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49 THE CROWN-GALL AND HAIRY-ROOT OF THE APPLE TREE. 
ments with twenty-four of the more valuable commercial varieties of 
apple trees were conducted in connection with the other lines of inves- 
tigation. The data from the trees that were grown in these experi- 
ments from ordmary whip grafts have been carefully recorded and 
compiled for five years. <A larger number of grafts were planted and 
dug during the last three years than during the first two of the period 
of experimentation. During the first two years, 1903 and 1904, the 
work of experimentation was confined to one locality. During the 
last three years each stock of root grafts was made at a single nursery 
and divided into eight equal lots and planted in cooperative plats 1 
to 8, thus getting the combined results of the effect of growing a 
variety under as many different conditions. The results from these 
series of experiments for five years are combined and given in Table 
XXI, in the appendix. 
The conclusion from this series of experiments is that the different 
varieties of apple trees show a wide variability in their susceptibility 
to the disease; that some varieties are more apt to develop forms of 
crown-gall and others forms of hairy-root; and that none of the varie- 
ties were free from either type of the disease. 
It will be noted that the varieties Wealthy, Yellow Transparent, and 
Wolf River developed a high percentage of crown-gall, and the Ben 
Davis, Wolf River, and Northern Spy varieties a high percentage of 
hairy-root. In both cases the forms of disease occurred chiefly on the 
scion portions of the trees. (See Pl. I1,fig.3.) It follows that the dis- 
ease must be considered more with reference to the scion than to the 
seedling or root portions of the root graft; also that m root-grafted 
trees, susceptibility or resistance is inherent in the variety of scion, 
and methods for the prevention of the disease must be directed 
primarily to the selection of varieties resistant to the disease, and 
secondarily to the selection of resistant seedling stocks. 
The tendency toward the development of the disease appears to be 
in no way related to the character of the growth of any particular 
variety. Certain of the varieties experimented with are vigorous 
growers, such as Wealthy, Walbridge, Wolf River, Minkler, Ben 
Davis, Northwestern Greening, Northern Spy, and Baldwin, while 
others, like Yellow Transparent, Whitney, Red June, Maiden Blush, 
and Jonathan, are much slower growers. It will be seen from Table 
XXYVI, in the appendix, that some of the more vigorous varieties 
showed a high percentage of disease, as, for example, the Wolf River, 
with an average of 46.4; the Wealthy, with 43.3 per cent; and the 
Ben Davis, with 36.1 per cent. On the other hand, equally vigorous 
growers were the Baldwin, with only 16.6 per cent; the Minkler, with 
16.8 per cent; the Walbridge, with 14.1 per cent; and the Martha, 
with only 10.7 per cent. It must be added, however, that there was 
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