Feb. 15, 1925 
Control of Mycelial Neck Rot of Onion 
367 
It is unforunate that the writer was 
unable to make parallel tests with a 
white variety. Later studies 5 have 
shown that the basic principle in 
resistance to neck rot among colored 
varieties of onion is due to a host toxin 
which is affiliated with the scale pig¬ 
ments and becomes functional as the 
outer scale tissue dies and permits its 
diffusion into drops of moisture on the 
exterior. The exposure of the suc¬ 
culent neck tissue of the immature 
bulbs often allows the neck-rot organ¬ 
ism to gain entrance without contact 
with the toxin of the dead pigmented 
scale tissue, whereas, in the case of the 
mature neck, the parasite is more 
liable to come in contact with this 
toxin. With the white varieties the 
effect of this toxin does not enter in. 
Although the mature neck tissue is un¬ 
doubtedly less favorable to infection 
than the immature neck tissue, the 
difference is not due, as it is in part 
in the colored varieties, to the presence 
or absence of the inhibitory toxin. 
More experimental work on this point 
is contemplated. 
EFFECT OF REMOVAL OF TOPS UPON 
INFECTION 
In order to estimate the bearing of 
the removal of tops at harvest time 
upon infection with the mycelial neck- 
rot fungus, some artificial inoculations 
were first performed. Bulbs of White 
Portugal, Yellow Globe, and Red 
Globe varieties, the tops of which had 
fully matured, were harvested at 
Madison, Wis., on September 3 from 
soil which had not grown onions for at 
least six years. The tops were left 
intact on a portion of each variety, 
while from a similar amount of each 
variety the tops were cut close to the 
bulb so as to expose the succulent 
tissue. The bulbs were then sprayed 
with a spore suspension of the fungus. 
A number of each variety, topped and 
untopped, were sprayed with distilled 
water as controls. All were then 
placed in a moderately humid chamber 
at about 18° C. After 19 days the 
bulbs were examined, the percentage 
of neck rot recorded being given in 
Table II. 
In the experiment a much larger 
percentage of infection occurred in 
topped than in the untopped bulbs, 
and there is little doubt that the 
removal of tops offers greater oppor¬ 
tunity for infection. It is to be noted, 
however, that the fungus was not 
entirely excluded by leaving the tops 
intact, and the question naturally 
arises whether under more favorable 
environment even more infection might 
not have occurred in the untopped 
bulbs. The function of the inhibitive 
toxin already cited 6 in the pigmented 
neck tissue also enters into considera¬ 
tion when the amount of infection in 
colored and uncolored untopped bulbs 
is noted. It is significant in this con¬ 
nection that the highest percentage of 
infection in untopped bulbs occurred 
in the white variety. This difference 
becomes more striking in the field 
experiments to be discussed presently. 
Table II.— Development of mycelial 
neck rot in topped and untopped 
bulbs of White Portugal , Yellow 
Globe , and Red Globe onions 19 days 
after artificial inoculation 
Variety 
Method of 
handling 
tops 
Inoculated 
Control 
Total number 
of bulbs 
Per cent neck 
rot 
Total number 
of bulbs 
Per cent neck 
rot 
White Globe. 
Topped.. 
21 
90 
20 
0 
Do_ 
Untopped 
19 
16 
20 
0 
Yellow Globe 
Topped _. 
10 
80 
9 
11 
Do... 
Untopped 
10 
10 
10 
0 
Red Globe... 
Topped.. 
10 
80 
10 
0 
Do. 
Untopped 
10 
0 
10 
0 
During the years 1917, 1918, and 
1920 some comparative field studies 
were made. Since the Red Globe 
onion is the type grown almost ex¬ 
clusively in the Racine section, observa¬ 
tions during the first two years were 
limited to that variety. In 1920 
special plantings of Yellow Globe and 
White Globe were made alongside Red 
Globe for the purpose of this study. 
In each season when the crop was 
mature several bushels were taken from 
a small portion of the field and the 
bulbs were impartially divided into 
two lots, those of one being placed in 
crates with the tops intact and those 
of the other with the tops clipped. 
The crates were stacked in the field to 
cure for several weeks and were then 
removed to storage. Final notes were 
taken two to three months later. 
The data accumulated during three 
seasons is given in Table III. There 
® Walker, J. C., and Lindegren, C. C. further studies on the relation of onion scale pig¬ 
mentation to disease resistance. Jour. Agr. Research 29: 507-514. 1924. 
