Image credit: Andy Lassiter, Clemson University. Asexual fruiting bodies called pycnidia of the gummy stem blight fungus Stagonosporopsis citrulli. Fruiting bodies are rare on muskmelon fruit but may be present in large lesions on watermelon fruit.įigure 7. Leaf hairs, however, are two to three times as wide as fruiting bodies. Dead leaf hairs in lesions may be mistaken for fruiting bodies. Under high relative humidity, a mass of conidia may ooze out the opening in the center of the pycnidium (figure 7). More commonly, tan to brown asexual fruiting bodies (called pycnidia) with conidia are present when gummy stem blight is observed during the growing season. At 50 to 80 × magnification under a dissecting microscope, hyaline (clear) sterile hairs may be seen protruding from the fruiting bodies under high relative humidity. Gummy stem blight can be confirmed if dark brown to black sexual fruiting bodies (called pseudothecia) are present with asci or ascospores inside them. 3 On leaves, fruiting bodies form first in the center of spots (figure 6). Signsįruiting bodies form on all parts of diseased cucurbit plants, although fruiting bodies are more common on leaves and crowns than on fruits. Signs of the pathogen described below can be used to distinguish black rot from blossom end rot. Black rot lesions tend to be larger and moister than blossom end rot lesions. It commonly starts at the blossom end of the fruit, so it may be mistaken for blossom end rot since both rots are dark, leathery, and slightly sunken. The flesh becomes watery and discolored (figure 5). External symptoms on melon are not diagnostic. The rot normally starts at the stem end or in cracks in the fruit. Black rot also is more common on western-type cantaloupes, cultivars that produce lightly sutured, heavily netted fruit with firm ripe flesh, than on eastern-type muskmelons, cultivars that produce deeply sutured, lightly netted fruit with soft ripe flesh. Melon fruit are more likely to have black rot than watermelon fruit. Image credit: Virginia DuBose, Clemson University. 2 The centers are more likely to drop out of leaf lesions due to anthracnose than gummy stem blight (figure 4).įigure 5. Anthracnose leaf spots are angular, irregular, or star-shaped, while leaf spots of gummy stem blight generally are round. On watermelon, gummy stem blight can be distinguished from anthracnose by the larger leaf spots, larger lesions that completely encircle petioles, and the presence of crown cankers, which do not form with anthracnose. Although this symptom gives gummy stem blight its common name, this symptom is not diagnostic stem injuries and Fusarium wilt also cause gumming. Drops of gummy, amber plant sap may appear on cankers (figure 3). On melons, young cankers start as water-soaked, green lesions, then become dry, rough, and cracked. Cankers are light brown, beige, or off-white. Image credit: Virginia DuBose, Clemson University.Ĭankers on crowns, main stems, or vines are more common on melons ( Cucumis melo varieties) than on watermelon ( Citrullus lanatus). Leaf blight phase of gummy stem blight on Athena muskmelon not sprayed with fungicides. Lesions on petioles may cause the entire leaf to collapse.įigure 2. As leaf spots expand, they merge, which leads to leaf blighting (figure 2). Leaf spots sometimes are found along the midvein. More than half the leaf spots start at the margins or touch the margins of leaves (figure 1). Leaf spots may have dark and light brown rings. The centers of spots often are lighter brown than the surrounding portions. 1 Leaf lesions are round or triangular, particularly those at leaf edges, brown on melons and dark brown on watermelon. Gummy stem blight affects all above-ground parts of diseased cucurbit plants, including leaves, petioles (leaf stems), vines, crowns, tendrils, pedicels (flower stalks), and peduncles (fruit stems). However, gummy stem blight starts as distinct dark spots. The common name “blight” indicates the disease often affects the entire plant. Gummy stem blight appears in three distinct phases: leaf spots, crown cankers, and fruit rots. Growers and Extension agents should focus management and education efforts on crop rotation and careful fungicide selection to effectively manage this disease. How severe the disease becomes depends on rainfall and dew periods. Gummy stem blight is one of the most common foliar diseases on watermelon, muskmelon, honeydew, and other specialty melons in the southeastern United States.
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