Feb. io, 1933 
Genetics of Bunt Resistance in Wheat 
455 
tissues in which it is most frequently found in this case, and the fungus 
no doubt produces toxins which bring about the deformity characteristic 
of a smutted plant. (PI. 1, B; 2, A.) 
According to Woolman, 4 bunt infection takes place during the early 
seedling stages, through the walls of the coleoptile. He is unable to 
find any trace of mycelium except in the coleoptile until the seedlings 
are about 8 days old. By the time they are 15 days old the mycelium 
has penetrated to the inner side of the second leaf sheath. As the plant 
grows older the mycelium penetrates farther and is carried upward 
mechanically with tie growing point, living filaments being found most 
frequently at the nodes and in or near the actively growing parts. The 
older parts of the mycelium disappear, apparently being digested and 
absorbed. He finds that the so-called immune varieties are readily 
infected and that resistant and susceptible seedlings show no difference 
during the first two weeks. By the time they are a month old, however, 
the mycelium in the resistant varieties is more restricted in its rami¬ 
fications and appears lacking in stainable cell contents. The “re¬ 
sistance” is evidently chemical in nature and begins to restrict the growth 
of the mycelium shortly after the genesis of photosynthesis. He rarely 
finds evidence of infection at more than one point, although he found 
one head containing spores of both Tillitia tritici and T. few, which 
proves that two infections did take place, each functioning at maturity. 
Heald (17) believes that multiple infection is required to develop a 
smutted plant. He found that Marquis wheat, which is very resistant 
to bunt, could endure from 500 to 1,000 spores per grain without pro¬ 
ducing bunted heads in the succeeding crop. When seed carrying 
100,000 or more spores per grain were planted half the resulting crop 
consisted of bunted heads. Jenkins Club, which is very susceptible, 
produced a trace of bunt with as few as 100 spores per seed. When the 
inoculation reached 40,000 or more the resulting crop produced from 80 
to 100 per cent of bunted heads. 
If the multiple infection hypothesis be the correct explanation of this 
phenomenon, it might be compared to.induced immunity in animals 
brought about by sublethal injections of fully virulent organisms. In¬ 
fection in a great number of different places—that is, a lethal dose—would 
use up the “antitoxin” already present in the plant so rapidly that the 
pathogenic relationship could be established. The resistant varieties 
would either contain more of the “antitoxin” or a greater capacity for 
developing such material. Much more work will be required to estab¬ 
lish this point. The data as published (17) might be interpreted as 
increasing the chances for infection or producing the external conditions 
that would make possible the penetration of the host cuticle (7). It 
has been shown that the amount of bunt is greatest in wheat planted for 
September 15 and only slightly less on October 1, at Pullman, Wash., 
the amount growing less and less in plantings either earlier or later. 
In view of this fact, as well as the fact that varieties differ in relative 
susceptibility, makes the proposition of determining by a spore count 
whether a given lot of seed needs treating, a difficult and precarious one, 
for winter wheat. A general prediction of the amount of smut to be 
expected in spring wheat would be less hazardous. 
4 My former colleague, Mr. H. M. Woolman, has done much cytological work on the relationships of bunt 
and wheat, and he has kindly permitted me to make the above statements of his unpublished work. 
