12 BULLETIN 209, TJ. S. DEPARTMENT OP AGEICULTURE. 



cate the phylloxera has been tried in this and other countries, but it 

 is conceded that, with the exception of vineyards which can be flooded 

 cheaply and sufficiently to kill the insect, the only way successfully to 

 reestablish Vinifera vineyards is by growing the vines on phylloxera- 

 resistant stocks. 



FACTORS IN RESISTANCE. 



The resistance of vines to phylloxera depends upon two factors: 

 (1) The inherent resistant character of the vine and (2) its adaptation 

 to soil, climatic, and other conditions. 



The inherent or natural characteristics of the plant repel or invite 

 the attacks of the phylloxera. The number of swellings, nodosities, 

 and tuberosities from insect punctures and the rotting of the root 

 occasioned by them progress more or less rapidly and deeply in 

 accordance with the texture and character of the root attacked. 

 The weakening and ultimate death of the vine are determined by the 

 extent of the punctures and the progress of the rot upon the roots. 



The nodosities rot more or less rapidly in the different grape species. 

 In the Vinifera they are larger and usually rot in a very short time. 

 In the American species the nodosities are smaller and do not rot so 

 quickly, various species differing in this respect. The number of 

 nodosities varies greatly on the different vine varieties, and when the 

 insects have multiplied sufficiently to cover the smaller rootlets with 

 them, eventually the larger roots are attacked, and if cancerous 

 patches of decomposition are found on the more developed roots 

 tuberosities occur. 



Some varieties have nodosities on practically all rootlets, while 

 some of the American species are not injured by the phylloxera 

 except that a few nodosities form on their roots. In fact, the 

 resistant ratings of different species are based on a determination of 

 the relative number and size of the nodosities found on their roots. 



In order to indicate with some degree of definiteness the resistance 

 to phylloxera (not the value of the stock), scientists have provision- 

 ally adopted an arbitrary scale of ratings. In this scale the maximum 

 of resistance or immunity is taken as 20, while absence of resistance 

 (or no resistance) is reckoned as zero. 



The necessary degree of resistance for the production of good crops 

 varies with the character of the soil, stocks rating above 16 being 

 considered sufficient for all soils, 14 to 16 for fairly'good soils, and 10 

 to 14 for rich, moist, sandy soils. 



ADAPTATION TO SOIL, CLIMATIC, AND OTHER CONDITIONS. 



The ability of a vine to withstand or resist the attacks of the 

 phylloxera without serious injury is greatly influenced by the 

 adaptability of the vine to the climatic and soil conditions under 

 which it is grown. These may increase or diminish the vigor of the 

 plant, retard or favor the reparation of the insect injuries, and affect 



