1916] 
GILMAN—CABBAGE YELLOWS 1e 
Discussion.—Exaetly why the raising of the temperature 
should bring on this disease is still not clear, but in view of 
our present knowledge some correlation should be made be- 
tween the relations found and the other work that may shed 
light on this point. First, it should be pointed out that many 
of the so-called vascular parasites behave in a very similar 
manner toward temperature. As Smith (714) has shown with 
Bacillus Solanacearum, Humphrey (714) with Fusarium or- 
thoceras, and the present investigation with F. conglutinans, 
high temperatures facilitate the destruction of the host. To 
what extent this destruetiveness may be attributed to changes 
in the parasite and to what extent to changes in the host plant, 
it is diffieult to determine. Smith and Humphrey both are 
inclined to consider the changes in the host the primary fac- 
tors concerned, and as will be pointed out, the same opinion 
may be taken in the case of the cabbage disease. Nevertheless, 
the change in the fungus must be looked into also. 
Among diseases of plants that are partially dependent on 
temperature relations for their oceurrence, in many of the 
cases the relation is not one of loss of virulence on the part of 
the fungus but a limitation in the temperature range of ger- 
mination of the fungous spores. This sort of limitation was 
best illustrated by the work of Melhus (212, 713) on Phytoph- 
thora infestans as related to the potato blight. This author 
showed that, although the spores germinated only at low tem- 
peratures, the mycelium which wintered over in the tuber, at- 
tacked the new shoots from such tubers only at high tempera- 
tures. Other cases of similar nature, where the temperature 
for spore germination differed from that of mycelial growth, 
are found among many of the obligate plant parasites. Exam- 
ples that might be cited are Cystopus candidus, Plasmopara 
Viticola, Ustilago Avenae, U. Tritici, Uromyces Trifolii, 
Peridermium Strobi, Puccinia graminis, P. rubigo-vera, P. 
dispersa, and P. coronata. 
That Fusarium conglutinans is not dependent on germina- 
tion temperatures for its destructive attack is clearly shown 
by the fact that germination occurs readily at 17°C., which 
temperature is close to the lower limits of its destructiveness 
