356 



THALLOPHYTES 



common and important one of the many Downy Mildews and 

 often causes much loss in grape-growing districts. Its downy 

 white growth occurs most commonly on the leaves, but the Fun- 

 gus often attacks the green shoots and fruit (Fig. 306). Some- 

 times it destroys the fruit crop and weakens the vines. 



The mycelium consists of coenocytic hyphae, which extend 



through the tissues of the part at- 

 tacked. The hyphae grow between 

 cells and send into the cells short 

 branches (haustoria) which absorb 

 the cell contents of the host (Fig. 

 807). The death of the leaf cells re- 

 sulting from the attack is indicated 

 by the occurrence of yellow or brown 

 areas which may involve much of 

 the leaf. This destruction of leaf 

 tissue diminishes the carbohydrates 



FIG. 306. A bunch 

 of Grapes partially de- 

 stroyed by the Downy 

 Mildew. From Farmer's 

 Bulletin 284, U. S. Dept. 

 of Agriculture. 



FIG. 307. The haustoria of the 

 Downy Mildew reaching 'into the 

 cells of the grape, h, hypha; a, 

 haustoria. From Bulletin 214, Ohio 

 Agr. Exp. Sta. 



furnished by the leaves and as a result both fruit and vine 

 may suffer. Often, the fruit is directly attacked and de- 

 stroyed. After the Mildew is well established within the tissues 

 of the hosts, it sends through the stomata numerous branches 

 which constitute the superficial downy patches characteristic 

 of the parasite (Fig. 308). On the tips of these protruding 

 hyphae are produced small globular bodies known as conidio- 

 spores or conidia, and the hyphae bearing them are called 

 conidiophores which means " conidia bearing." 



The conidia are really small sporangia which break off and are 



