Reports on Plant Diseases
RPD No. 904
- Fusarium Wilt of Watermelon and Muskmelon
] [ Disease Cycle ] [ Control
Fusarium wilt of watermelon is caused by
the fungus Fusarium oxysporum f. sp. niveum;
and muskmelon wilt by the fungus Fusarium oxysporum
f. sp. melonis. Both fungi are widely distributed
in Illinois soils. The watermelon form attacks only watermelon, citron,
and summer squash. The muskmelon form infects only Cucumis melon which
includes muskmelon, cantaloupe, crenshaw melon, and honeydew melon. Several
distinct races of both fungi have been identified. Before resistant varieties
were developed, it was not unusual for growers to lose up to 100 percent
of a watermelon or muskmelon crop where the soil was thoroughly infested
with the Fusarium wilt fungus.
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Watermelons and muskmelons are attacked at all stages
of growth. Seeds may decay in the soil. Seedlings often wilt, collapse
(damp-off), and die before or after emergence. Older plants wilt, wither,
and die any time during the growing season (Figure 1). If fruit forms,
it is usually small and unmarketable. Necrotic lesions are often formed
on the roots at sites of penetration. Wilting of runners usually progresses
slowly, showing at first only during the heat of midday. Such plants recover
at night, but after a few days they wilt permanently and die. Plant wilting
is promoted by a high air temperature combined with high light intensity,
a high rate of evaporation, and a low relative humidity. Before death
the leaf margins may appear scorched. The water-conducting tissue (xylem)
within the main stem is often discolored yellow to dark brown.
In muskmelon wilt, narrow streaks of dead tissue, beginning
at or near the soil line, may extend for some distance along a runner.
The streaks are water-soaked at first, turning a yellowish tan to dark
brown. In wet weather, a white or salmon-pink growth of the Fusarium
fungus develops on the surface of dead watermelon and muskmelon stems.
Resistant watermelon plants can become infected, grow slowly, and appear
stunted or dwarfed. Affected older plants often first show a severe stunting
or yellowing of the leaves on one or more vines. The roots appear normal
at first, but later turn reddish brown and finally die. Muskmelon fruit
on systemically infected plants may develop sunken, irregular rot lesions
due in part to the entrance of secondary fungi and bacteria.
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1. Late stage of Fusarium wilt of watermelon.
All leaves have wilted except a few in the center of the vine.
2. Fusarium oxysporum, the fungus that causes Fusarium wilt
of watermelon and muskmelon, as it would be seen under a high-power
laboratory microscope: (a) conidiophores, many bearing macroconidia;
(b) macroconidia; microconidia; (d) thick-walled chlamydospores (drawing
by Lenore Gray)
The two wilt fungi may survive on seed for 2 years
or longer, live from season to season in old infected vines, and are able
to remain alive indefinitely in wilt-infested soil. The watermelon form
is an aggressive saprophyte capable of colonizing and reproducing on most
soil organic matter and the roots and stems of a number of living plants,
including peanut, rice, sorghum, tomato, and several weeds. High populations
of the muskmelon form are usually associated specifically with susceptible
melon tissue that is decaying. Both fungi are commonly spread from one
field or garden to another by infested soil, compost and manure adhering
to machinery, tools, and the feet of humans and animals; urface-drainage
water; and infected seeds.
The Fusarium fungi grow in
soil at any degree of moisture favorable for their host crops. The watermelon
form causes infection between 60° and 95°F (15° to 35°C)
with an optimum of 75° to 83°F (23° to 28°C). For the
muskmelon form, the optimum infection temperature is about 70°F (21°C).
Muskmelon wilt symptoms, however, are most severe at 65° to 77°F
(18 °to 25°C). The greatest damage to both watermelon and muskmelon
may occur below 80°F (26°C). Here the temperature slows down the
growth of the plants and apparently increases their susceptibility more
than it reduces the activity of the fungus. The fungi enter the root tips
and older roots through natural wounds, nematode feeding punctures, and
other wounds. After penetration, the fungi grow into the water-conducting
vessels (xylem tissue), invade other parts of the plant, plug up the vessels,
and produce typical wilt symptoms in the foliage. Disease development
is favored by high nitrogen, low calcium, and low potassium levels in
After infected plants wilt, the Fusarium
fungi produce masses of microscopic spores on and in dead vines. Three
types of spores are formed: small, colorless, one-celled, oval to elliptical
microconidia; larger, slightly curved, septate macroconidia; and rounded,
thick-walled chlamydospores (Figure 2) which can survive long periods
in the soil being resistant to unfavorable environmental conditions. The
conidia may be splashed and blown from diseased vines resulting in expansion
of areas infested by the fungi.
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The only practical control in wilt-infested soil
is to grow wilt-resistant varieties. A number of watermelon and muskmelon
varieties are available that are resistant and well adapted to Illinois.
For information on recommended varieties, refer to Midwest Vegetable
Production Guide for Commercial Growers (C1373) available at your
nearest Extension office or ITCS, University of Illinois P345, 1917
S. Wright St. Ext., Champaign, IL 61820.
Sow best quality, disease-free seed from a reputable
Keep plants growing vigorously by means of a good
fertility program based on a soil test.
Grow seed for transplanting in soil that has been
disinfested by steam or a soil fumigant.
In fields and gardens where the disease has not
appeared, extreme caution is needed to exclude the wilt fungi. Purchase
only certified, disease-free seed or transplant of resistant varieties.
Keep the fungi out of wilt-free fields and gardens by preventing the
spread of infested soil carried on equipment, tools, feet, and running
water. Do not put melon vines in compost or manure piles.
Information concerning insecticides, weed control, varieties, and other recommendations
can be found in the Illinois Homeowners' Guide to Pest Management, available
at your nearest Extension office.
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