48 INTERNATIONAL CONGRESS OF VITICULTURE 



Factors in Resistance. 



For the benefit of those not familiar with the subject it should be stated 

 that the resistance of vines depends on (1) the inherent resistant character 

 of the vine, and (2) its adaptation to soil, climatic and other conditions. 

 These are greatly influenced by the cogeniality of the graft with or to the 

 stock. 



The inherent characters pertain to the nature of the plant, which more 

 or less enhance or restrain attacks of the phylloxera; causing the punctures 

 of the insect more or less rapidly to produce swellings, nodosities and tuber- 

 osities, varying in size and number, upon roots of different texture, causing 

 such swellings to rot more or less easily, rapidly and deeply, and by that 

 rot determining the extent and rapidity of the enfeeblement of the roots 

 and, where it is sufficient, the death of the vine. 



The roots of the different grape species vary greatly in the effects the 

 insect injury has on them, as well as in the likes and dislikes the insects 

 seem to have for them. Thus, on vinifera varieties, the nodosities usually 

 rot quickly and are about three times as large as those on the most resistant 

 American species. Vines upon the roots of which the phylloxera does not 

 remain and produces no injury at all would be called "immune". Varieties 

 of some of the American species are not injured any further than the forming 

 of a few nodosities on their roots. Such vines have a very high resistance. 



In fact, on a determination of the relative number and size of nodosities 

 found on the roots of the different species, the resistant ratings of these 

 species are based. 



The necessary degree of resistance for the production of good crops 

 varies with the character of the soil and the congeniality of scion to stock; 

 otherwise stocks rating 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 the attacks of the phylloxera without 

 serious injury is influenced greatly by the climatic and soil conditions in 

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

 plant and retard or favor the reparation of the insect injury. 



The soil and climate also affect the resistance by being favorable or 

 unfavorable to the approach, dissemination or activity of the phylloxera. 

 For instance, sand of a certain fineness hinders the insect in traveling from 

 the root of one vine to that of another, etc. Climatic variations also affect 

 the multiplication of the insect. 



Then again, a vine variety which resists splendidly in one locality 

 perishes in another having the same soil but a different climate, or in another 

 having the same climate but a different soil. This is due not only to the 

 adaptability of some species to a moist and others to a drier soil or to a 

 moist or dry climate, but also largely to the root systems of the species, 

 which vary from horizontal to vertical, from thick to thin, and from soft to 

 hard with intermediate grades between these extremes. 



For instance, how could a horizontal root system thrive in a dry, hot 

 climate and what could a deep rooting system do in a shallow, hard soil, 

 or a moisture loving variety where there is but little moisture, or vice versa. 



