Advances in the Biology and Management of Monosporascus Vine Decline and Wilt of Melons and Other Cucurbits
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Monosporascus root rot and vine decline (sudden wilt), caused by the soilborne fungus Monosporascus cannonballus, has become one of the most important diseases of melon and watermelon worldwide. The fungus infects the roots early in the growing season, causing severe necrosis and ultimately resulting in a sudden and severe collapse of the vines late in the season. Melon (Cucumis melo) and watermelon (Citrullus lanatus) are the most severely affected, but all cucurbits tested to date can be infected; severe disease, however, is rarely observed except on melon and watermelon. The extensive root systems of the Cucurbita spp. and Lagenaria spp. offer some tolerance to the disease, which makes them suitable to serve as rootstocks. Fruit load, heat, drought, and other stresses may exacerbate disease symptoms and cause a collapse of the vines. Monosporascus cannonballus is somewhat unique as an ascomycete as it produces only one large ascospore per ascus, while two other species in the genus produce more than one. All species of the genus are pseudothermophiles, growing optimally at 25o-30oC, and are presumably native to hot, semiarid climates. Monosporascus spp. have been isolated from roots of numerous plant species, including dicots and monocots, although disease symptoms in plants other than cucurbits are rare. Monosporascus was described as a new genus and species in 1974, but pathogenicity to cucurbits was not established until 1983. Disease development in the field requires warm soil temperatures above 25oC. Ascospores are the overseasoning propagule and germinate in the presence of host root exudates and soil microflora. Initial infection occurs in the fine feeder roots, causing severe necrosis. The stimulation of tyloses in the xylem tissue leads to reduced water uptake and translocation, causing the vines to wilt suddenly. Management of sudden wilt has relied predominantly on preplant soil fumigation with methyl bromide. While effective, this method is not sustainable, given the phase-out of methyl bromide. Sanitation techniques that remove infected roots from the soil immediately after harvest can reduce the inoculum buildup in the soil and disease the following cycle but could be cost prohibitive in some production areas. Timely application of fungicides through the drip irrigation system during the growing season offers a cost-effective alternative to fumigation. Grafting melon or watermelon onto tolerant Cucurbita spp. rootstocks is gaining acceptance; however, there may fruit quality issues in some instances. The development of host plant resistance utilizing exotic C. melo germplasm has been pursued with promising results, but it is a slow process and has focused on just a few commercial melon types. An integrated strategy continues to evolve for control of sudden wilt and includes enhancing plant root system development, timely fungicide application, irrigation and soil management practices, and adoption of improved rootstocks and resistant melon lines. 2012 Wiley-Blackwell. Published 2012 by John Wiley and Sons, Inc.
