374 
Journal of Agricultural Research voi. xxvn, no. 6 
early enough to escape infection to some extent. The differences be¬ 
tween these kinds of resistance, on the one hand, and those phenomena 
on the other hand which clearly can not be defined as resistance, should 
be kept distinctly in mind, as they probably have been confused fre¬ 
quently in field studies similar to those reported in this paper. This 
confusion, together with the fact that opportunities for infection often 
are affected considerably by density of stand and other factors, has given 
rise to the view that resistance to rust can be modified easily by changing 
environmental conditions. 
Resistance of wheat varieties to Puccinia graminis may not be rigidly 
immutable. One would not expect it to be. As plant characters are the 
resultant of the interaction of genetic and environmental factors, fluctua¬ 
tions are bound to occur. Otherwise every plant of the same pure line 
should always be exactly like all the other plants of that pure line. But 
such is not the case. Neither are all of the individuals of a pure-line 
wheat variety affected equally severely by stem rust, even when they are 
growing under apparently identical environmental conditions. Yet, two 
plants seldom if ever grow under identical conditions. It is necessary, 
therefore, to distinguish as clearly as possible between the effects of 
genetic factors and those of environmental factors. The results pre¬ 
sented in this paper lend practically no support to the opinion that the 
genetic resistance of wheat varieties is changed by fertilizers. The 
type of infection remained the same on susceptible varieties 11 like Glyn- 
don Fife, Haynes Bluestem, and Marquis and on resistant varieties like 
Iumillo and the resistant hybrids, regardless of fertilization. More and 
larger uredinia sometimes developed on wheat in some fertilizer plats 
than on that in others. This seemed to be due, however, to increased 
opportunity for infection or to structural changes in the plants which 
made it possible for the rust mycelium to spread more extensively. 
Morphologic resistance may be changed somewhat by fertilizers. It is 
clear from the data obtained that the height of the plants and stiffness 
of straw are affected greatly by different fertilizers. The stiffness of 
straw, of course, is an index of the amount of supportive tissue in the 
plant. P . graminis and other cereal-rust fungi grow almost entirely in 
chlorenchymatous tissues. If the fertilization is such as to increase the 
amount of mechanical tissue, the rust mycelium can not thrive as well 
as it can in plants containing large amounts of chlorenchyma. But the 
writers found little evidence that, under field conditions, anything ap¬ 
proaching normal fertilization changed the morphologic resistance to 
stem rust greatly. There was much more convincing evidence that fer¬ 
tilizers affected the final degree of rustiness of plants by hastening or 
delaying ripening and by affecting the stand. 
The degree of infection by orange leaf rust seemed to be increased by 
heavy applications of nitrogen in some plats, but the rust was not abun¬ 
dant enough in most of them to make it possible to draw final conclu¬ 
sions. 
Any fertilizers which prolong the growing period and thus delay ma¬ 
turity are likely to increase indirectly the amount of rust. Late matur¬ 
ing plants are exposed to inoculation longer than are those which mature 
early. A few days' difference between the dates of ripening of two fields 
of wheat may have a marked influence on the amount of rust which 
11 This statement applies to susceptibility to the biologic forms used and to the general reaction o 
the varieties in the field at University Farm, St. Paul. 
