960 
Journal of Agricultural Research 
Vol. XXXI, No. 10 
EXPERIMENTAL METHODS 
For several years inoculation tests have been made each week from 
May to September (4), using pure cultures of Pseudomonas tumefa- 
ciens isolated from almond, peach, or plum. The artificial inocula¬ 
tions were made in series of 10 or a multiple of 10 on the current 
season's growth by punctures with a steel needle. No protection was 
given the puncture inoculations. In each series a susceptible species 
was included to test the virulence of the culture. The results were 
very consistent, in that a resistant species usually showed the forma¬ 
tion of but few galls, while a susceptible one showed a large number. 
This is well illustrated in Table VI, which presents the results of 
several series of inoculations. 
In addition to the method outlined above, pits of such species 
(Table VII) as could be obtained were planted in soil with fragments 
of galls taken from diseased orchard trees, usually peach roots (pi. 1). 
These galls were broken up into small fragments by pounding with a 
heavy tool. The pits were sown the proper distance apart in a furrow 
3 or 4 inches deep, and the gall fragments scattered upon them. 
The pits in the upper part of each row were not inoculated, being 
left as a control. The furrow was then filled with soil. In another 
experiment one-year-old trees were planted in soil, mixed with 
broken pieces of crown gall. 
RESISTANCE IN INOCULATION TESTS 
Table I presents a summary of the results obtained from the 
artificial inoculations on species of Prunus. These results agree with 
those previously published (5 ). In some cases the number of inocu¬ 
lations was too small to make the results entirely conclusive; these 
results may be considered as only suggestive of the actual resistance. 
A resistant species usually shows the characteristic at once, and a 
large number of inoculations, while making the test more con¬ 
clusive, generally does not change very materially the actual per¬ 
centage of galls produced. 
The name Prunus is used in this paper in its inclusive sense, and 
the species tested are very diverse. Three species, Prunus ilicifolia, 
P. integrifolia, and P. caroliniana , are evergreens, and are placed by 
some authorities in Laurocerasus. It will be noted that these showed 
strong resistance. They can not, however, be successfully budded 
or grafted with any of the cultivated varieties of the stone fruits. 
Another group of native subdesert species, P. andersoni , P.fasciculata , 
P. fremonti (P. eriogyna ), P. minutifolia , and P. microphylla , while 
differing among themselves, have certain common characteristics. 
They are about equally resistant, yielding about 50 per cent of 
infections. With the exception of the last two species, they grow 
in the Pacific and Rocky Mountain States. They are interesting 
forms, but are probably too small to be valuable as standard stocks. 
These, together with P. sub cor data, the Sierra plum, are worthy of 
experimental testing as dwarf stocks. 
The varieties of the species Amygdalus persica and A. communis 
thus far tested have shown no marked resistance. The bitter almond 
is the most promising, but in actual nursery practice this variety 
often is very susceptible. Other species of Amygdalus, as A. mira 
