370 



Minnesota Plant Diseases. 



Later in the year the winter spores are formed in the usual way 

 for the downy mildews, i. e., from breeding organs. The winter 

 spore has a thick coat and remains in the leaf after the latter falls. 

 In the spring the decay of the leaf sets the spore free, and in moist 

 conditions it produces numerous swimming spores ; the infection 

 of the vines follows in the usual way. The destruction of fallen 

 leaves, to avoid future infection, is therefore seen to be of im- 

 portance. 



Fig. 198. Downy mildew of grape. A. Section of a leaf with spore-bearing threads emerg- 

 ing from an air-pore. B. Greatly enlarged sucker threads, h. C. Formation of swarm 

 soores in the so-called summer spores; 1, summer spore (is really a spore-case) ; 2, same 

 with protoplasm divided into regular areas; 3, areas of 2 are seen separated, and the 

 whole mass escaping from the spore-case; 4, an escaped and free area of the protoplasm 

 which becomes the swarm spore, 5. D. Formation of egg-spores by the breeding act. 

 Highly magnified. After Millardet. 



The treatment for black rot is usually recommended for this 

 disease. The sprayings may be at longer intervals. (See Black 

 Rot of Grapes.) 



Disease-resisting varieties may also be used. It has been rec- 

 ommended that the vine be treated in the fall or early spring, be- 

 fore the buds commence to open, with an iron sulphate solution. 



