September, 1955 ForSBERG: 
might be distinguished by differences in 
spore sizes. As pointed out by Harter 
(1939), several factors influence the size 
of Fusarium spores produced in laboratory 
culture. It is often difficult, if not impos- 
sible, by any means of manipulation of the 
culture to obtain a morphological agree- 
ment with the description of a given spe- 
cies. It seemed reasonable to assume, how- 
ever, that a fair comparison could be made 
of spores from cultures which had been 
kept under the same conditions. 
Six isolates were selected for use in 
making spore measurements and were 
grown on Wellman’s agar in Petri dishes 
for 14 days in a location where the dishes 
were exposed to diffused light during day- 
light hours. All measurements were made 
within 2 or 3 days after the cultures were 
14 days old. Cultures from which meas- 
urements could not be made immediately 
were placed in a refrigerator at 3-5 de- 
grees C. until measurements could be 
made. ‘The spores to be measured were 
mounted on 2 per cent plain agar on a 
microscope slide. Thirty spores of each 
septation type were measured from each 
culture in which this number of spores was 
available. The 4- and 5-septate types were 
so rare in most of the cultures that it was 
impossible to find 30 spores for measure- 
ment. 
Mean measurements and ranges of 
measurements of spores from the six se- 
lected isolates are shown in table 13. The 
differences in spore sizes are not great 
enough to place the isolates in distinct 
groups on the basis of spore size. 
PATHOGENICITY TESTS 
In this investigation, the pathogenicity 
of the isolates of Fusarium was tested both 
in the laboratory and in the greenhouse. 
All 40 isolates were tested in the labora- 
tory; 27 of them were tested in the green- 
house. 
Laboratory Tests 
Large mature gladiolus corms, free 
from blemishes, were selected for the lab- 
oratory inoculation tests. After the husks 
had been removed, the corms were washed 
well with water and then left on a table 
FUSARIUM DISEASE OF GLADIOLUS 481 
until dry. Four small wounds, approxi- 
mately equal distances apart, were made 
on the basal area of each corm with a 
three-sixteenths-inch metal drill bit turned 
rapidly with thumb and index finger. Uni- 
form disks of inoculum were cut from agar 
plate cultures with a sterile metal tube 
three-sixteenths inch in diameter. One disk 
of inoculum was pressed gently into each 
wound. Each corm was inoculated with 
four isolates; each isolate was used on four 
corms in each of the various tests. 
Inoculations were identified by paper 
tags pinned near the points of inocula- 
tion, fig. 19. Immediately after they had 
been inoculated, the corms were placed in 
moist chambers and left for 48 hours. 
‘They were then removed and placed on a 
table in the laboratory. The corms were 
examined frequently, and at the end of 4 
to 6 weeks final disease readings were 
made. 
These inoculations resulted in the de- 
velopment of three general types of le- 
sions: severe, in which the rot progressed 
steadily from the point of inoculation un- 
til most of the corm was rotted, shown in 
the Bit o’ Heaven corm in fig. 19; mild, 
in which the rot progressed very slowly 
and only a narrow brown band appeared 
around the point of inoculation, shown in 
the Acca Laurentia corm in fig. 19; healed, 
in which no rot developed and the inocu- 
lation wound corked over, shown in the 
Annamae corm in fig. 19. 
A basis on which to compare virulence 
of the isolates of Fusarium, as well as to 
compare susceptibilities of gladiolus vari- 
eties, was derived from a modification of 
McKinney’s (1923) formula for disease 
evaluation. Class values of 0, 1, and 2, 
respectively, were assigned to the healed, 
mild, and severe types of lesions. Indexes 
of rot severity were calculated by use of 
the following formula: 
__N,O + Nol + N,2 
rai 2t 
x 100 
Rot severity index 
where Ni, No, Nz = number of corms in disease 
classes 1, 2, and 3, respec- 
tively 
0, 1,2 = values assigned to disease 
classes 1, 2, and 3 respec- 
tively 
t = total number of corms used 
