June 23,1923 
Disease Resistance to Onion Smudge 
1035 
a. m., July 31, 1920, and determinations made directly by the electro¬ 
lytic method.® The tops and the scales were chopped up separately and 
the expressed juice used for the determinations. The results are given 
in Table VIII. It is to be remembered, of course, that this method does 
not necessarily give the actual reaction which the fungus hyphae en¬ 
counter as they invade the plant. It is conceivable that there may be 
natural variations in reaction in different parts of the bulb and at different 
hours of the day, while secretions of the fungus itself might change its 
immediate surroundings. Valleau (19) suggests in the case of brown- 
rot of plums {Sclerotinia cinerea) that, **it is very probable that in the 
slow development of the fungus in the resistant fruits enough oxalic acid 
is produced by the hyphae to actually become toxic to them.’' However, 
there was no evidence of wide variations in acidity between resistant and 
susceptible onion bulbs, and the acidity of the juice expressed from either 
was much lower than that of the onion decoction in which germination 
and growth of the fungus were inhibited. Moreover, the fungus tended to 
change very acid solutions to a more alkaline, and thus more favorable, 
reaction. 
Tablb VII. —Growth of Colletotrichum circinans in onion decoction adjusted to^ various 
Ph values {duration of experiment, 54 days) 
Flask No. 
Before ster¬ 
ilization. 
Ph value. 
After ster¬ 
ilization. 
At end of 
experiment. 
Amount of growth. 
I. 
2. 0 
I. 6 
1.8 
None. 
2. 
2. 0 
I. 6 
1.8 
Do. 
.3 . 
3-0 
3-0 
2. 0 
Do. 
4 . 
3 - 0 
3-0 
2. 0 
Do. 
5 . 
3-2 
3-0 
2. 6 
Very slight. 
6. 
3- 2 
3-0 
2. 6 
Do. 
7 . 
3.8 
3-6 
5-2 
Good growth and sporulation. 
8. 
3.8 
3*6 
4. 8 
Do. 
9 . 
4-6 
4.6 
6. 2 
Do. 
10. 
4. 6 
4. 6 
6. 2 
Do. 
II. 
5-2 
5-4 
7.2 
Good growth. 
12. 
5-2 
5-4 
7.0 
Do. 
13. 
6.4 
6.4 
7-4 
Do. 
14 . 
6.4 
6.4 
7-4 
Do. 
Tabi^E VIII. —Acidity of expressed juice of scales and tops of Red, Yellow, and White 
Globe onions 
Variety. 
Ph values. 
Tops. 
Succulent 
scales. 
Red Globe. 
5-2 
4.8 
5-2 
5-3 
5-4 
S -4 
Yellow Globe. 
White Globe. 
« The writer is indebted to Mr. O. C .Bryan of the Department of Soils, University of Wisconsin, for mak¬ 
ing the hydrogen-ion determinations. 
