PLANT DISEASES 



437 



the spots indicating that they have been attacked by some 

 fungus (Figs. 83-87). Among the parasites that produce 

 spotted leaves are the " mildews." One kind of mildew is 

 called the " downy mildew" because it appears on the sur- 

 face (usually leaf) of the host plant as small downy patches. 



FIG. 86. A spot disease of strawberry 

 leaves. After MASSEE. 



These patches are numerous minute branches bearing spores, 

 that have arisen from the parasite deep within the host, 

 where it is destroying living cells. Before the downy patches 

 appear, the presence of such a parasite in a leaf is shown 

 by the dying and dead spots. This attack on leaves reduces 

 their ability to manufacture food, and it may be so general 



an attack that the leaves are destroyed entirely. Such 

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