68 
Journal of Agricultural Research 
Vol. XXIV, No. I 
INCUBATION PJ^RIOD OF THE FUSARIUM DISEASE IN CABBAGE SEED¬ 
LINGS AT different TEMPERATURES 
In all of the experiments reported in this paper a great difference in 
the incubation period of the disease was observed in the plants grown 
at the different soil temperatures, and also to some degree between 
plants in the same receptacle. The incubation period, as spoken of in 
this connection, was reckoned from the time the seed was planted rather 
than from the date the plants emerged from the soil, because at tempera¬ 
tures below 23° C. two or three days more were required for the plants 
to emerge from the soil than at the higher temperatures, although the 
seed coats ruptured practically as soon at the lower temperatures. 
Because of this fairly uniform breaking of the seed coats, the seedlings 
at the different temperatures were exposed to the fungus for about the 
same length of time. 
In every case the disease appeared first at 26® to 32® and last at 17° C., 
with a gradation between these extremes. The incubation periods at 
these temperatures were 7 days and 18 days, respectively. At 26® 
to 32® the disease appeared in some plants within i or 2 days after the 
plants emerged from the soil, while others in the same receptacle ap¬ 
peared perfectly normal and healthy for several days longer. At the 
lower temperatures the disease was much longer in becoming manifest 
but at the same time showed a similar variation in length of incubation 
among individuals. The variation among plants grown in artificially 
inoculated soil was not as great as that among plants grown in naturally 
infested soil. A comparison of the results in Ae two cases is shown in 
Tables VI and VII. Gilman {8y p, 43y Table VI) also showed that, al¬ 
though inoculations were made with parts of the same culture on plants 
from the same pot and under conditions as nearly identical as possible, 
the incubation period varied widely. The cause of this variation has 
not been definitely determined, but the results so far obtained indicate 
that it is a difference in the genetic composition of the individuals. 
Under the conditions of these experiments the plants must have been 
exposed to essentially like opportunities for attack by the fungus. 
When the plants were grown in sick” soil for 20 days at 14° and then 
transferred to 26° C., the incubation period was only 4 to 6 days, al¬ 
though a smaller percentage of disease developed in the resistant seed¬ 
lings than when the plants were grown from seed in “sick” soil at 26®. 
This short incubation period is very probably due to the fact that the 
root hairs were in close contact with the fungus mycelium at the low 
temperature, and when subjected to a more favorable temperature the 
fungus immediately began invasion of the roots through the root hairs. 
The Wisconsin Hollander plants treated in this manner showed the dis¬ 
ease mostly in incipient form, whereas the susceptible commercial strain 
was as susceptible as in the em-lier stages of development. 
When the plants were grown for 30 to 36 days in noninfested soil and 
then transplanted to “sick” soil the incubation period was longer in all 
cases than when the seed was planted in “sick” soil. Under these 
conditions the incubation period was shortest at 26° C. and longest at 
17°, being ii and 17 days, respectively. It is probable that this longer 
period was due to the inability of the fungus to make a successful attack 
until new root hairs developed on the roots. 
