460 
Journal of Agricultural Research 
Vol. VIII, No. za 
Griffin (1911), Barss (1913, 1915), and Rees (1915), writing separately 
of the gummosis of cherry trees, agree that the disease is caused by a 
motile bacterium, and suggest that sucking insects are the probable agent 
in its dissemination. Barss (1915, p. 239) says: 
The exact manner in which the disease is disseminated is not known, but indica¬ 
tions seem to point to the possibility that sucking insects may be largely responsible 
for infections. 
No data are available in the literature as to the susceptibility of any 
part of the plant without previous injury, all inoculations having been 
made by needle puncture. 
Stewart and Leonard (1913) show clearly that certain sucking insects, 
among them the tarnished plant bug, are capable of inoculating tender 
shoots of pears when Bacillus amylovorus is present at the immediate 
point of puncture or upon the mouth parts of the insect. These authors 
(1915) conclude that certain flies are not active agents of inoculation, 
but, because of their abundant presence in the orchards and among nur¬ 
sery stock, they are probably a factor in spreading the bacteria from 
place to place. Attention is attracted by Stewart (1913, 1915) to the 
increased disease following rainy weather, due to both the increased sus¬ 
ceptibility of the host because of the more succulent growth and the 
increased activity of the bacteria because of more favorable environment. 
Heald (1915) is of the opinion that leaf infection of pears by fireblight 
is possible through water pores, while Hotson (1915, 1916) offers further 
evidence in support of this. 
Rolfs (1915), writing on the disease of stone fruits caused by Bacterium 
pruni, states that infection takes place through stomata and that a film 
of moisture is necessary to successful inoculation. He says (p. 416, 421): 
Rain and dew are not only important factors for inoculation, but they also carry the 
bacteria to the healthy leaves, twigs, and fruits, and thus frequently serve as agents of 
transportation. * * * The warm, slow, continued rains of the summer furnish the 
best conditions for the rapid spread of the disease. Heavy, driving rains of short 
duration followed by sunshine and winds are not favorable to its spread, since many 
of the bacteria are washed to the ground, and the leaves are quickly dried off and 
the few bacteria that may be spread will be quickly dried and killed. 
Rolfs (1915), in his discussion of the angular leafspot of cotton, states 
(p. 17-18): 
Wet weather, of course, materially aids in the dissemination of the organism. Even 
if the weather is excessively dry, the dew at night will often furnish sufficient moisture 
for inoculation. * * * If for any reason the first leaves fail to become inoculated 
in this way [by contact in presence of dew] the movement of the leaves in the wind 
especially during a storm, will soon bring them in contact with some of the virus on 
diseased leaves. * * * The rain carries large numbers of the organism to the new 
tissue and to the ground under the plants. * * * The soil under the infected plants 
may thus become an important means of inoculating many of the lower leaves. 
Hasse's (1915) conclusion that Pseudomonas citri is the cause of Citrus 
canker has been confirmed by Wolf (1916). The latter states (p. 94), 
regarding its dissemination: 
