September, 1955 FORSBERG: 
Coons & Strong (1931) scheme shown on 
pages 458 and 459. 
With the exception of isolates 50-26, 
49-8, and 50-24, the isolates that were 
the most sensitive to the copper salts were 
also the most sensitive to mercuric chlo- 
ride. Isolates from the three disease forms 
could not be separated on the basis of their 
reactions to mercuric chloride. 
Although the isolates used in this study 
varied considerably in their sensitivity to 
mercuric chloride, as well as to other 
chemicals, more intensive work would have 
to be done before a conclusion could be 
reached that differences in sensitivity are 
responsible for differences in disease con- 
trol obtained in tests involving chemical 
treatment of corms. However, sensitivity 
differences possibly could be factors con- 
tributing to control differences. 
Color Reactions on Steamed Rice 
Steamed rice was recommended by Wol- 
lenweber e¢ al. (1925) as being especially 
useful in identification of groups of Fu- 
sarium because of the distinctive colors 
produced by cultures grown on it. Nelson, 
Coons, & Cochran (1937) reported that 
isolates from two forms of Fusarium yel- 
lows of celery fell into two groups when 
grown on steamed rice; in all instances the 
cultures which formed color on rice pro- 
duced only form I of Fusarium yellows, 
while the cultures that were colorless pro- 
duced only form II. 
In the present study on the gladiolus 
Fusarium, 2 grams of rice and 6 milli- 
liters of distilled water were placed in 
each of 80 test tubes. The tubes were 
plugged with cotton and steamed 1 hour 
on each of 3 successive days. Then a small 
amount of an agar slant culture was trans- 
ferred to each tube of steamed rice. Two 
tubes of rice were used for each isolate. 
The test was run twice. In one trial the 
inoculated tubes were kept in the dark in 
an incubator held at 25 degrees C. In the 
other trial the tubes, in wire baskets, were 
placed on a cabinet shelf so that they were 
exposed to diffused light during the day. 
Colors produced in the rice substratum 
were, after 3 weeks and again after 11 
weeks, compared with plates in the color 
manual of Ridgway (1912). The colors 
FuSsARIUM DISEASE OF GLADIOLUS 469 
produced in the two trials were almost 
identical. 
As shown in table 9, the 40 isolates of 
gladiolus Fusarium varied a great deal in 
the colors they produced on steamed rice. 
As will be described more fully in a section 
on pH studies, isolates of Fusarium caused 
progressive pH changes to occur in the cul- 
ture medium. In the first 3 weeks, when 
the cultures were acid in reaction, the col- 
ors in most cases were forms of pink or 
red, although one isolate remained white 
and several others were shades of violet or 
purple. In the last 6 or 7 weeks, when the 
cultures were alkaline in reaction, the col- 
ors changed to blues, violets, or purples, 
in many of which olives and browns ap- 
peared. There was no relation between 
the disease form from which the isolates 
were obtained and the colors the isolates 
produced on rice. 
VARIATIONS IN CULTURE 
TYPES AND PATHOGENICITY 
Variability in culture types and in path- 
ogenicity within a given species of Fu- 
sarium seems to be of almost universal oc- 
currence. The only exception found in a 
search through the literature was in the 
report by Blank (1934) that different 
strains of the cabbage yellows organism 
were uniform in their cultural behavior 
and pathogenicity. 
The relation between growth types of 
cultures of Fusarium on laboratory media 
and differences in pathogenicity has been 
studied by many workers. Ullstrup (1935) 
reported that with the Fusarium stage of 
Gibberella saubinetii (Mont.) Sacc. rapid 
mycelial growth and abundant aerial my- 
celium are directly correlated with a high 
degree of pathogenicity in the majority of 
cases. Earlier, Brown (1928), working 
with certain fruit-rotting species of Fu- 
sarium, had pointed out that the mycelial 
type of culture is the most pathogenic. He 
stated that this type of growth is the form 
generally obtained in first isolations from 
diseased tissues. Harvey (1929) reported 
that high virulence in various strains of 
Fusarium fructigenum Fries was corre- 
lated with vigorous mycelial growth. Arm- 
strong, MacLachlan, & Weindling (1940) 
reported that in the cotton-wilt organism, 
