Many areas of the United States produce cucurbits (Cucurbita spp.) for local and regional consumption. In the southeastern United States, weather is often conducive to the development of fungal diseases and subsequent yield loss. Two of the most important fungal diseases of cucurbits are powdery mildew (PM) and downy mildew (DM) (Zitter et al., 1996). These foliar diseases reduce fruit number, fruit quality, and the length of time crops can be harvested. Fungicide application to vegetable crops is generally preventive in nature rather than curative, and many states publish pesticide spray schedules for high-value crops. Thus, most fungicides are applied at a set schedule during some or all stages of a crop's production cycle or are applied based on integrated pest management models that forecast environmental conditions conducive to growth and infection by one or more pathogens. Models have been developed to forecast weather conditions favorable for disease occurrence such as TOM-CAST for tomato early blight (Alternaria solani), anthracnose (Colletotrichum coccodes), and septoria leaf spot (Septoria lycopersici) (Gleason et al., 1995) and cucurbit DM spore movement (Holmes et al., 2006). Some growers schedule sprays using these models and, as a result, often benefit from reduced fungicide applications and increased yield. However, adoption of these models by growers and consultants has been slow. Early application of fungicides has been shown to reduce gummy stem blight [Didymella bryoniae (synonym Mycosphaerella melonis)] in watermelon (Citrullus lanatus) compared with delayed applications, but differences in final spray date did not influence the area under the disease progress curve (Keinath, 2000). In a later paper, the same author reported fungicide scheduling increased marketable watermelon fruit yield and quality when the severity of gummy stem blight was high, but had little influence on yield or quality when severity was moderate or low (Keinath, 2001).
In addition to several common fungicides, several types of less traditional and potentially synergistic products are available. Phosphonic acid, phosphorus acid, or phosphonate fungicides used alone or in combination with traditional fungicides have been evaluated for the control of PM and DM in cucurbits and cole crops (Brassica spp.). Two such products, Biophos (Foliar Nutrients, Westminster, Colo.) and Phostrol (Nufarm Americas, Burr Ridge, Ill.) (Raid, 2005), reduced DM severity in chinese cabbage (Brassica rapa var. pekinensis) when used alone or in combination with mancozeb. Prophyt (Helena Chemical Co., Collierville, Tenn.), another P-based product, reduced the severity of DM of lettuce when used alone or with traditional fungicides (Raid and Meister, 2006). Potassium bicarbonate reduced the amount of PM on the upper leaf surface of muskmelon (Cucumis melo) (Turini et al., 2003). These products were reported to be systemic in plant tissue (Cooke and Little, 2001), but various plant species differ in their ability for uptake and distribution. Phosphonic acid compounds are reported to be effective against oomycete fungi by inhibiting oxidative phosphorylation (McGrath, 2004).
There is a paucity of information about fungicidal activity of other compounds primarily sold for foliar fertilizer use. Foliar nitrogen products, often composed of a slow-release and proprietary mixture of urea, diurea, or methylene urea, have been reported to slightly increase lint yield of cotton (Gossypium hirsutum) (White et al., 1995). It is not known how a foliar N product will affect pumpkin yield or disease severity when combined with K and phosphite products and traditional fungicides in a disease management program.
Other researchers that have compared selected approaches to manage foliar diseases of vine crops have published similar conclusions—that is, fungicide application reduced disease severity compared with no fungicide application. This simple, but basic conclusion was reported regardless of the management approach under investigation—for example, a modeling approach for managing DM of pumpkin (Langston and Garner, 2001), a preventive fungicide spray schedule for controlling PM of squash (Cucurbita pepo) based on crop growth stage (Langston and Kelley, 2001), or an integrated pest management threshold method to schedule preventive applications for PM of pumpkin (McGrath, 2002). Results from such studies have produced varied results about which method of fungicide application would be best suited for managing foliar diseases of cucurbits, but all reported fungicide applications greatly improved vine growth, yield, and fruit quality compared with untreated controls.
Fungicide resistance management is an important aspect of any fungicide program for the control of foliar diseases of cucurbits. Benomyl, triadimefon (McGrath, 2001), and two strobilurin fungicides have been shown to control cucurbit PM inadequately in field research plots (McGrath, 1999). Fungicide-insensitive strains of the PM fungus developed after repeated use of the same product. Thus, any fungicide program to manage foliar diseases of cucurbits should use tank mixes or rotation of multiple products with different modes of action. The objective of this study was to compare low-input (LI) and high-input (HI) management approaches (early or delayed time of first application) with combinations of fungicide products to reduce losses associated with PM and DM on pumpkin.
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