May 17,1924 
A Fusanum Bulb Rot of Onion 
689 
RELATION OF SOIL TEMPERATURE AND MOISTURE TO THE 
DISEASE 
The importance of studying the relation of environmental conditions to the 
occurrence of this disease is obvious. Since the organism is a soil fungus and 
infection occurs normally in the subterranean parts of the plant, soil temperature 
and soil moisture were considered first. 
Temperature studies were carried out in Wisconsin soil temperature tanks. 
Inoculations were made, as previously described, by first placing the wounded 
bulbs in a spore suspension and then planting in metal cans of soil, four bulbs to a 
can. The soil moisture was adjusted to 80 per cent of the water-holding capacity, 
and the cans were then placed in the constant temperature tanks held at 30°, 26°, 
22°, 19°, 15°, and 12° C. An equal number of uninoculated controls were 
included in each case. The air temperature ranged from 15° to 20° and was 
uniform for all tanks. 
No signs of the disease developed at 12° C. even after several weeks. All of 
the inoculated plants held at 15° to 30° eventually succumbed to the typical 
wilt of the tops and decay of the bulbs, but a decided difference prevailed in the 
progress of the disease at the various temperatures. The bulbs all sent up tops 
very quickly. The first signs of the wilt were noted on the tenth day at 26° and 
at 30° and the disease progressed very rapidly at these temperatures, resulting 
in entire collapse of the plants by the twentieth day. The controls at these tem¬ 
peratures had reached a height of eight inches at this time. The plants at 22° 
and at 19° were proportionately slower in succumbing to wilt. At 15° there 
was no sign of wilt until the twenty-fourth day and the last of the four plants was 
not affected until the thirty-sixth day. The experiment when repeated and ex¬ 
tended to 32° gave confirming results and the disease was nearly as active at 
32° as at 28°.. (See PI. 3.) The controls all remained healthy with the excep¬ 
tion of one plant at 30° which wilted slightly. 
The results show the fungus capable of producing the disease at a range of 
considerable width extending from 15° to 32° C. or above. Within these limits 
there is an optimum at about 28° and a gradual retardation of progress as the 
temperature lowers. The disease, as affecting the growing plant, is apparently 
inhibited at 12°. 
Constant soil moistures are not so readily obtained. To avoid as much as 
possible variations in the can due to force of gravity upon the soil water, shallow 
metal pots were chosen (6 inches deep and 5 inches in diameter). Sterilized 
greenhouse clay loam soil was adjusted in various lots to 100 per cent, 85 per cent, 
50 per cent, and 26 per cent of its water-holding capacity, as determined by the 
standard 10-inch cylinder. The soil was then potted, care being taken to so pack 
it that equivalent amounts by dry weight occupied equivalent volumes throughout 
the series. In the case of the highest moisture, final adjustment was made after 
the soil was potted. Constant moisture content was approximated by daily 
weighing and adjustment. At the end of the experiment moisture determinations 
of samples from the upper 2 inches of each of the pots were made. 
In pots of soil thus prepared, bulbs wounded and inoculated in the usual 
manner were planted, 4 bulbs to each pot. These pots were placed in a green¬ 
house held at 20° to 25° C. Although the range of soil used extended from a very 
dry soil to one practically saturated, little difference was noted in the rate at 
which the disease developed. (See Table IV.) It appears that soil moisture 
within the range at which the onion plant will normally develop, does not materi¬ 
ally affect the Fusarium wilt or decay after infection has once occurred. 
