Jtme 23,1923 
Disease Resistance to Onion Smndge 
1025 
germinated normally and growth soon covered the plate. In the other 
plate the volatile oil prevented germination of all the spores except those 
around the edge of the plate. In a few days the extract was decomposed 
by bacteria, since no attempt was made to keep it sterile; then the growth 
of the fungus proceeded from the margin toward the center of the plate. 
The fact that the spores in the center of the plate did not germinate 
showed that they had been killed by the volatile oil. 
Another experiment was performed to determine the effect of the vola¬ 
tile oil on the growth of the mycelium. Three plates of potato agar were 
poured, and to the center of each was transferred a bit of mycelium from 
a pure culture of the fungus. The colonies were allowed to develop for 
four days, when they measured 18, 19, and 15.5 mm. in diameter, respec¬ 
tively. One plate was then designated as a control. Within the inverted 
covers of the other two were placed small amounts of onion extract as 
described in the previous experiment. In Table III are given the meas¬ 
urements of the colonies for the next five days. The colony in the control 
plate continued to grow at a normal rate, but those colonies exposed to 
the volatile oil practically ceased growth. At the end of the third day 
the extract was removed from one plate; the fungus colony in that plate 
resumed growth, showing that the mycelium had not been killed, but had 
merely been checked by the volatile oil. In the third plate a slight 
amount of growth had taken place by the fifth day, after which, as the 
onion extract decomposed, the fungus resumed the normal growth rate. 
This experiment shows that the growth of mycelium is markedly checked 
in the presence of the volatile oil, but that, unlike the spores, its 
growth processes are resumed when the inhibitive substance is removed. 
This fact is of significance in considering the effect of the onion oil upon 
the fungus hypha invading the scale tissue. One might expect in this 
case a slowing up or cessation of growth of the hypha, temporarily at 
least, but not necessarily a killing of the fungus filament. 
Tabi.® III .—Effect of volatile oil from onion extract on growth of Colletotrichum circinans 
Time of measurement. 
Diameter of thalli. 
Control. 
Exposed. 
Exposed. 
Beginning of experiment. 
Afw. 
18 
27 
33 
39 
41 
Mm. 
19 
19 
19 
a 20 
23 
Mm. 
15-5 
15-5 
15*5 
16. 0 
Second day... 
Third day. 
Fourth day. 
Fifth day. 
0 Extract removed at end of third day. 
ONION OIL FROM RESISTANT AND FROM SUSCEPTIBLE VARIETIES 
It should be kept in mind that the foregoing experiments were con¬ 
ducted entirely with extracts from white onions, which have been shown 
to be susceptible to the disease. It was of interest to know what degree 
of toxicity was to be found in the extract from the yellow and the red 
varieties, the so-called resistant types. Accordingly, extracts were 
made from Red Globe and Yellow Globe bulbs, as well as from the White 
Globe, a single bulb being used in each case. It should be pointed out 
