66 MANUAL OF AMERICAN GRAPE-GROWING 



The value of a species or variety for a resistant stock may be 

 judged somewhat by the visible effect of the phylloxera on the 

 roots of the vines. On susceptible species, the punctures of 

 the insects rapidly produce swellings w r hich vary in size and 

 number in accordance with resistance of the species. Techni- 

 cally, the first swelling on the young tender rootlets of the vine 

 is called a nodosity. The presence of a few nodosities on the 

 root system does not indicate that a vine is not a valuable 

 resistant stock. When the nodosity begins to decay and 

 becomes of a cancerous nature, it is called a tuberosity. These 

 tuberosities decay more or less rapidly and deeply, and when 

 they rot deeply cause enfeeblement or death to the vine. Thus, 

 on Vinifera varieties the tuberosities are several times larger 

 and decay sets in much more quickly than on American species 

 which show these tuberosities. Ratings as to resistance of 

 species are usually made from the size and number of the 

 tuberosities, though when these are found producing a scab-like 

 wound which scales off, there may be high resisting power. 



In order to convey with some degree of definiteness the power 

 of resistance to phylloxera, an arbitrary scale has been agreed 

 on by viticulturists. In this scale, maximum resistance is 

 indicated by 20 and minimum by 0. Thus, the resisting power 

 of a good Vulpina is put as 19.5 and that of a poor Vinifera 

 variety as 0. 



ADAPTATIONS OF RESISTANT STOCKS TO SOILS AND CLIMATES 



Resistance, of course, counts for naught in a stock which 

 comes from a species unsuited to the soil and climate or other 

 circumstances of the locality in which the vineyard is to be 

 planted. The several species used for stocks differ widely 

 in the requirements affecting growth so that the grower must 

 make certain that the resistant stock he selects will find con- 

 genial surroundings. Stocks in congenial circumstances are 



