496 
a value of 0. The formula used was as 
follows: 
NiO + Nal +N2Q4N3 
1 
3 x 100 
Disease index = 
where N,, No, Ns, Ny = number of corms in each 
severity class 
0, 1, 2, 3 = values assigned to severity 
classes 
t = total number of corms 
counted 
The disease indexes obtained from these 
calculations are shown in tables 23 and 24. 
The isolates varied in their effects on the 
test varieties, but the variations did not 
follow any definite pattern. 
The results obtained in the inoculation 
tests show that an isolate does not always 
produce the same form of the disease. For 
example, in the 1953 tests, all three forms 
of the disease appeared in plants inocu- 
lated with isolates +7-32 and 50-26. In 
most cases, more than one disease form was 
obtained from inoculations with a single 
isolate. A summary of the disease forms 
produced in plants inocu‘ated with the va- 
rious isolates is given in table 25. 
Although the same methods were not 
employed, these results are in general 
agreement with those obtained by Mc- 
Clellan (1948), who used mass culture 
isolates of Fusarium for inoculations on 
the Picardy and Dr. F. E. Bennett varie- 
ties. Some isolates caused a vascular rot, 
some a surface rot, and others caused a 
combination of symptoms. McClellan 
mentioned that the pathogenicity of iso- 
lates varied from year to year but he did 
not describe the nature of the variation. 
DISCUSSION AND 
CONCLUSIONS 
The occurrence of different forms of a 
single fungus in laboratory cultures has 
been reported by many workers in many 
parts of the world. The terms “varia- 
tion,’ “‘saltation,” “dissociation,” ‘‘muta- 
tion,” and “‘sectoring’ have been used to 
designate this phenomenon. 
Most of the studies of variations in the 
genus ['usarium have been made on the 
relation of pathogenicity to forms of 
growth. Most of the workers have re- 
ILLiNois NATURAL History SURVEY BULLETIN 
Vol. 26, Art. 6 
ported that cultures of Fusarium vary be- 
tween a type with abundant aerial growth 
and a type with all the mycelium appressed 
in the nutrient substrate. The type of cul- 
ture having aerial mycelium generally has 
been found to be most pathogenic, the type 
having only appressed mycelium least 
pathogenic, but observers on this point 
have not agreed unanimously (Armstrong, 
MacLachlan, & Weindling 1940, Burk- 
holder 1925). 
Numerous attempts have been made to 
account for these variations, but none has 
been entirely satisfactory. Leonian (1929, 
1932) considered different cultural forms 
merely as phases in an orbit of variation of 
a species. Hansen (1938) explained varia- 
bility in many of the Fungi Imperfecti as 
a “dual phenomenon”’ resulting from a 
segregation of genetically different nuclei 
in a multinucleate mycelium. Hansen & 
Smith (1932) earlier concluded that vari- 
able forms of the Fungi Imperfecti may 
owe their instability to nuclear heteroge- 
neity, and that this condition can be 
brought about by nuclei of one strain en- 
tering the cells of another strain through 
anastomoses, and that reassortment of di- 
verse nuclei can be accomplished by such 
mechanisms as anastomosis and unequal 
cell division. 
Anastomoses, or fusions, between the 
hyphae and between the germ tubes of va- 
rious fungi have been described by other 
workers. Zeller (1926) found conjugat- 
ing macroconidia of Nectria sanguinea 
(Sibth.) Fr. in sporodochia on apple bark. 
He observed some comparatively long con- 
jugating tubes connecting cells of one spore 
with those of another. In each case the nu- 
cleus had migrated from one cell to the 
other, producing a binucleate condition. 
Also, Zeller saw conjugation between two 
cells of the same spore. He did not discuss 
the significance of these conjugations. 
Dickinson (1932) studied hyphal fu- 
sions in Fusarium fructigenum (Fries) 
and FI. vasinfectum (Atk.). In no case 
did he observe fusion between segments 
(cells) of a conidium and only once did he 
find fusion between two segments of ad- 
jacent conidia. He concluded that in these 
species saltation was due to mutation 
rather than to heterocaryosis or cytoplas- 
mic inheritance. 
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