June 15, 1921 
Varietal Susceptibility of Beans to Rust 
397 
The greatest variability is shown in the varieties Horticultural Pole ? 
Horticultural Dwarf, Brockton, Early Refugee, Marblehead, and Mexican 
Red. All had a considerable percentage of clean plants, some that 
developed sori, and some intermediates. 
size; of the; sorus as an index of susceptibility 
In addition to the degree of infection and the production of flecks, the 
size of the sorus produced on different varieties also provides an index 
of relative susceptibility. As noted previously, great variation exists in 
the size of the sori produced on different varieties. The size of the sorus 
in turn is an index to the spore production of the sorus, and any marked 
curtailment in the number of spores produced may readily be conceived 
to be a limiting factor of considerable importance in the propagation of 
the pathogene. The production of a small sorus is evidently to be con¬ 
sidered in the same light as the production of flecks—as a response to an 
unfavorable medium. 
The average size of the uredinium on each variety has been determined 
by measurement of a number of uredinia. The relative spore-producing 
capacity is then determined by calculating the number of urediniospores 
produced in a layer one spore in thickness covering the sorus. This 
does not represent the total spore-producing capacity of the sorus but 
gives a basis for comparison between varieties. The main objection to 
this expression is that it does not magnify the differences between the 
more susceptible and more resistant varieties sufficiently. It seems 
evident from examination that the more susceptible varieties produce 
more layers of spores in a uredinium than the more resistant ones. No 
differences have been found in the size of the urediniospores borne on 
different varieties. 
In determining the number of spores produced by a sorus, the number 
in the diameter is determined by calculation. The urediniospore is 
approximately 20 \i in diameter, and 30 such spores would be contained 
in the diameter of a sorus 600 /x broad. The square of the number in 
the diameter multiplied by 0.7854 gives the number in one layer covering 
the sorus. The sorus on Tennessee Green Pod, which is one of the 
largest, averages 600 n in diameter and, therefore, has a spore-producing 
capacity of 706.8 spores in one layer, while that on Bird Eye, averaging 
300 /x, has a spore-producing capacity of only 176.7. The spore- 
producing capacity of the sorus on different varieties has been reduced to 
percentages for purposes of comparison. The sorus on Tennessee Green 
Pod is again used as the standard, and its spore-producing capacity is 
rated 100 per cent. The varieties on which data were obtained are 
grouped in Table VII according to spore production. 
It will be noted in a comparison between Tables VII and III that varieties 
with high rating in percentage of fertile infection also show rate high in 
spore-producing capacity. Tepary is the only variety rating above 50 
