Alan
McNab
Professor of Plant Pathology |
|
| CUCURBIT
DISEASES |
|
|
Powdery
Mildew, caused by the fungus Erysiphe cichoracearum, affects
cucumber, muskmelon, pumpkin, and squash. It is caused by a fungus
that appears as a white powdery growth on leaves. Crown leaves are
affected first and may wither and die. The fungus can be introduced
on greenhouse-grown platns or by wind from areas with relatively warm
winter climate where the fungus can over-winter. Disease development
is favored by high temperatures. |
|
|
Downy
Mildew on cucumber and muskmelon is caused by the fungus Pseudoperonospora
cubensis. Irregularly shaped yellowish to brown spots appear on upper
sides of leaves, usually at the center of plants. Under moist conditions,
a purplish midew develops on the underside of leaf spots. Leaves die
as spots increase in size. Spread is rapid from the crown toward new
growth. The causal fungus overwinters in areas with a relatively warm
climate and can be introduced to other areas by wind. Moist conditions
favor disease development. |
|
|
Target
Spot, caused by Corynespora cassiicola, is similar to downy
mildew in appearance. Leaf spots begin as small yellow flecks which
become angular or as circular spots with light brown centers and dark
brown spots. Spots vary from 1/8 to 3/8 inch in diameter. Lesion coalescence
may result in large areas of dead tissue, which may shred and fall
out. |
|
|
Cercospora
Leaf Spot, caused by Cercospora citrullina, affects cantaloupe
and watermelon. Leaf spots are small, dark brown to black with a white
center, and may have a yellow halo. |
|
|
Alternaria
Leaf Spot, caused by the fungus Alternaria cucumerina, occurs
primarily on muskmelons but also on cucumbers and other cucurbits.
Leaf spots are small, circular, and water-soaked at first, and then
expand up to 1/2 inch in diameter with dark concentric rings within
the spots. Spots coalesce to affect large areas of leaves and cause
defoliation that begins on crown leaves. This disease-causing fungus
overwinters on and in seed, as well as in residue from diseased plants.
Fungus spores (reproductive structures) are spread by wind, by running
and splashing water, on workers, and on tools and implements. Weak
and sensecing plants are more susceptible to Alternaria leaf spot
than are vigorous plants. |
|
|
Anthracnose,
caused by the fungus Colletotrichum lagenarium, affects cucumbers,
muskmelons, and watermelons. Muskmelon and cucumber leaf spots begin
as yellowish or water-soaked areas that enlarge rapidly, turn brown,
and shatter to form a ragged hole within the spot. Leaf spots on watermelons
turn black. Elongated dark spots with light centers often develop
on petioles and stems and can cause death of tissue beyond these spots.
Young fruit may be killed, but large fruit usually develop depressed
dark-bordered cankers with creamy pink colored ooze in the center.
The fungus overwinters in seed and in residue from diseased plants,
and is spread in splashing water. Humid weather and frequent rains
promote disease development and spread. |
|
|
Angular
Leaf Spot, caused by the bacterium Pseudomonas lachrymans,
affects cucumber, squash, and pumpkin and is characterized by conspicuous
leaf symptoms. Stem and fruit symptoms occur, but are less conspicuous.
Leaf spots are angular and irregular in shape and size. spots, water-soaked
at first, later turn gray or tan and finally drop out, leaving ragged
holes. Fruit infections appear as small sunken water-soaked spots;
fruit rot soon follows. Bacteria which cause this disease overwinter
on seed and persist in crop residue from diseased plants. Splashing
rain and workers spread the bacterium within fields. |
|
|
Scab,
caused by the fungus Cladosporium cucumerinum, affects muskmelons
and susceptible cucumber varieties. Dry corky spots, up to 1/2 inch
in diameter develop on cucumber and green muskmelon fruit. Under moist
conditions a dark olive-green velvety growth covers the spot; this
growth distinguishes scab from angular leaf spot on cucumber fruit.
Spots also develop on young terminal stem growth and on petioles.
When spots girdle young stems and petioles, growth beyond the spot
dies. Affected areas on very young leaves are irregular in shape;
the areas dry up. The disease-causing fungus overwinters on seed and
in residue from diseased plants. Disease development is promoted by
moist humid conditions and by cool night temperatures. |
|
|
Pythium
Cottony Leak is caused by Pythium aphanidermatum. The fungus
can cause damping-off or vine cankers during unusually wet seasons,
but fruit rot usually is the most prevalent symptom. The fungus penetrates
fruit wounds and old flower parts, as well as plant parts touching
the soil. Fruit rot begins as a dark green water-soaked area. Wet
mushy rot develops rapidly and may become covered with white cotton
mold during wet weather. The fungus will spread by fruit-to-truit
contact and can cause rapid fruit rot in transid. |
|
|
Belly
Rot is caused by Rhizoctonia solani, a common soil-borne fungus.
The rot develops on the belly (that area of the fruit in contact with
the soil) of the cucumber fruit. Young fruit exhibit a yellowish brown
superficial discoloration which develops into sunken irregular spots
on the underside of the fruit. Large water-soaked decay areas develop
on mature fruit. Belly rot proceeds rapidly at temperatures exceeding
82 ° F; in periods of high humidity, a dense mold growth may develop
on fruit spots. |
|
|
Choanephora
Wet-Rot, caused by the fungus Choanephora cucurbitarum, is
a summer squash fruit rot that affects wilted blossoms and spread
to attached fruit. Infect fruit rot rabidly and fungus mold appears
on therotted area. The fungus growth reselmbes small black-headed
pins stuck into the fruit. The disease-causing fungus is spread by
insects, wind, and splashing water. Disease development is promoted
by high moisture conditions. |
|
|
Fusarium
Fruit Rot on muskmelons usually is caused by Fusarium roseum,
a soil-borne fungus. Usually, ripe fruit are affected. The fruit spots
are from 1/2 to 1 inch in diameter and about 1/2 inch deep. Internal
rotted tissue is whitish to rose-colored, dry and spongy; this rotted
tissue can be removed easily from surrounding unaffected tissue. White
mold develops on the surface of the fruit spot during wet conditions
and during storage. |
|
|
Black
Rot and Gummy Stem Blight apparently are caused by the same
fungus, Didymella bryoniae (also called Mycosphaerella melonis). Different
symptoms develop on different cucurbits. Winter squash, pumpkins and
gourds are affected most by black rot. Black rot starts as irregular
circular fruit spots that are faded green or yellow, later gray to
brown, and finally black. The fungus penetrates the rind and causes
a dry rot. Other rot organisms may follow and cause a wet rot that
affects the entire fruit. Muskmelons, cucumbers, and watermelons are
affected most by gummy stem blight, which begins as pale brown or
gray spots on leaves, petioles, and stems. Stem spots appear first
at the nodes and elongate into stem streaks, a gummy exudate frequently
appears near stem streaks. Leaves on affected vines turn yellow and
die. Entire plants occasionally are killed. Tiny black pycnidia (pimple-like
fungus reproductive structures) develop on fruit, stem, and leaf spots.
The fungus overwinters in seed and residue from diseased plants. |
|
|
Mosaic
Viruses are important on muskmelons, summer squash, and cucumbers.
Three viruses that affect cucurbits are cucumber mosaid virus (CMV),
watermelon mosaic-2 (WM-2) and squash mosaic virus (SqMV). CMV affects
many vegetables and other plants unrelated to cucurbits. WM-2 affects
legumes in addition to cucurbits. Vines on mosaic-infected plants
are stunted, and new leaves are dwarfed, mottled, and sometimes distorted.
On CMV-infected muskmelons and cucumbers, new leaves sometimes wilt
and die; old crown leaves may turn yellow and dry up, resulting in
a slow decline of affected plants. CMV and WM-2 overwinter in some
biennial and perennial plants and usually are carried to new plantings
by aphids. SqMV overwinters and is introduced to new plantings in
infected seed. Within plantings, CMV is spread chiefly by aphids (but
also by cucumber beetles), WM-2 is spread by aphids, and SqMV is spread
by cucumber beetles and workers. |
|
|
Bacteria
Wilt, caused by the bacterium Erwinia tracheiphila, is characterized
initially by wilting and drying of individual leaves which also may
exhibit cucumber beetle injury. Later, leaves on one or more laterals
or entire plants wilt. Wilted parts may appear to recover at night,
but they wilt on successive sunny days and finally die. Two diagnostic
tests for bacterial wilt are as follows:
1) Cut a wilted stem near the crown and squeeze sap from the newly
cut stem, watching for a white exudate from the vascular bundles.
Then touch a clean knife blade to the cut surface and slowly withdraw
the blade from the stem. Watch for a white ooze that strings out in
a fine thread between the newly cut stem surface and the knife.
2) Put two cut ends together and squeeze; then separate the ends and
look for sticky strands. Presence of the white exudate and stringing
out of this ooze is a bositive test for bacterial wilt.
Positive test results appear to be easier to obtain for cucumber and
for some squash than for muskmelon. THis disease is more prevalent
on cucumber and muskmelon than on pumpkin and squash. Wilt-causing
bacteria overwinter in cucumber beetles. The bacteria are carried
to plants when beetles feed. |
|
|
Fusarium
Wilt of Muskmelon, caused by the fungus Fusarium oxysporum
f. sp. melonis, is characterized by stunting, yellowing, wilting,
and dying of vines. A streak, at first water-soaked and later turning
yellow to tan and finally dark brown, often appears at the soil line
on one side of the vine; this symptom is diagnostic. The disease-causing
fungus survives in soil for many years. High soil temperatures favor
disease development |
|
|
Late
Collapse of Muskmelon is characterized by a sudden collapse
of plants. Collapse occurs on bright sunny days late in the season
following heavy rains and cold nights that cause a signifigant drop
is soil temperature. Temperaturs of 60 ° F or lower at the 4-inch
soil debth have been correlated with the collapse. Collapse can be
severe after cool wet weather because i) the soil cools to the critical
level quickly and ii) cool wet weather favors root rotting, an additional
stress that promotes wilting. |
|
|
Fusarium
Wilt of Watermelon, caused by Fusarium oxysporum f. sp. neiveum,
is characterized by seedling damping-off, and by stunting, wilting,
and death of established plants. Wilt symptoms develop in one or more
laterals. usually starting at vine tips, and continue until the plant
dies. Vascular tissue may be discolored, and a white mold may develop
on dead vines. The disease-causing fungus is soil-borne and can persist
in soil for many years. The fungus can be introduced on seed or in
soil that is transproted by equipment, drainage water, or workers.
|
|
|
Root
Knot caused by the Meloidogyne nematode, appears as swelling
on roots. The disease is discussed with tomato diseases. |
|
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}