THE GRAPE PHYLLOXERA IN CALIFORNIA. 17 



Too few American varieties, either nonresistant or resistant, are 

 grown in the State of California at this time to have been con- 

 sidered in this investigation. 



Intrinsic vigor of vines. — Vines of great intrinsic vigor always re- 

 sist phylloxera attack better than naturally weak plants. 



Varieties of vines. — Amongst vinifera varieties grown in Cali- 

 fornia, a few have shown certain resistance when inoculations have 

 taken place several years after planting. Such are, in order, Flame, 

 Tokay, Mission, and Muscat (Fresno district), and in a lesser degree 

 Grenache, Chasselas, and Burger. Laboratory tests with certain 

 varieties in which phylloxera lesions rotted rapidly have shown that 

 Zinfandel, Thompson's Seedless, Carignan, Burger, and Muscat suc- 

 cumbed more rapidly and Tokay and Grenache less rapidly. 



Destruction of a highly susceptible vine. — Under favorable con- 

 ditions for rapid phylloxeration, the hypothetical progress of de- 

 struction of a highly susceptible vine, as Zinfandel, with established 

 roots may be set down as follows: During summer and fall a few 

 larvae settle on a part of the root system; the following year in- 

 festation spreads to the surface fibrous and fleshy roots, and to a 

 certain extent to the large roots near the crown, and nodosities and 

 tuberosities are formed. The third year the subterranean infesta- 

 tion spreads pretty well throughout the root system, although it 

 is rare to find year-old wood much attacked, for it appears that 

 the habit of roots of this age to slough the outer layer of bark pre- 

 vents the phylloxerse from retaining a hold, and compels those 

 already settled to move to other more hospitable portions of the root 

 system. In this year some of the larger roots decay under combina- 

 tion of phylloxera attack and excessive moisture in the subsoil or 

 become dried out from phylloxeration combined with too great 

 drought, and thus the flow of sap between the feeding rootlets and 

 the aerial portion of the vine is more or less cut off. This results 

 in a shortening of cane growth and sometimes in an abnormally 

 large crop of grapes. During this third summer as the larger roots 

 die an emigration of young larva? takes place. Many winged forms 

 also may be developed. The fourth year finds the larger roots in 

 great part destroyed, the cane growth correspondingly reduced, and 

 a large number of fibrous and fleshy rootlets sent out from the 

 trunk just below the soil surface. The phylloxeras colonize these 

 rootlets in spring, but leave them in summer, when they decay. There 

 is also a heavy migration from the decaying roots farther down 

 in the soil. In the autumn it is hard to find phylloxera on such a 

 vine, and this explains the maxim that the best type of phylloxerated 

 vine on which to look for the insect is not one badly stunted, but 

 rather one with slight stunting of the canes; in fact, one in the 

 1900°— 21 2 



