. It is obvious that the foundation of the art
of growing resistant vineyards is exact knowledge of the immunities
and susceptibilities of the many varieties and species of grapes. From
the first use of resistant vines, experimenters everywhere have set
themselves at work to determine not only what the most resistant vines
are, but what the causes and conditions of immunity. In spite of a
wealth of empirical discoveries as to what grapes can best resist the
root-louse, causes and most of the conditions of immunity are still
little understood. Definite, useful knowledge, so far, goes little
further than the establishment of lists of species and varieties, the
latter subject to change, that are most useful in setting resistant
vineyards.

Phylloxera does little damage to species of Vitis native to the same
general region in which the pest has its habitat, but nevertheless
there are some differences in resistance in American grapes. Munson,
one of the best American authorities on the resistance of species to
phylloxera, says:[7] "Rotundifolia is entirely immune, then Rupestris,
Vulpina, Cinerea, Berlandieri, Champini, Candicans, Doaniana,
AEstivalis and Lincecumii are so high in resistance as to be
practically uninjured, though they may be attacked, while Labrusca is
low in resistance and is much weakened in clay soils, if infested, and
Vinifera is entirely non-resistant." Some of these species are hard to
propagate and difficult to suit in soil and climate so that but two of
them are much used for resistant stocks. The two most used are
Rupestris and Vulpina (Riparia), of both of which there are varieties
which give satisfaction. Bioletti, a leading authority on resistant
stocks in California, says:[8]

"Varieties of resistant stocks which will in all probability be used
in California are Rupestris St. George (du Lot), Riparia x Rupestris
3306, Riparia x Rupestris 3309, Riparia Solonis 1616, Mourvedre x
Rupestris 1202, Aramon x Rupestris 2, Riparia gloire, and Riparia
grande glabre. These are all varieties which have given excellent
varieties for years in Europe, and have all been tested successfully
in California. Among them are varieties suitable for nearly all the
vineyard soils of California, with perhaps the exception of some of
the heavier clays.

"The only one of these varieties which has been planted extensively in
California is the Rupestris St. George. There can be little doubt,
however, that it will fail to