Minnesota Plant Diseases. 331 



This is not usually of serious importance to crops in this 

 state. Diseased plants should be destroyed. 



Downy mildew of mustards, cabbage, etc. [Peronospora para- 

 sitica (P.) DeBy.]. This disease usually accompanies the white 

 rust of the same plants. It is found in general upon the same 

 plants as white rust and often causes deformations of the host. 

 The summer spores are produced on filmy patches of a downy 

 nature and can thus be distinguished from the white rust. 

 They are not produced in chains but in clusters on much- 

 branched threads, which protrude from the leaf of the host 

 and give the downy appearance to the infected regions. The 

 winter spores, which are similar in appearance to those of white 

 rust, do not, however, produce swimming spores, but germinate 

 into an infection tube. 



For preventives see White Rust of Mustards, etc. 



The downy mildew of potato. Potato blight [Phytophthora 

 infestans (Mont.) DeBy.]. This disease has proved an exceed- 

 ingly destructive one both in the United States and in Europe. 

 It was first known in the United States between 1840 and 1845 

 and was introduced into Europe about 1845. It probably 

 came originally from South America, where it grows on many 

 wild plants of the potato family. Shortly after its introduction 

 into Europe, it caused complete failure' of the potato crop in 

 many districts. In America it causes most damage in the 

 eastern states and is apparently not so destructive in Minne- 

 sota, though by no means unknown in this state. It may 

 sometimes be found growing on close relatives such as tomato 

 and other members of this family. The parasite has the typical 

 habit of the downy mildews and hence grows best in moist 

 seasons or in low-lying, damp situations. The mycelium is de- 

 structively parasitic and as soon as it is established in the 

 leaves of the host plant causes a diseased condition. The first 

 indication is the appearance of brownish spots, which rapidly 

 grow darker and finally become blackish. The discoloration is 

 often accompanied by a crumpling of the leaves. The diseased 

 leaves finally suffer complete decay and produce an offensive 

 odor. 



The lower surface of the leaf-spots is seen to have a very 

 delicate downy coat, which increases to a white band around 



