434 Wisconsin Academy of Sciences, Arts, and Letters . 
clearly defined interest, but to the difficulties commonly met in the 
attempts to control the essential environmental factors with any 
degree of precision. The more obvious parasites like the rusts and 
mildews are air-borne, attacking the host foliage. Moisture and 
temperature are evidently factors profoundly influencing their at¬ 
tack, but no experimenter has yet succeeded in exactly measuring 
these influences separately for the simple reason that when he at¬ 
tempts precise regulation of any such factor surrounding the func¬ 
tioning green plant he necessarily disturbs other variables, and thus 
almost hopelessly complicates his problem. 
Early in our own studies of plant parasites, we became interested 
in those invading the underground structures. This was in part 
because the hidden diseases had been relatively neglected and be¬ 
cause when root attacks occur they cause the most serious types of 
maladies. In these studies the importance of environmental factors 
soon became obvious. Let us consider, as examples, the yellows 
disease of cabbage and the root rot of tobacco, each due to a specific 
soil fungus which is locally capable of the complete annihilation of 
its host. Both parasites persist and possibly multiply indefinitely 
in the soil, rendering it permanently ‘ ‘ sick ’ ’ for that special crop, 
Yet it is a matter of common experience in southern Wisconsin 
fields, where both occur, that the cabbage yellows is serious only 
with a hot dry midsummer, whereas with the tobacco the factors 
are exactly reversed. Cool, moist weather favors the root rot, and 
with a period of dry heat a tobacco crop sick with this disease 
quickly recovers. When we met like conditions with potato and 
other 4 ‘root diseases”, the question arose whether the fundamental 
problems of the relation of environment to parasitism may not be 
handled by more simple and direct experimental methods with 
these subterranean parasites than with the aerial ones as hereto¬ 
fore studied. Certain difficulties of manipulation, and compli¬ 
cations from the light factor inherent with the leafy shoots, are 
lessened or disappear in a study of the organs below ground. 
Work upon such problems has been carried on in the Department 
of Plant Pathology almost uninterruptedly during some six years 
past. The first problem, that of the relation of climate to cab¬ 
bage yellows, was defined during our early study 1 of this disease 
in which Dr. J. C. Gilman, then a graduate student assistant, 
cooperated. Soil temperature was an evident factor, and Gil- 
1 See later list of publications concerning this and other matters referred to 
above. 
