74 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
leaves supplied by infected bundles, those leaves progressively higher up 
on the stem being attacked later. When, however, the fungus reached the 
succulent tissues of the upper portion of the plant, there was a sudden wilting 
of the entire plant. The stem and the older petioles were always the last 
to succumb and remained green and turgid for a time after the other parts 
had shriveled and blackened. 
The symptoms varied somewhat, however, under conditions which were 
not favorable for the maximum development of the disease. Thus, if the 
plants were resistant, if the fungus was lacking in virulence, or if the en¬ 
vironmental conditions were unfavorable to the growth of either host or 
parasite, the tendency was for the disease to appear as a slow blight rather 
than as a wilt. In this blight the leaves yellow and die slowly, while in the 
wilt they droop suddenly and die without yellowing. 
Symptoms in Relation to Temperature. If a susceptible variety of tomato, 
such as Chalk’s Jewel, is inoculated with a virulent strain of F. lycopersici, 
and the general growth conditions are optimum for the development of 
the disease, the following classification may be made on the basis of soil 
temperature: 
First: Temperature optimum for the disease, i.e., 25 0 to 31 0 C.—a 
sudden wilting which appears first in the lower leaves, then in those progres¬ 
sively higher up, and is rarely accompanied by yellowing of the leaves 
affected. 
Second: Temperature just above or just below the optimum, i.e., 33 0 
to 34 0 C., or 20° to 24 0 C.—a wilting accompanied, and often preceded, by 
a yellowing of the leaves. The appearance may frequently be that of a 
slow blight rather than of a wilt for, as above stated, there is more yellowing 
than actual wilting, and the plants often show stunting of growth. 
Third: At temperatures above 34 0 or below 20° C. there is no external 
evidence of the disease. 
Fourth: In addition to the manifestations of disease mentioned above, 
the fungus may enter the host but penetrate the bundles in only the lower 
portions of the stem. This condition is often the result of a short exposure 
to temperatures favoring the disease, followed by a drop in temperature 
sufficient to check further development of disease. Plants thus infected 
are lighter in weight than uninfected plants grown under similar conditions, 
and the greater the amount of infection, the greater the loss in weight. 
Growth Differences of Host and Parasite in 
Relation to the Disease 
Since a parasitic disease is the result of interactions between host plant 
and parasite, external influences which modify the appearance of the disease 
are necessarily effective through the changes which they produce either in 
the host, in the parasite, or in both. If those complexes, called host 
resistance on the one hand and virulence of the parasite on the other, 
