Dec. 8, 1923 
Fusarium Stem and Rootrot 0} Peas 
469 
pathogenic strains were obtained from districts where rootrot is not 
severe, it seemed possible that there might be a relation between the 
pathogenicity of the strain of Fusarium present there and the severity 
of the disease. The following year the collection of cultures was enlarged, 
not by direct isolation of the fungus from diseased stems—a procedure 
which is difficult when materials are not fresh—but by placing diseased 
stems in sterile soil, maintaining optimum conditions for infection, and 
isolating the fungus from diseased seedlings which resulted. Two new 
cultures obtained in this way from the Bitter Root Valley were much 
more pathogenic than that of the previous year, though one of them 
was much less pathogenic than the strains from Wisconsin and Michigan 
that were used as standards of comparison. Repeated inoculations 
have established beyond doubt the fact of the slight pathogenicity of 
the cultures enumerated, and have indicated that there is a constant, 
though often slight, difference between the more pathogenic cultures. 
If, then, this difference in parasitism, whatever its physiological sig¬ 
nificance, has not been produced by the method of isolation or by con¬ 
ditions of culture, but inheres in the fungus in the field, such a fact is 
of importance, inasmuch as upon the degree of parasitism depends tffie 
degree of injury that the disease may cause. The existence of parasitic 
and of nonparasitic varieties of fungi are well known; but instances of 
intermediate degrees of parasitism have not been extensively investi¬ 
gated. We can not, then, obtain clues from past experience which will 
incline us to expect to find these differing strains constant in parasitism 
in the soil as they appear to be in culture, or mingled together in the 
same field, or constant over considerable areas. Neither do we know 
whether the constant presence of a susceptible host increases the degree 
of parasitism of any portion of the potential parasite in the field. Un¬ 
fortunately, the writer has not secured a sufficient number of cultures 
for comparison to obtain a clue to the answer to any of these implied 
questions. However, the finding of such variability in one species is 
not likely to be a unique experience; and thus the result of inoculation 
with a single or even a few local isolations, at least of a Fusarium, does 
not necessarily determine more than a local pathological significance of 
that fungus. 
RELATION OF SOIL CONDITIONS TO THE PERSISTENCE OF THE 
FUNGUS 
The experimental inoculations conducted under controlled environ¬ 
mental conditions in the greenhouse have presented convincing evidence 
of the pathogenicity of this Fusarium when newly infested soil is planted 
with peas. The relation of the fungus to the host plant when the fungus 
is abundant in the soil can be worked out with comparative ease; but 
the relation of the fungus to its natural environment in the field—that 
relation which determines not only the abundance but the persistence 
of the fungus in the soil—can not be determined in a brief space of time 
by laboratory methods. The absence of conidia by which the fungus 
can be distributed, and its apparent absence from seed, leave us to assume 
that it persists chiefly as mycelium, which can only be detected by the 
presence of a host plant in which it may produce lesions. The number 
or extent of lesions becomes, then, the only criterion whereby we can 
determine from month to month or from year to year in inoculated or 
infested soil the vegetative activity of the fungus. 
