June 23,1923 
Disease Resistance to Onion Smudge 
1037 
that such an analysis breaks down to some extent the complex balance 
of compounds as liiey exist in the tissue. 
The marked toxicity of the expressed juice of fleshy scales of both the 
susceptible and the resistant varieties is also of interest. It seems^to 
be due largely to the volatile onion oil, but apparently it does not con¬ 
tribute to the noted differences in varietal susceptibility of the northern 
Globe types. The juice from colored scales and that from white scales 
are essentially equal in toxicity. Moreover, as stated above, infection 
of the fleshy scales seems to progress as readily in resistant as in sus¬ 
ceptible varieties. The relation of the volatile oil to the aggressiveness 
of the parasite may well be considered, however. The smudge organ¬ 
ism causes an exceedingly slow decay of the bulb compared with 
certain other bulb parasites, such as Botrytis allii, for instance, although 
penetration by the former is accomplished very readily and easily and 
in fact much more generally than in the case of the latter fungus, 
which usually requires a. wound in order that invasion may take place. 
When we consider the marked toxicity of the onion oil to the smudge 
fungus, we are led to wonder that invasion occurs at all. However, 
several possible explanations at once present themselves. In the first 
place, the oil as it is released from the expressed sap may be in a differ¬ 
ent state than that in which it actually exists in the living cell. In 
the allied case of the mustard oil of the crucifers, allyl isothiocya¬ 
nate, for instance, the oil exists in certain cells as the glucoside, sinigrin, 
while in adjoining cells is contained a glucoside-splitting enzym, myro- 
sin. When the tissue is crushed the enzym comes into contact with 
the glucoside and the volatile oil is released. Does a similar con¬ 
dition prevail in the onion? In the second place, under ordinary 
natural conditions the mycelium of the smudge fungus apparently does 
not invade the living host cells, but weakens and kills them, usually 
slightly in advance of the hyphae, by means of some diffusible sub¬ 
stance. Moreover, the change in color and texture of the cell con¬ 
tents and, in the case of red scales, the tendency toward formation of 
coloring matter in the parenchymatous cells both indicate certain effects 
on host metabolism ahead of the parasite. It may be that through 
these changes in the cells the volatile oil is either broken down or released 
and hence that its toxic effects are reduced, at least to the point where 
invasion is merely retarded and not entirely checked. 
The study of penetration indicates that the fungus is capable of 
penetrating the cuticle and dissolving the cellulose of the walls equally 
well in all varieties. The acid range tolerated in culture media is wider 
than the range occurring in the host. It is assumed, therefore, that 
differences in cell membranes and in acidity of cell sap do not materially 
affect resistance to the smudge organism. 
SUMMARY 
(1) Onion smudge is a common disease of the bulb scales. In general, 
white varieties are susceptible to the disease while colored varieties 
show a high degree of resistance. 
(2) When the dry outer scales of colored varieties were removed the 
fleshy scales were found to be very susceptible, invasion taking place in 
the same manner as in the white varieties. The highly colored dry 
outer scales apparently serve as a barrier to the entrance of the parasite. 
(3) In the seedling stage all varieties tried were equally susceptible. 
