486 
Journal of Agricultural Research voi. xxm, No. 6 
tion that wounding by insects or other means has played any part in 
infection. It seemed rather that infection in the field was dependent 
upon the occurrence of favorable environmental conditions. Every 
attempt to duplicate such conditions experimentally has thus far given 
negative results. The writer has as yet, however, done nothing as regards 
the intrinsic predisposition of the plant itself to infection, and it is not 
improbable that the host grown under different conditions as regards 
chemical and physical relationships may be considerably altered thereby 
as regards predispostion. This belief is strengthened by the fact that 
similar experience has been had with frequent attempts at securing good 
infection with the wildfire organism by spraying under greenhouse 
conditions, while with this organism we know that under other conditions 
gopd infection may be secured in this manner. 
The method of inoculation by wounding has been essentially that of 
puncturing the leaf with a needle point which has been dipped in a 
suspension of the organism in water and permitting a small droplet of 
the watery suspension to cover the wound. In this manner, when a 
virulent strain of Wisconsin leafspot organism is used, infection is 
secured in two to five days, and symptoms develop which compare 
favorably with those from Bacterium tabacum in size of chlorotic area or 
lesions obtained (PI. 2, B). On the other hand, it is certain that under 
field conditions the Wisconsin leafspot organism is not as virulent as the 
wildfire organism, and that the former can be pathogenic only under 
more limited conditions than is the latter. Under field conditions the 
chlorotic area or halo formed by the Wisconsin leafspot organism is not 
normally as marked as that of wildfire (PI. 1, A) and frequently may 
not occur at all (PI. 3, A) on older leaves, where conditions are seemingly 
more favorable for a rapid collapse of the leaf tissue than is the case on 
the younger leaves. 
Considerable uncertainty has been experienced throughout the progress 
of this investigation as to the continued pathogenicity of the organisms 
carried in culture on potato-dextrose agar, and as a result frequent 
inoculations have been made to test this point with various cultures. 
In large measure, the same has been true of the wildfire organisms carried 
along simultaneously. 
A large number of subcultures have died, lost their pathogenicity 
completely, or in considerable part (PI. 2, B). 
This has been due apparently in most cases to an unfavorable cultural 
medium, although in many cases this occurred on potato-dextrose agar 
made according to the same formula as other batches in which organisms 
have been kept alive and virulent through transfers kept in the refriger¬ 
ator for three years or more. 
A limited number of inoculations have been made upon other host 
plants aside from ordinary tobacco {Nicotiana tabacum). Infection has 
been secured upon various other species of Nicotiana, especially N . 
glauca and N. rustica, and also upon the tomato, together with some 
indication of infection upon certain cereals. 
CULTURAL CHARACTERS 
Morphology. —The organism is a short motile rod with rounded 
ends, occurring singly, in pairs, or occassionally in short chains. Meas¬ 
urements under various conditions have ranged from 0.5 to 0.8 microns 
in width by 1 to 2.4 microns in length, averaging about 0.6 by 1.8. 
