PLANT DISEASES 383 



they do very little harm to many of the plants upon which 

 they live ; on the other hand, if conditions are favorable for 

 their growth they may cause much damage. Powdery 

 mildews are found from the tropics to Greenland. The 

 leaves of the host plant are usually the parts affected, but 

 the mildew may appear on the young branches and on the 

 fruit as well. The plant body of the fungus, composed of a 

 tangled mass of delicate light-colored threads, is found on 

 the surface of the host ; projecting from these threads into 

 the host cells are variously shaped short branches which 

 absorb food. On upright branches spores are borne, which 

 spread the fungus during the growing season. Later there 

 usually appear little dark spherical fruiting bodies with 

 peculiar projections from their lower portions. Within the 

 fruiting bodies one or more sacs are formed, which contain 

 usually eight spores each. One of these mildews sometimes 

 causes considerable damage to young peach trees ; another 

 attacks the gooseberry. The apple and strawberry are also 

 affected, and the powdery mildew that attacks grapes has at 

 times caused immense losses both in the United States and 

 in Europe. 



387. Black Knot. — This disease affects plimis and cherries 

 and is so vigorous in its attacks that it has entirely ruined 

 the plum and cherry orchards in various parts of New York 

 and of some of the other Atlantic states. In the eastern 

 portions of the United States the fungus thrives well both 

 on wild and cultivated forms ; but in some regions farther 

 west it is found only on wild varieties, and cultivated plums 

 and cherries are often not affected, even though growing near 

 diseased trees of related wild species. The knot appears 

 in early spring as a conspicuous sweUing, usually on one of 

 the smaller branches of the tree (Fig. 218). The bark cracks 

 and breaks off and the exposed tissues turn to a deep olive- 

 green color. At this time abundant spores are formed from 

 short threads arising from the surface of the knot. These 



