Minnesota Plant Diseases. 



333 



Fig. 167. — Potato blight. Later stages on the 

 leaves. After Clinton. 



fore unsafe to use for seed 

 potatoes tubers harvested 

 from an infected crop. 



"Spray with bordeaux 

 before the trouble appears, 

 about July 7th to loth, and 

 keep vines well covered, es- 

 pecially from the middle of 

 July to the middle of Au- 

 gust. Unless season is 

 very moist three sprayings 

 should suffice. If this 

 treatment is impossible 

 plant early varieties only." 

 (Conn. Ex. Sta. Bull. 142 — 



1903-) 



It has been claimed by 

 various authors that the bordeaux spray not only destroys the 

 parasite but improves the foliage of the potato as well. Dis- 

 eased plants and tubers should be burned. Wet soil should be 

 avoided, if possible, for infection can take place in the tubers in 

 spring as weU as in the leaf. Thick-skinned potatoes have 

 been recommended as more resistant to the fungus than thin- 

 skinned ones so that, for storage in particular, these varieties 

 should be selected. (See also Fig. 39.) 



Downy mildew of onion {P eronospora schleideni Ung.). The 

 mildew of onions has long been known in Europe where it is 

 much feared. It has now become established in many places 

 in the United States and has appeared in abundance in Wiscon- 

 sin. The fungus, like the other downy mildews, produces sum- 

 mer and winter spores. The former are produced on the leaves 

 upon threads in a manner similar to that in the grape mildew. 

 They give to the leaf a grey to green, moldy appearance and 

 the leaf-gloss appears to be lost. The spores in moist condi- 

 tions produce swimming spores, but are very sensitive to, and 

 easily destroyed by, drying. The winter spores are provided 

 with a thick protective coat. They are very resistant and in 

 the spring following their formation produce swimming spores 

 in a manner common to the downy mildews. The fungus is 



