358 THALLOPHYTES 
The conidia and zodspores which they produce spread the 
disease very rapidly in moist weather. Since the zodspore 
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 
Fic. 309. — Leaf of Irish Potato affected with the Late Blight. From 
Bulletin 140, Cornell University. 
organs and odspores occur in the seed coat or cotyledons of the 
seed, in which case the odspore is planted with the seed. In the 
Phytophthora cactorum,! which is destructive to Ginseng, the sex 
organs and odspores 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 odspores. 
One is able to check the spread of the disease by spraying the 
plants with a solution that is poisonous to conidia and zodspores. 
1 Phytophthora Disease of Ginseng. Bulletin 363, Cornell Agr. Exp. Sta., 
1915, 
