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Developing a Management Program for Powdery Mildew in Winter Squashes with Resistant Varieties 2003

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Project Leader:  Meg McGrath, Dept. of Plant Pathology, Riverhead

Cooperators:

Ted Blomgren, Vegetable Specialist, Capital District Vegetable Program

Alan Erb, Vegetable Specialist, Lake Plains Vegetable Program

Julie Kikkert, Vegetable Specialist, CCE Ontario-Wayne-Yates Counties

Joseph Sieczka, Dept. of Fruit & Vegetable Science, Ithaca

Dale Moyer, Vegetable/Potato Specialist, CCE Suffolk County

George Moriarty, Dept. of Plant Breeding, Ithaca

Molly Kyle Jahn, Dept. of Plant Breeding, Ithaca

Bill Johnson, plant breeder, Seminis Vegetable Seeds, Inc., California

Type of grant: Pest-resistant crops; allelopaths

Project location: National application

Abstract:

Powdery mildew is an important disease of winter squash and other cucurbit crops. It occurs throughout New York every year. Management is usually needed to avoid a reduction in yield. Application of fungicides has been the main practice. Several winter squash varieties with resistance to powdery mildew are now commercially available. The source of this gene for resistance is CornellÕs Department of Plant Breeding. Growers need to know how well these resistant varieties perform compared to horticulturally-similar, fungicide-treated susceptible varieties, and whether there are benefits to an integrated program with reduced fungicide inputs (fungicides applied every 14 days compared to the standard 7-day interval). Two experiments were conducted to evaluate acorn and butternut squashes. Growing varieties with resistance to powdery mildew was shown to be an effective and economic means to manage powdery mildew. Powdery mildew was managed better with chemical control than with heterozygous-based genetic control (resistance from one parent): fungicide-treated susceptible Taybelle had less powdery mildew than non-treated resistant Taybelle PM. However, homozygous-based genetic control (resistance from both parents) was more effective than chemical control: non-treated resistant Autumn Delight and resistant Bugle had less powdery mildew than fungicide-treated susceptible Table Ace and susceptible Waltham, respectively. Efficacy of chemical control, however, was likely compromised by the presence of pathogen strains that were resistance to the two main groups of systemic fungicides used. Resistance to QoI fungicides was very common when assessed on 31 Aug, one month after fungicide applications were started on the susceptible varieties, and most of the pathogen population also was moderately insensitive to DMI fungicides. Control of powdery mildew was improved significantly by applying fungicides to the resistant acorn squash varieties but not to the resistant butternut variety. Regardless of disease control benefit, an integrated program is recommended to reduce selection pressure for new races of the pathogen able to overcome the resistance in these varieties and for new strains of the pathogen that are able to tolerate the fungicides. A reduced-sprays fungicide program with a 14-day spray interval was as effective as the conventional program with a 7-day interval when applied to resistant varieties but not when applied to susceptible varieties. The greater cost of seed of Bugle, $48.50/lb versus $15.15/lb for Waltham, is offset by the additional fungicide applications needed to affectively control powdery mildew in Waltham. It will cost about $16 more to grow an acre of Waltham sprayed seven times than an acre of Bugle sprayed thrice. Although seed of Taybelle PM and Autumn Delight is priced slightly higher than seed of Taybelle and Table Ace, overall production costs are lower because of the cost difference between a 7- and 14-day fungicide program and because fungicide treatment is needed earlier in susceptible varieties. It will cost about $109 less to grow an acre of Autumn Delight sprayed thrice compared to Table Ace sprayed seven times and $131 less for Taybelle PM compared to Taybelle. The resistant varieties evaluated are commercially available, thus growers can implement an integrated program (fungicides applied on a reduced schedule) for managing powdery mildew now.

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