154 



FUNGOUS DISEASES OF PLANTS 



not known on the Pacific Slope during the early history of 

 grape-growing in that region, but it is now not uncommon. The 

 fungus was apparently introduced into Europe from America, 

 and it became a serious pest within a very short time after it was 

 first noted in that country. This greater injury under European 

 conditions had been predicted by Farlow on account of the early 

 spring and the relatively slight growth which is made by Vitis 

 I'inifera, the cultivated grape of Europe. 



The grape mildew has been found abundantly on practically all 

 species of cultivated or wild grapes, that is, upon such species as 

 Vitis (zstivalis, Vitis cordifolia, Vitis Labnisca, and Vitis vinifera. 

 It occurs, therefore, on the smooth-leaved species as well as on 

 those possessing a downy lower surface, and there are few varie- 

 ties which are notably resistant under all conditions. 



This fungus attacks practically all young or green portions 

 of the growing vine, leaves, shoots, and berries. The vine may, 

 therefore, under conditions favorable for the development of the 

 fungus, show the disease abundantly. Under ordinary condi- 

 tions, however, it is largely confined to the leaves, and its in- 

 jurious action consists in the production of discolored spots which 

 prevent or inhibit normal physiological activities (Fig. 52). A 

 slight attack of this fungus may, however, cause no perceptible 

 diminution of the amount of the grape crop. Under ordinary 

 circumstances the fungus may be found in the early summer, 

 particularly if the season is moist, and it may reach its greatest 

 intensity during August or as late as September. 



Symptoms. Upon the leaves the first indications of the disease 

 are indefinite spots, which from the upper surface are yellowish 

 in color and irregular in size and form. Upon the lower surface 

 of the leaf the spots are not so evident, but almost simultaneously 

 with the spots above, the sporophores of the fungus are produced 

 on the lower surface. Later in the season the spots may turn 

 brownish above, and upon some varieties of grapes they may 

 be almost brown from the beginning, finally appearing as small, 

 angular brown spots, visible on both surfaces of the leaf. It is 

 only when the fungus is very severe that the leaf dries and falls 

 prematurely. Upon the shoots depressed areas, dark in color, are 

 produced, and these therefore bear no resemblance to anthracnose, 



