SUSCEPTIBILITY AND RESISTANCE TO CROWN-GALL, 15 



In New Mexico, Arizona, and western Texas the vines are usually cut 

 back and grown by the stump system (PL IV, fig. 2), in order to cover 

 the vines with soil during the winter as a protection against cold. In 

 spite of the fact that the vines are protected, they are injured in the 

 spring by frost after they are uncovered and severely pruned. The 

 bark is often burst by freezing near the surface of the ground, and 

 this enables the disease to easily gain an entrance on the canes. The 

 vine soon dies above the point of attack (PI. I). New shoots then 

 develop from the canes or roots below the diseased portion, as shown 

 in this illustration. These shoots are never so vigorous as the original 

 ones and have a tendency to sprawl over the ground, it being a difficult 

 matter to establish a new erect stump or vine (PL IV, fig. 1). After 

 the vine dies back a couple of times it is usually worthless, although 

 it may remain alive at the roots. This condition frequently obtains 

 at the end of four to five years under conditions favorable to the 

 disease. In California, Oregon, and Arizona it usually requires a 

 longer period. A large number of vine}^ards in localities in California 

 where vines suffer little from late frost were found almost free from 

 the disease. 



THE SUSCEPTIBILITY AND RESISTANCE OF SPECIES AND 

 VARIETIES OF GRAPES TO CROWN-GALL. 



Two factors apparently enter into the problem of the resistance 

 and immunity of grapevines to the disease: 



(1) The susceptibility of the species or variety to frosts, etc., in 

 the locality where it is grown. This varies not only with the species 

 but also with the variety grown; for example, the Concord, Catawba, 

 Delaware, and other American varieties are perfectly hardy and 

 usually free from the disease in sections where varieties of Vitis vini- 

 fera are seriously injured by freezing in winter and late frosts in the 

 spring and become badly diseased. 



(2) The variation in the chemical constituents of the sap of different 

 varieties of grapes may enter into the problem of resistance. The 

 sap of the vines of some of the resistant varieties is especially acid to 

 the taste, much more so than that of some of the more susceptible 

 varieties. This acidity may increase the resistance. Unfortunately 

 no chemical determination has been made to fully establish this 

 opinion, and it is offered as a working hypothesis for future investi- 

 gators. 



In the Southern and Western States it was found by observation 

 that varieties of Vitis vinifera are less resistant than those of Vitis 

 labrusca and V. aestivalis, although in Arkansas in the Ozark region 

 two species became slightly diseased, one variety of which, Goethe, a 

 hybrid of Vitis vinifera and V. labrusca, became badly diseased in a 



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