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Diseases of Cucurbits and Their Control with Genetic Resistance Thomas A. Zitter Department of Plant Pathology Cornell University Ithaca, NY 14853 March 2002   Use of disease resistant varieties for disease control is one of the most effective tools available to home gardeners and commercial growers alike to produce healthy vegetables with extended shelf life, and limit the use of pesticides. Plant Breeders have made great strides in incorporating multiple disease resistance into many crops, including cucurbits. As shown in Table 2, cucumber has the highest level of disease resistance; however, other cucurbits (zucchini and yellow squash, muskmelon, pumpkin and winter squash) also have some resistance to viruses, bacteria and selected fungi.

Cucurbit Viral Diseases

Since we do not feel that seed transmission plays a role in the spread of viruses infecting cucurbits in New York, we need to look elsewhere for the virus source. In addition to infecting a diverse group of horticultural crops (Table 1), vegetable viruses commonly infect many important weeds, which serve as the principle reservoirs for these viruses. A complete listing of weeds as natural reservoirs for plant viruses in the Northeast can be found at http://VegetableMDOnline.ppath.cornell.edu/Tables/WeedHostTable.html. For some weeds, color pictures have been included to aid in their identification

Table 1. Viruses reported to infect cucurbit crops grown in New York.

Virus key: AMV = alfalfa mosaic virus; BBWV = broad bean wilt virus; BCMV = bean common mosaic virus; BYMV = bean yellow mosaic virus; ClYVV = clover yellow vein virus; CMV = cucumber mosaic virus; PRSV = papaya ringspot virus; PVY = potato virus Y; TSWV = tomato spotted wilt virus; TurMV = turnip mosaic virus; WMV = watermelon mosaic virus; ZYMV = zucchini yellow mosaic virus. UNDERLINED = most consistently recovered from infected plants in New York.

Cucurbit Bacterial Diseases

The three prominent bacterial diseases of cucurbits are bacterial wilt (Erwinia tracheiphila), angular leaf spot (Pseudomonas syringae pv. lachrymans), and bacterial leaf spot (Xanthomonas campestris pv. cucurbitae). By far the most important is bacterial wilt because the bacteria are readily vectored by the massing of striped and spotted cucumber beetles, often in the early stages of seedling development. Control depends upon the control of the beetles, and use of imidacloprid (Admire) has been shown to be an effective insecticide treatment. Beetles preference is displayed among cucurbit species. Consult the latest Vegetable Guidelines for a list of some squash and pumpkin varieties that are arranged in order of increasing preference. Unfortunately, some commonly grown cucurbit varieties are quite susceptible to bacterial wilt, and insect control should be considered; pumpkin (Merlin and Magic Lantern); Burgess buttercup squash; Turk’s Turban gourd are particularly vulnerable. In the case of angular and bacterial leaf spot diseases, copper fungicides are relied upon to reduce the spread of both. These particular pathogens are known to be seed transmitted in cucurbits.

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Cucurbit Fungal Diseases

Powdery mildew (PM), (Sphaerotheca fuliginea), is annually the most important fungal disease affecting most cucurbits (see Table 2 for varieties resistant to PM). A few varieties are available with partial tolerance to PM, but commercially most growers rely upon the use of foliar fungicides. It is critical when using synthetic fungicides (Quadris, Benlate [last year of usage in 2002 for NY], Nova, Topsin M, and Flint) that these products should be alternated with broad-spectrum fungicides like Bravo (Equus, etc.), maneb/mancozeb, sulfur or other products to reduce the risk of fungicide resistance from developing during the growing season. The opportunity for developing resistance increases with pathogens that have repeating disease cycles (polycyclic), and applies for powder mildew, gummy stem blight/black rot, and others.

Diseases that are particularly important under periods of frequent rainfall, heavy dews, standing water, and high humidity include gummy stem blight (Didymella bryoniae), scab (Cladosporium cucumerinum), and Phytophthora blight (Phytophthora capsici). Fungicides are necessary for the control of gummy stem blight and scab, and are used in conjunction with materials needed for control of powdery mildew. Phytophthora blight is far more difficult to control, and growers must rely essentially on cultural practices that minimize the accumulation of excessive soil moisture. Procedures such as subsoiling, raised beds, well-drained fields, along with a minimum rotation of 3 years with crops other than peppers, eggplants, tomatoes, and cucurbits are required. Contamination of pond water with Phytophthora spores is a distinct possibility, and only adds to the difficulty of maintaining farms free of Phytophthora.

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Table 2. Cucumber, Summer Squash (Zucchini and Yellow), and Cantaloupes (Eastern muskmelon) with Disease Resistance. (based upon available sources) TAZ 2002.

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Key — General: T = tolerant; GMO = transgenic plant. Viruses: CMV = cucumber mosaic virus; WMV = watermelon mosaic virus (formerly WMV-2); PRSV = papaya ringspot virus (formerly WMV-1); ZYMV = zucchini yellow mosaic virus. Fungi: A = anthracnose (race noted); DM = downy mildew; F = Fusarium wilt of melon (race noted); PM = powdery mildew (race noted). Bacteria: ALS = angular leaf spot. Seed Breeder/Source: CU = Cornell University; HM = Harris Moran; H = Harris, J = Johnny’s; R = Rupp; Se = Seedway; Si = Siegers; St = Stokes.

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