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BACTERIAL DISEASES

ANGULAR LEAF SPOT
BACTERIAL FRUIT BLOTCH
BACTERIAL FRUIT ROTS
BACTERIAL LEAF SPOT OF CUCURBITS
BACTERIAL WILT

ANGULAR LEAF SPOT

CAUSAL AGENT:
Pseudomonas syringae pv. lachrymans
DISTRIBUTION:
Worldwide
SYMPTOMS:
This disease can occur on most cucurbits but is of greatest importance on cucumbers.
Foliar symptoms initially appear as small, water-soaked areas on the underside of the
leaf, which develop an angular appearance due to restriction by the small leaf veins.
Under humid conditions, a milky exudatte may appear from the water-soaked areas
on the lower leaf surface. As this exudate dries, a white crust is left behind. Leaf spots
turn brown and may develop yellow haloes. The centers of the spots may eventually
disintegrate, giving leaves a tattered appearance. Infection on stems, petioles and
fruit first appears as water-soaked spots, which may also produce the milky exudate
under humid conditions and corresponding white crust upon drying. Infection of young
fruit may result in deformation at maturity. Secondary soft rots often develop on
infected fruit.


CONDITIONS FOR DISEASE DEVELOPMENT:
This disease can originate with infested seed, infected transplants, or in the field from
infested crop residue or infected volunteer plants. Infection occurs through stomata,
hydathodes and wounds. On sandy soils, wind-blown sand can be particularly conducive
to infection by abrasion of plant tissues. Humid conditions favor disease development.
The bacterium can be spread from plant to plant by splashing water, insects, farm
equipment and workers. Moisture on the leaves is especially conducive to spread by
equipment and when workers contact plants.
CONTROL:
Rotate out of cucurbits for at least two years. Avoid overhead irrigation and entry into
the field when foliage is wet. Copper-based sprays may help limit spread. Resistant
varieties of cucumber are available.

BACTERIAL FRUIT BLOTCH

CAUSAL AGENT:
Acidovorax citrulli (synonym = Acidovorax avenae subsp. citrulli )
DISTRIBUTION:
Worldwide
SYMPTOMS:
Watermelon: The disease may first appear in the nursery on cotyledons as irregularly
shaped water-soaked tissue which progresses to brown-black lesions. On young
expanding true leaves, small discrete brown lesions may develop along leaf veins.
Additional symptoms on seedlings may include chlorosis, pin-point lesions, veinal or
interveinal necrosis and damping-off. In the field, lesions that develop along leaf veins
eventually dry and may turn reddish-brown to black. Watermelon fruit symptoms first
appear as dark, gray-green, water-soaked lesions or blotches on rind surfaces not in
contact with the soil. Blotches that develop on fruit tissue in contact with soil are most
often associated with fungal infection. As the disease progresses, infected areas on the
fruit rinds may rupture or crack.

Atypical bacterial fruit blotch symptoms have been observed on fruits of watermelon
grown for edible seed in dry, cool climates. Lesions initially appear on the epidermis
as small, pinpoint-sized necrotic spots. As lesions enlarge, brownish-black, star-shaped
cracks form in the centers. While light green chlorotic haloes may surround lesions,
water-soaking is typically not observed. Beneath the external lesions, flesh of the fruit
often disintegrates into dry, firm rotten cavities. At advanced stages, fruit may become
misshapen and deformed .


Melon: Cotyledon and leaf lesions on melon are tan-brown in color. Necrosis usually
develops sooner and is more prevalent in melon compared to watermelon. Symptoms
vary with fruit type. Lesions on smooth-skinned fruit can range from pinpoint spots
to small raised or sunken circular areas. Net formation may be disrupted and
water-soaking may occur around sunken lesions. While lesions do not necessarily
expand externally on the rind, lesions initiating from the fruit surface often expand
internally to a conical shape. Secondary fruit rot may develop from internal lesions.
Additional fruit symptoms for all melon types may include epidermal cracks and
scab-like lesions.
Squash/Pumpkin: Symptoms on cotyledons range from water-soaking to dry necrotic
lesions. Damping-off of seedlings may also occur. Pumpkin foliar symptoms may include
extensive chlorosis as well as elongated tan lesions along the leaf veins. Shot-holing of
leaves is also commonly observed. Fruit symptoms on pumpkin are similar to those
found on melon and include water-soaked areas, cracks in the rind and internal rotting
of the fruit.
CONDITIONS FOR DISEASE DEVELOPMENT:
Acidovorax citrulli is a seed-borne and seed-transmitted pathogen. Contaminated seed
or infected transplants are often the primary source of inoculum leading to outbreaks.
Volunteer plants and wild cucurbits species such as citron can also serve as inoculum
sources. Acidovorax citrulli does not survive for long periods in soil in the absence of
host tissue. Infection and disease development are favored by high relative humidity,
heavy dew formation or rainfall, combined with warm temperatures. The bacterium is
spread by splashing rain, irrigation water, people and equipment. Fruit may be infected
through stomata early in development. Infection occurs before formation of the waxy
layer in watermelon fruit. Hence, unwounded mature fruit are not considered to be
susceptible to infection, although abrasions and other wounds may allow entry of the
pathogen leading to fruit infection. Acidovorax citrulli is not known to move systemically
within the plant. Foliar symptoms can often be mistaken for symptoms caused by other
cucurbit pathogens (e.g., Didymella bryoniae ).
CONTROL:
Use seed that has tested negative for the presence of Acidovorax citrulli using
a validated seed health testing method. Incorporate crop residues to accelerate
breakdown of debris and rogue volunteer seedlings. Rotate out of cucurbits for a
minimum of three years and implement a sanitation program for cultivation equipment
and field crews. Applications of copper-based products to transplants and throughout
the growing season can help minimize disease outbreaks and spread.

 

BACTERIAL FRUIT ROTS

CAUSAL AGENTS:
Soft Rot: Pectobacterium carotovorum subsp. carotovorum (synonym = Erwinia
carotovora subsp. carotovora ), Pseudomonas spp. and several other bacteria.
Brown Spot: Pantoea ananas (synonym = Erwinia ananas )
DISTRIBUTION:
Worldwide
SYMPTOMS:
Soft rot manifests as a water-soaked area of the fruit, developing very quickly to
complete softening and tissue collapse.
Brown spot has been reported on cantaloupe and honeydew melon types. Lesions
are typically smooth, firm and yellow-brown in color. Lesions may extend one to two
millimeters into the epidermis, sometimes entering the cavity of the fruit. Symptoms
are less conspicuous on netted types.


CONDITIONS FOR DISEASE DEVELOPMENT:
Soft rot occurs most commonly under hot and wet or humid conditions. Other
diseases or disorders (e.g. angular leaf spot, anthracnose, blossom-end rot) can
predispose fruit to soft rot bacteria. Wounds created during harvest or packing
can also be sites for soft rot to develop.
Brown spot develops under similar conditions as bacterial soft rot.
CONTROL:
Avoid bruising, puncturing and other mechanical damage to fruit during harvest
and packing. The use of chlorinated fruit dips or sprays in packing houses has been
shown to lower incidence of soft rot. Store fruits at an appropriate temperature and
relative humidity to prevent condensation on fruit surfaces.

 

BACTERIAL LEAF SPOT OF CUCURBITS

CAUSAL AGENT:
Xanthomonas cucurbitae
DISTRIBUTION:
Worldwide
SYMPTOMS:
Symptoms initially appear on the underside of the leaf as water-soaked lesions, which
are mostly angular in shape but may be somewhat rounded. Leaf veins do not appear
to define the lesion shape in all cases. Yellowish spots form on the upper surface of the
leaf. These spots eventually turn brown or become translucent, retaining a distinct
yellow halo. Foliar symptoms may resemble those of angular leaf spot (Pseudomonas
syringae pv. lachrymans ). Leaf spots caused by Xanthomonas cucurbitae are initially
smaller than those caused by the angular leaf spot pathogen, but may resemble angular
leaf spot more as they coalesce. The appearance and size of lesions on fruit can vary
with rind maturity and the amount of moisture present. Symptoms generally initiate as
small, rounded, slightly depressed lesions with tan centers surrounded by dark haloes
(somewhat scab-like in appearance). Lesions may become sunken as they progress,
resulting in cracking of the rind and fruit rot in the field or in storage.
CONDITIONS FOR DISEASE DEVELOPMENT:
Xanthomonas cucurbitae is known to be associated with seed and can overwinter
in crop debris. Infection is favored by high temperatures [25–30ºC (77–86ºF )] and
high relative humidity. Occurrence is common following heavy rains, dew or
overhead irrigation.
CONTROL:
Avoid overhead irrigation and entry into the field when foliage is wet. Copper-based
sprays applied prior to infection may help limit spread. Destroy infected crop debris by either plowing into the soil or burning. Rotate out of cucurbits for at least two years.

 

BACTERIAL WILT

CAUSAL AGENT:
Erwinia tracheiphila
VECTORS:
Acalymma vittatum (striped cucumber beetle)
Diabrotica undecimpunctata howardi (spotted cucumber beetle)
DISTRIBUTION:
North America, Asia, Africa, Europe
SYMPTOMS:
This disease is severe on cucumber and melon, but is less damaging to squash
and watermelon. Symptoms begin with wilting, which may be confined to individual
runners or may involve the entire plant. Plants may wilt at any growth stage, but
wilting is often most severe during periods of rapid growth. Affected leaves display
marginal chlorosis and necrosis. In time, the entire plant becomes necrotic and
dies. A diagnostic procedure for identifying this disease in the field is to make a cut
through a symptomatic stem, rejoin the ends and slowly pull the pieces apart. In
infected plants, bacteria from the vascular tissue will cohere as filamentous strands
between the two pieces.
CONDITIONS FOR DISEASE DEVELOPMENT:
Erwinia tracheiphila is transmitted by cucumber beetles. Environmental conditions
have little effect on incidence and spread of the disease, but can influence symptom
expression. The bacterium is short-lived in dried plant debris, and does not typically
survive in crop debris from one season to the next. Weeds and volunteer cucurbits
serve as alternate hosts and facilitate survival between crops.
CONTROL:
Control cucumber beetles which vector Erwinia tracheiphila to help control this
disease. Eliminate all weeds and volunteer cucurbits. Remove and destroy infected
plants as soon as they are identified. Rotate out of cucurbits for 2-3 years.

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