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| BACTERIAL
WILT OF CUCURBITS
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Introduction
This vascular wilt disease caused by the bacterium Erwinia tracheiphila
affects only certain members of the cucumber family including cucumber,
squash, muskmelon, pumpkin, and gourd. Watermelon is immune and certain
varieties of cucumber and squash show varying degrees of resistance
to this disease. Bacterial wilt causes losses of 10 to 20 percent
in unsprayed plantings of susceptible cucurbits.
Symptoms
Symptoms of the disease first appear on a single leaf
which suddenly wilts and becomes dull green (Figure 1.). The wilting
symptoms spread up and down the runner sometimes as a recurring wilt
on hot, dry days. Soon infected runners and leaves turn brown and
die. The bacteria spread through the xylem vessels of the infected
runner to the main stem, then to other runners. Eventually the entire
plant shrivels and dies. Less susceptible plants, such as certain
squash varieties, may show dwarfing of growth before the wilt symptoms
become apparent.
Figure 1: Symptoms of yellowing, wilting
and dieback of the foliage of squash. (provided
by the Plant Disease Diagnostic Clinic,
Cornell University) |
Figure 2: Close up of the symptoms
that are capable of infecting roots as well. (provided
by the Plant Disease Diagnostic Clinic,
Cornell University) |
A creamy white bacterial ooze consisting of thousands
of microscopic, rod-shaped bacteria may sometimes be seen in the xylem
vascular bundles of an affected stem if it is cut crosswise near the
ground and squeezed. This bacterial ooze will string out forming fine,
shiny threads (like a spider's web) if a knife blade or finger is
pressed firmly against the cut surface, then slowly drawn away about
1 cm. Two cut stem ends can also be put together, squeezed, then separated
to look for shiny strands of bacteria. The sap of a healthy plant
is watery and will not string. Sometimes it helps to wait several
minutes after cutting to perform the test. This technique is useful
in field diagnosis to separate this disease from other vascular wilts.
Beware, however, that the technique may not always work (i.e., no
bacterial strings occur yet the plant is still infected). The test
works better for cucumbers than for muskmelons.
Fruit may also show symptoms. Small water-soaked patches form on the
surface. These patches eventually turn into shiny decayed spots on
the fruit.
Disease Cycle
Bacterial wilt is an unusual disease in
that the bacterial pathogen can survive the winter only in the digestive
tract of striped cucumber beetles and spotted cucumber beetles. In
spring the overwintered bacteria land on cucurbit leaves within the
fecal droppings of beetles which have begun to feed on the plants.
The bacteria can then infect the plant through wounds produced by
the feeding of the beetles or other chewing insects. Bacteria cannot
infect the plant through normal plant openings (stomates and hydathodes)
nor are they carried on or in seed. The beetles' mouthparts become
contaminated with the bacteria while feeding on infected leaves. In
this manner the beetles carry the bacteria to the next three or four
plants on which they feed. Beetles apparently prefer to feed on plants
with bacterial wilt symptoms. Fortunately, only a small number of
beetles become active carriers of the bacteria and infection can only
take place when there is a film of water on the leaf sufficient for
the bacteria to reach a wound and gain entry into the inner leaf tissue.
Weather conditions have an indirect effect on the disease. Environmental
conditions which favor the ovenwintering, feeding, and reproduction
of the cucumber beetles will affect the prevalence of bacterial wilt.
A year with a good winter snow cover followed by a warm March and
April could be expected to increase the number of beetles and therefore
increase the incidence of bacterial wilt.
Management Strategies
Bacterial wilt may be reduced by controlling the cucumber beetles.
They may be prevented from feeding on plants in the home garden by
enclosing the plants in wire or cheesecloth tents supported by a wooden
or wire framework as soon as they come through the ground in the spring.
The cheesecloth is stretched over the frame and its lower edges weighted
with stones or soil to prevent beetles from crawling underneath. This
tent should be removed as soon as the plant becomes well established.
Larger plantings must be protected by insecticides. Some carbaryl
(Sevin), malathion, or rotenone insecticides or combination products
are registered to treat cucumber beetles in the home garden in New
York State. They will provide control of the beetles if applied when
beetles first appear in the spring. Early control, beginning as soon
as the plants emerge, is most important as a single beetle can introduce
the bacteria. One to four generations of the beetle may occur on unprotected
plants and applications of these insecticides at weekly intervals
may become necessary. Apply a light even coating of the insecticide
over the entire plant, especially where the stem emerges from the
soil (that is where the beetles often congregate).
Last updated, KLS, 8/99
This publication contains pesticide recommendations. Changes in pesticide
regulations occur constantly, some materials mentioned may no longer
be available, and some uses may no longer be legal. All pesticides
distributed, sold, and/or applied in New York State must be registered
with the New York State Department of Environmental Conservation (DEC).
Questions concerning the legality and/or registration status for pesticide
use in New York State should be directed to the appropriate Cornell
Cooperative Extension Specialist or your regional DEC office. READ
THE LABEL BEFORE APPLYING ANY PESTICIDE. __________________________________________________________________________________
The Plant Disease Diagnostic Clinic
at Cornell University is located at 334 Plant Science Building,
Ithaca, NY, 14853. Phone: 607-255-7850, Fax: 607-255-4471,
Email: kls13@cornell.edu
or slj2@cornell.edu
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