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Fusarium crown and foot rot of squash and pumpkin is caused by a forma
specialis of Fusarium solani (Mar.) Sacc. The pathogen exhibits
host specificity and exists as races, like the formae speciales of F.
oxysporum causing vascular wilts of cucurbits. Fusarium crown and
foot rot of squash was first described in detail in South Africa in 1932.
It has also been reported in Australia, Canada, and the United States;
however, its known distribution in the United States is limited. The
disease has been described by various names, including wilt, root rot,
foot rot, root and stem rot, and cortical rot.
Symptoms
Fig. 1. Fusarium crown and foot rot of
watermelon caused by Fusarium solani f. sp. cucurbitae.
Courtesy R. D. Martyn. (Click image for larger
view).
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Usually, the first symptom noticed in the field is wilting of the
leaves. Within several days, the entire plant wilts and dies. If the soil
is removed from around the base of the plant, a very distinct necrotic rot
of the crown and the upper portion of the taproot is evident. The rot
develops first as a light-colored, water-soaked area, which becomes
progressively darker. It begins in the cortex of the root, causes cortex
tissue to slough off, and eventually destroys all of the tissue except the
fibrous vascular strands (Fig. 1). Infected plants break off easily
about 2–4 cm below the soil line. The fungus is generally limited to the
crown area of the plant. The main and lower portion of the taproot are not
affected, except under extremely wet conditions. Likewise, the stem is not
affected, except for the lower 2–4 cm immediately above the soil line.
Plants showing symptoms develop numerous sporodochia and macroconidia,
giving the mycelia a white to pink color on the stem near the ground
surface. Fruits are attacked at the fruit-soil interface (Fig. 2); the
severity of the fruit rot is dependent on soil moisture.
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Fig. 2. Fusarium fruit rot of pumpkin caused
by Fusarium solani f. sp. cucurbitae. Courtesy T. A. Zitter.
(Click image for larger
view). |
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Causal Organism
Fusarium crown and foot rot of squash is caused by F. solani f.
sp. cucurbitae W. C. Snyder & H. N. Hans. Two races have been
described. Race 1 causes a root, stem, and fruit rot and occurs worldwide.
Race 2 causes only a fruit rot and has been reported only in California
and Ohio. Recent work, however, has separated these two races into
different mating populations, which most probably represent different
species. F. solani f. sp. cucurbitae forms all three
asexual spore types (microconidia, macroconidia, and chlamydospores)
typical of Fusarium
and a sexual stage when proper mating types are introduced;
however, perithecia have not been observed in nature. Pathogenic isolates
of F. solani are generally heterothallic (having two or more
mating types). The teleomorph is Nectria haematococca Berk. &
Broome (syn.
Hypomyces solani (Rke. & Berth.) W. C. Snyder & H. N.
Hans.). Numerous strains of F. solani f. sp. cucurbitae
have been found, and there is a high degree of variability in the fungus.
Disease Cycle
Both races can be internally or externally seedborne. However, F.
solani f. sp. cucurbitae apparently survives for only 1–2
years in seed. Infection does not appear to affect seed viability or
germination. Although the fungus produces abundant chlamydospores, it
apparently survives for only 2–3 years in soil, which is much less than F.
oxysporum or even other formae speciales of F. solani. Since
the perfect stage has not been observed in nature, ascospores probably do
not play a significant role in survival. Plants and fruit of any age can
be infected. In the field, the disease is generally a problem only in
summer squash and certain pumpkin cultivars; however, laboratory studies
have shown that most cucurbits are susceptible, including melon and
watermelon. Mature cucumber is much less susceptible than other cucurbits,
but all are equally susceptible in the seedling stage.
Control
Fusarium crown and foot rot occurs sporadically in most areas, and
disease severity is dependent on soil moisture and inoculum density.
Because the fungus survives in the soil for only 2–3 years, a 4-year
rotation is usually adequate for disease control. Planting
fungicide-treated seed is also effective in reducing the incidence of
disease initiated from infected seed.
Selected References
Champaco, E. R., Martyn, R. D., and Miller, M. E. 1993. Comparison of Fusarium
solani and F. oxysporum as causal agents of fruit rot and
root rot of muskmelon. HortScience 28:1174-1177.
Nash, S. M., and Alexander, J. V. 1965. Comparative survival of Fusarium
solani f. cucurbitae and F. solani f. phaseoli
in soil. Phytopathology 55:963-966.
Paternote, S. J. 1987. Pathogenicity of Fusarium solani f. sp. cucurbitae
race 1 to courgette. Neth. J. Plant Pathol. 93:245-252.
Sumner, D. R. 1976. Etiology and control of root rot of summer squash
in Georgia. Plant Dis. Rep. 60:923-927.
Tousson, T. A., and Snyder, W. C. 1961. The pathogenicity,
distribution, and control of two races of Fusarium (Hypomyces)
solani f. cucurbitae. Phytopathology 51:17-22.
Van Etten, H. D., and Kistler, H. C. 1988. Nectria haematococca
mating populations I and VI. Pages 180-206 in: Advances in Plant
Pathology. Vol. 6. D. S. Ingram and P. H. Williams, eds. Academic Press,
New York.
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