the potatoes are 

 stored, especially if 'sweating' takes place, and in this 

 manner frequently spreads rapidly through the mass of 

 tubers huddled together. 



The sexual mode of reproduction is not developed in 

 this species, hence there are no oospores or resting-spores. 

 The bodies once described as the oospores of this species 

 were proved to belong to a species of Pythium living as a 

 saprophyte on tubers killed by the Phytophthora. 



PREVENTIVE MEANS. Much has been written on this 

 phase of the subject, and the outcome is not all that 

 could be desired. Spraying with Bordeaux mixture has, 

 in the hands of many experimenters, proved beneficial, 

 and furthermore, apart from its action on the fungus, has 



