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GRAPE REGIONS AND THEIR DETERMINANTS 25 



Humidity of summer weather. 



The grape is very sensitive to moisture conditions, and 

 grows best in regions where the summer rainfall is compara- 

 tively light. A damp and cloudy summer brings disaster 

 to the vineyard in several ways; as small growth of vine, 

 small set of fruit, a crop of poor quality, and the development 

 of the several fungous diseases. Although the grape stands 

 drought, a superfluity of moisture in the soil may do little 

 harm, as is shown in irrigated vineyards, but a humid air 

 is fatal to success especially if the air is both warm and wet. 

 Moist weather during the time of maturity is particularly 

 disastrous to the grape, as are frequent fogs. Cold wet 

 weather in blooming time is the grape-grower's vernal bane, 

 since it most effectually prevents the setting of fruit. It 

 may be laid down as a rule that the grape lives by sunlight, 

 warmth and air it often thrives on the desert's edge. These 

 considerations make it manifest that the monthly and sea- 

 sonal means of precipitation must be considered in selecting 

 a locality to grow grapes. 



Spring and autumn frosts. 



The average date at which the last killing frost occurs in the 

 spring often determines the limit in latitude at which the grape 

 can be grown. Even in the most favored grape region of the 

 continent, killing frosts occasionally destroy the grape crop, and 

 there are few seasons in which frost does not take some toll. 

 Thus on May 7, 1916, frost all but ruined the crop of wine- and 

 table-grapes in the great grape region ol northern California 

 where frosts are seldom expected in May. Little or nothing 

 can be done to protect grapes from frost. Windbreaks as 

 often favor the frost as the vine, and smudging or heating 

 the vineyards is too expensive to be practical. In growing 

 grapes, therefore, the commonly recognized precaution of 



