498 INSECTS AFFECTING VEGETATION 



fungous parasites. It is usually regarded as too laborious and ex- 

 pensive for the large vineyards, but may be profitably practiced 

 where only a small number of vines are grown. 



Downy Mildew. This disease in certain seasons and in north- 

 ern localities sometimes causes more loss than black-rot and is a close 

 rival for first place among the fungous enemies of the grape. It at- 

 tacks all the tender growing parts of the vine. Usually it is at first 

 most noticeable on the foliage, producing greenish yellow, irregular 

 spots upon the upper surface, which become reddish brown. At the 

 same time there appears on the under surface of the leaf a thin, 

 loose, white, downy growth, suggestive of hoar frost. This growth 

 consists of the fertile fungous filaments bearing the summer spores 

 which under favorable conditions are distributed by the wind and 

 water to the berries and other parts, where they germinate, pene- 

 trate the tissues, and continue their destructive work. The young 

 shoots are also frequently attacked and killed. 



The fruit, if attacked when young or only partly grown, shows 

 first a brownish spot, and later becomes covered with the gray, downy 

 growth of the fungus. This form of the disease is sometimes called 

 gray-rot by vineyardists. When the berries escape the disease until 

 they are half grown or more it appears as a brownish or brownish 

 purple spot which spreads and soon involves the whole berry. The 

 affected fruit becomes soft and wrinkled and falls to the ground 

 when disturbed. This stage of the disease is sometimes called 

 brown-rot. 



Besides the summer spores mentioned, there is also produced 

 within the diseased tissues another form of reproductive body, some- 

 times called a winter or resting spore. These spores are produced 

 in much smaller numbers than the summer spores and are provided 

 with a rather thick, dark-colored outer covering apparently intended 

 for their protection during the winter. 



This disease, like the black-rot and many others, develops most 

 rapidly and does most injury during hot, wet weather. It is de- 

 sirable to destroy as many as possible of the diseased leaves, shoots, 

 and berries, which may contain the winter spores. Thorough spray- 

 ing, as recommended for the black-rot, will effectually control this 

 disease. (F. B. 284.) 



Powdery Mildeiu. This rarely causes great loss to American 

 varieties of grapes. It is most severe on the European, or vinifera, 

 grapes. This mildew belongs to a group of fungi quite different 

 from the downy mildew. It differs from all other parasites which 

 attack the grape in its superficial habit of growth. The filaments of 

 the fungus do not invade the tissues of the plant to destroy them. 

 The parasite obtains its nutriment by means of sucker-like organs 

 which penetrate the cell walls of the surface layer of tissue only. 

 The fine, white filaments of the fungus, which constitute the vegeta- 

 tive portion of the parasite, spread over the surface of 'the leaves, 

 shoots, and fruit, and send up short, irregular branches upon which 

 immense numbers of summer spores are produced in short chains. 

 These are most noticeable upon the upper surface of the leaf, giving 



