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!!!
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 and Signs
Disease Symptoms Whitish ooze
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 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.
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
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. Disease Symptoms and Signs
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.
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.
Pathogen: Erysiphe cichoracearum, Sphaerotheca fuliginea Cylindrical, hyaline conidia are produced in chains on conidiophores that arise from surface mycelial growth.
Symptoms and Signs
Signs of the Pathogen
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. Symptoms and Signs:
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