Transcript Document

Diseases of Cucurbits
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Types of Cucurbits
Those grown in IL
Production facts
Etc.
Diseases Covered
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Watermelon fruit blotch
Bacterial wilt
Anthracnose
Powdery mildew
Production of squash seed
Harvesting for seed
DISEASE: Watermelon Fruit Bloch
CROP: Watermelon, other cucurbits are hosts
PATHOGEN: Acidovorax avenae subsp. Citrulli
DISTRIBUTION: FL, SC, NC, MD, IN, LA
PATHOGEN DESCRIPTION: Bacterium
Gram-negative, straight rod, motile, peritrichous flagella.
Disease Symptoms
Foliage
Infected transplants- dark, water-soaked areas on the underside
of cotyledons and first leaves.
Necrotic lesions may appear on the foliage.
Young seedlings may develop lesions on the hypocotyl, causing
eventual collapse and death.
Disease symptoms on foliage in the growing season may not be
obvious or confused with other diseases.
Symptoms on transplants can go away, and return with
symptoms on fruit!!!
Disease Symptoms
Disease Symptoms and Signs
Fruit
Begin as small, water-soaked areas (few millimeters in
diameter), rapidly expand into larger lesions with
irregular margins.
The entire surface of the fruit may become covered with
dark green, greasy-looking lesions.
Older fruit lesions become necrotic and may crack.
Whitish bacterial ooze may exude from the splits, later
infected fruit will rot.
Disease Symptoms
Whitish ooze
Disease Cycle
Disease Development
Introduced into fields with infested seed, infected
transplants, natural spread via alternate host (wild
cucurbits or volunteer watermelon).
Infected transplants represent most important means of
disease transmission – infected transplants may be
asymptomatic – lead to high numbers entering a field.
Warm, wet weather in May-June favors the bacterium and
disease.
Disease can develop quickly, 100% infection from just a
few primary infection sites.
Control of Fruit Blotch
Prevention
Avoid introduction of bacterium (pathogen-free seed)
Inspection of seedlings and destroy suspicious flats
Decontaminate if contact is made with infected plants
Chemical
Streptomycin (illegal, not labeled) is used in dire
situations in the greenhouse to stop the spread of the
pathogen
Control of Fruit Blotch
In the field:
Culls and plant debris should be plowed under
Rotate to new areas away from contaminated fields
Choose less susceptible varieties
Those with light green rinds = more susceptible
Light and dark green striped= more resistant
Solid dark green varieties are most resistant
Bacterium moved by wind-driven rain or by mechanical means.
Avoid contaminated fields when wet.
Copper-based fungicides can reduce incidence of fruit symptoms.
DISEASE: Bacterial Wilt
CROP: Cucurbits
PATHOGEN: Erwinia tracheiphila
DISTRIBUTION: North America, Europe,
Asia, Africa
PATHOGEN DESCRIPTION: Bacterium
Gram-negative, straight rod, motile, peritrichous
flagella.
Disease Symptoms and Signs
Crops affected are mainly cucumber and cantaloupe, but
also squash and pumpkin to a limited extent. Watermelon
is not affected.
Foliage wilts suddenly, frequently on a single runner at
first, followed by wilting of the entire plant.
Wilt is permanent. Bacteria are abundant in the vascular
tissue and exude in white droplets from vascular bundles
on cut stems.
The viscous bacterial mass will 'string-out' when the cut
ends of the stem are touched together.
Bacterial wilt of cucurbits
Bacterial wilt of cucurbits (pumpkin)
Conditions for Disease
Development:
The pathogen survives for extended periods in
its cucumber beetle vectors. It is transmitted
by the striped beetle, Acalymma sp. and the
spotted beetle, Diabrotica sp.; therefore,
conditions conducive to development of the
vectors favor the occurrence of the disease.
Control Measures:
Rogue diseased plants to prevent secondary
spread of the pathogen.
Control cucumber beetles with insecticides.
Some cultivars are more tolerant than others, if
available they should be used.
DISEASE: Anthracnose
CROP: Cucurbits
PATHOGEN: Colletotrichum lagenarium
(imperfect (asexual) stage): Glomerella
cingulata var. orbiculare (sexual stage)
DISTRIBUTION: Worldwide
Pathogen Description: Fungus
Cylindrical, hyaline conidia arc produced on lesions in
pinkish masses in acervuli also bearing two to three
septate, brown setae.
Pathogen Description: Fungus
Two-celled conidium
Formation of appresorium
Disease Symptoms and Signs
The disease is particularly damaging to watermelon,
cucumber, and cantaloupe, but also may occur on most
other cucurbit crops.
On cucumber and cantaloupe, leaf lesions are circular and
brown up to 1 cm in diameter, while on watermelon the
leaf lesions arc black and somewhat smaller.
Petiole and stem lesions arc elliptical in shape and sunken.
Fruit lesions appear at or near maturity as water-soaked
spots that develop into sunken, circular lesions lined
with dark fungal stroma bearing masses of pink spores.
Anthracnose of cucurbits
cucumber
watermelon
Anthracnose of cucurbits
(watermelon)
Sunken lesions with
pink/salmon colored
sporulation
Disease Cycle
Pathogen persists in crop debris, seedborne and may
survive on volunteer plants or cucurbit weeds
Conidia are the main means of in-field spread and are
dispersed by watersplash and wind blown rain.
Control
Cultural
1. Use commercially produced, disease-free seed.
2. Rotate vine crops with unrelated crops in a three-year rotation.
3. Practice good sanitation by plowing under fruits and vines at the
end of the season.
4. Choose anthracnose-resistant varieties if at all possible. Resistant
cucumber slicers include Dasher II and Slicemaster. Many pickling
cucumbers are tolerant or resistant, including Score and Premier.
Resistant watermelon varieties include Charleston Gray, Crimson
Sweet, and Dixie Lee.
Control
Fungicides
Apply approved fungicides to the crop at regular intervals,
more often if frequent rains occur.
Among fungicides available are chlorothalonil (Bravo),
benomyl (Benlate), and maneb and mancozeb formulations.
An effective spray treatment has been the combination of
Bravo with Benlate or mancozeb. If angular leaf spot should
be a problem, substitute a copper compound for Benlate in
the combination.
Powdery Mildew
Powdery Mildew
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Pathogen: Erysiphe cichoracearum, Sphaerotheca
fuliginea
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Cylindrical, hyaline conidia are produced in chains
on conidiophores that arise from surface mycelial
growth.
Symptoms
and Signs
Signs of the Pathogen
Symptoms and Signs:
All cucurbits are susceptible to powdery mildew
Symptoms appear first as pale yellow spots on
leaves and stems
Sporulation becomes evident as white powdery
masses of conidia are produced over the lesion
surface
Leaves and stems become chlorotic, then turn
brown and dry prematurely.
Conditions for Disease Development
The pathogens are obligate parasites and can persist on
wild cucurbits or crop plants
Disease development can occur over a wide range of
temperatures as long as there sufficient moisture for
spore germination and infection.
These conditions can be provided by high relative
humidity or dew formation in the absence of rainfall.
Inoculum is airborne for long distances
Control Strategies
Avoid crowding of plants
Resistant cultivars of many crops available,
pumpkin is exception
Fungicide sprays are available, but may not be cost
effective
Sanitation and practices that avoid lush growth help
to delay spread