Diseases of Cucurbits

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Diseases of Cucurbits

• Types of Cucurbits
• Those grown in IL
• Production facts
• Etc.

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Diseases Covered

• Watermelon fruit blotch
• Bacterial wilt
• Anthracnose
• Powdery mildew

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Production of squash seed
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Harvesting for seed
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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.

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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!!!

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Disease Symptoms
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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.

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Disease Symptoms
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Disease Cycle
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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.

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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 Str
eptomycin (illegal, not labeled) is used in dire
situations in the greenhouse to stop the spread
of the pathogen
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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.
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DISEASE Bacterial Wilt

• CROP Cucurbits
• PATHOGEN Erwinia tracheiphila
• DISTRIBUTION North America, Europe, Asia, Africa
• PATHOGEN DESCRIPTION Bacterium
• Gram-negative, straight rod, motile, peritrichous
  flagella.

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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.

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Bacterial wilt of cucurbits
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Bacterial wilt of cucurbits (pumpkin)
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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.

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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.

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DISEASE Anthracnose

• CROP Cucurbits
• PATHOGEN Colletotrichum lagenarium (imperfect
  (asexual) stage) Glomerella cingulata var.
  orbiculare (sexual stage)
• DISTRIBUTION Worldwide

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Pathogen Description Fungus

• Cylindrical, hyaline conidia arc produced on
  lesions in pinkish masses in acervuli also
  bearing two to three septate, brown setae.

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Pathogen Description Fungus
Formation of appresorium
Two-celled conidium
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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.

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Anthracnose of cucurbits
cucumber
watermelon
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Anthracnose of cucurbits (watermelon)
Sunken lesions with pink/salmon colored
sporulation
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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.
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Control

• Cultural
• 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.

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Control
FungicidesApply 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.
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Powdery Mildew
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Powdery Mildew

• Pathogen Erysiphe cichoracearum, Sphaerotheca
  fuliginea
• Cylindrical, hyaline conidia are produced in
  chains on conidiophores that arise from surface
  mycelial growth.

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Symptoms and Signs
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Signs of the Pathogen
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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.

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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

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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