June 15,1921 
Varietal Susceptibility of Beans to Rust 
387 
standards of the bacteriologist can only be approximated. The strain 
of the fungus used in the experimental work to be discussed was obtained 
from Kentucky Wonder beans in a garden at Blacksburg in the fall of 
1917. In an effort to obtain a single spore strain, bean plants were 
inoculated sparingly with urediniospores, and only a few well-isolated 
sori were obtained. Urediniospores from a single sorus were used again 
for a sparing inoculation on other plants. The repetition of this proce¬ 
dure through several spore generations provided a strain which could, 
with reasonable certainty, be considered to have arisen from a single 
urediniospore. Stock cultures of this strain were maintained on plants 
of Tennessee Green Pod, and all of the inoculum used was taken from 
this variety. 
The method of inoculation used throughout the work consisted in the 
atomization of the host with a urediniospore suspension followed by 
incubation in a moist chamber for 24 hours. One of the authors (3) in 
studies with the crownrust of oats had found that the best infection 
was obtained when dry spores from a culture of rusted oat seedlings 
were allowed to fall on plants which had been previously atomized with 
water. Melhus and Durrell (<?) have also found dusting the spores on 
the moistened plants preferable to the use of spore suspensions. They 
have found a considerable reduction in the percentage of germination of 
urediniospores of this rust after passing them through an atomizer. 
There seems, however, to be greater probability of securing uniformity 
of dosage with spore suspensions, and this method has, therefore, been 
followed in our work. 
A standard dosage was used throughout the experiments, the inoculum 
for each plant being the spores from one mature uredinium, grown on 
Tennessee Green Pod, applied in 1 cc. of water. In practice 40 plants 
were inoculated at one time, the dosage being the urediniospores from 40 
sori in 40 cc. of water. 
Since it had been determined previously that the susceptibility of 
the bean leaf varies somewhat according to the degree of maturity, the 
inoculations were made with the plants in as near the same stage of 
development as possible, the rule followed being to select the time when 
the first trifoliate leaf had reached a length of about inch. At this 
time the primordial leaves have attained practically their full size but 
are still young and in a susceptible state. The number of days required 
to reach this stage of development from the planting of the seed varies 
with the conditions in the greenhouse between 11 and 20 days, averaging 
about 14 days. 
In order to secure uniformity of exposure to infection an inoculation 
frame is employed to hold 40 four-inch pots each of which contains a 
single bean plant. Four varieties are inoculated in each test, 10 plants 
to a variety, the varieties being distributed through the frame in such a 
