BULLETIN 197. GRAPE CULTURE IN CALIFORNIA. 119 



Many of the vineyards are planted in sandy soil, where the progress 

 of the pest is always slow. The natural power of resistance to this 

 disease possessed by the Flame Tokay, though not sufficient alone to 

 save it permanently, is no doubt a factor in saving vineyards of this 

 variety from rapid destruction. There seems some reason to believe 

 that this variety under special conditions, when growing in deep, 

 rich soil, especially if somewhat sandy, might be kept sufficiently vigor- 

 ous by careful cultivation and fertilization indefinitely. It would 

 be unsafe to trust to this for immunity, however, unless the vineyard 

 were situated where it could, if necessary, be given a winter sub- 

 mersion of three or four weeks every few years. 



It is a mistake to suppose, however, that rich soil or ordinary irriga- 

 tion give any practical degree of immunity. Vineyards in the richest 

 and most copiously irrigated regions of the San Joaquin Valley have 

 already been destroyed by the pest. What the effect of alkaline soil 

 has in this respect is as yet undetermined. 



When we have made every allowance for the known factors which 

 operate to delay the spread of the Phylloxera in the great central 

 valleys of California, they do not seem sufficient to completely account 

 for its slowness. Even the fact that the vineyards and wine-growing 

 regions are widely separated by fields where no vines grow is insiiffi- 

 cient to account for the slow spread of the insect, when we know that 

 the winged form may be transported by the wind 20 or 30 miles and 

 still infect the vine on which it is deposited. 



The most plausible theory seems to be that the winged form is 

 absent or extremely rare in the interior of California. In fact, the 

 winged form is produced most abundantly on American species of 

 vines, growing in cool, moist situations, and especially when rains 

 occur in June and July. As such conditions never occur in the 

 interior valleys, it is not strange that the winged form should be rare. 

 No record exists of winged individuals having been seen in California 

 except in the coast valleys. 



If this theory is correct, the only means the insect has of spreading 

 from one vineyard to another in the- great valleys is by crawling from 

 vine to vine or by being carried on cuttings or roots. This makes the 

 delay of the extension of the pest by proper quarantine measures par- 

 ticularly useful and effective. 



These measures are of two kinds those which can be carried out 

 by each grape-grower himself, and those which require the enforcement 

 of ordinances by quarantine officers. The first are as important and 

 more generally practicable than the last. 



If no winged insects occur there is no danger of introducing the 



