358 THALLOPHYTES 



The conidia and zoospores which they produce spread the 

 disease very rapidly in moist weather. Since the zoospore 

 is a swimmer, it can function more efficiently during moist 

 weather. Moist weather also favors the germination of the 

 conidia. Little is known about the sex organs of the Potato 

 Blight, but in the form which causes the Bean Blight, the sex 



FIG. 309. Leaf of Irish Potato affected with the Late Blight. From 

 Bulletin 140, Cornell University. 



organs and oospores occur in the seed coat or cotyledons of the 

 seed, in which case the oospore is planted with the seed. In the 

 Phytophthora cactorum, 1 which is destructive to Ginseng, the sex 

 organs and oospores have been found in the stem and roots 

 (Fig. 312). 



There are many Downy Mildews which give us trouble. In 

 fact many of our plants such as Cucumbers, Melons, Beans, 

 Potatoes, Lettuce, Grapes, etc., are attacked and much damaged 

 by Downy Mildews. But the study of the Grape Mildew and 

 Potato Blight has given a general knowledge of their habits. In 

 combatting the Mildews one must reckon with conidiospores, 

 zoospores, and oospores. 



One is able to check the spread of the disease by spraying the 

 plants with a solution that is poisonous to conidia and zoospores. 



1 Phytophthora Disease of Ginseng. Bulletin 863, Cornell Agr. Exp. Sta., 

 1915. 



