Feb. 9.1924 
Effect of Fertilizers on Stem Rust of Wheat 
375 
may develop. In the hard red spring wheat region the stem rust epi¬ 
demic is likely to come just before harvest. Early ripening fields often 
escape great damage, whereas those which mature a week later may be 
damaged severely. Under fairly normal growth conditions for wheat, 
barnyard manure and other nitrogenous fertilizers may delay ripening 
for a few days or even for two weeks or more, depending on the soil type 
and amount of fertilizer used. It has been shown that the percentage of 
stem rust often could be correlated directly with the time required for 
maturity. The conclusion is that there is more rust on the late-ripening 
wheat than on that which ripens earlier. For practical purposes, there¬ 
fore, nitrogen has been conducive to rust, but it has not increased sus¬ 
ceptibility in the strict sense; it has only prolonged the growth period 
so as to expose the plants longer to the rust danger. But the effect may 
be just the opposite. In 1916, for instance, when the weather was very 
hot and dry after the wheat headed, the plants in the nitrogen plats 
actually ripened earlier than those in the other plats. It is true that it 
was not normal ripening, the plants being practically 11 burned up,” but 
the natural result was that the rust was checked also. 
Fertilizers also may make it possible for the rust fungus to infect 
plants more easily. When nitrogenous fertilizers increase the density of 
stand, succulence of the plants and consequent lodging, moisture is re¬ 
tained longer in the field than in those in which the stand is not so dense 
and the plants remain upright. The minimum incubation period for the 
uredinial stage usually is about six hours. There is an increase in the 
number of germ tubes which enter the plants as the incubation period is 
increased. Assuming that two fields have been inoculated equally heav¬ 
ily, other things being equal, the heavier infection will occur in the field 
in which moisture is retained longer. 
It is perfectly clear from the data obtained that the percentage of 
stem rust on wheat plants does not necessarily indicate the amount of 
damage which will be done. When wheat is grown in properly fertilized 
soil it often yields well in spite of heavy attacks of rust, while that grow¬ 
ing in soil with unbalanced fertilization may yield poorly regardless of 
the degree of severity of rust. It seems very likely, therefore, that much 
of the damage often attributed to rust when wheat is growing in soil 
containing an overabundance of nitrogen, actually is due to the direct 
effect of the fertilizer on the plants. It is quite likely also that a given 
amount of rust may injure plants too heavily fertilized with nitrogen 
more than it does plants growing in soil containing well balanced soil 
nutrients. This may be due partly to increased water requirement, 
still further increased by rust infection, and partly to weak straw which 
may be weakened still more by rust. The rational thing to do is to supply 
the soil with the nutrients which the plants need. Nitrogen decreased 
yields consistently on University Farm soils, but on the lighter soils on 
the Quinn farm and at Anoka it increased them. 
The final conclusion seems to be justified that wheat can not be predis¬ 
posed easily to Puccinia graminis , if we mean by predisposition the action 
of environmental factors in rendering a variety more susceptible than 
it normally would be. Nitrogen apparently may predispose plants by 
increasing succulence, density of stand and lodging, by delaying maturity 
and by decreasing yields on account of these factors, and sometimes by 
premature ripening in hot, dry weather. But the actual predisposing 
action apparently is of relatively minor importance. Since the rust 
fungi are highly specialized obligate parasites, a mere weakening of the 
