Acidovorax citrulli (synonym = Acidovorax avenae subsp. citrulli )
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 (see images on page 12).
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 ).
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.