Phytophthora Blight Caused by Phytophthora capsici on Pumpkin and Winter Squash in Texas.

Abstract

Phytophthora blight of pumpkin and squash (Cucurbita spp.) has increased in importance in many production areas of the United States in recent years. This disease was seen on a 36-ha commercial field of several cultivars of pumpkin and winter squash (including Cucurbita maxima cvs. Prizewinner, Atlantic Giant, and Casper and C. pepo cvs. Howden, Festival, Sweet Dumpling, and Magic Lantern) in Yoakum County in the High Plains of Texas during August of 2006. At that time, 2% of the field was affected. A month later, after unusually frequent rain showers and higher-than-average precipitation, 78% of the field was lost in spite of an aggressive fungicide program following the initial diagnosis. Symptoms consisted of white, velvety growth on fruit, fruit rot, leaf blight, and wilt. Sporangia on fruit were papillate, ovoid to ellipsoid, and measured 37 to 40 × 21 to 23 μm. Isolations were made from tissue onto water agar and two isolates used for further study were maintained on corn meal agar. When isolates were paired on V8 agar with two Phytophthora capsici isolates from pepper (3) of opposite mating type, amphigynous antheridia and plerotic oospores 25 to 27 μm in diameter were produced. The morphological characteristics of the isolates were consistent with P. capsici (1). Four-week-old plants of C. maxima cv. Cinderella and C. pepo cvs. Festival and Sweet Dumpling growing in Metro-Mix 366 were inoculated with each isolate by placing 2 cm3 of a 6-week-old culture from lima bean agar (0.2% agar) adjacent to the base of each stem. The soil was then watered to saturation. There were four plants per 2.8-liter pot and two pots per isolate-cultivar combination. Plants were grown in the greenhouse at 18 to 27°C. The test was repeated once. Wilting and stem collapse were seen on inoculated plants 2 to 7 days later, sometimes with production of sporangia on stems. The pathogen was consistently reisolated from symptomatic plants, fulfilling Koch's postulates. These isolates were also pathogenic to pepper (Capsicum annuum). The affected field had been planted to pumpkins and winter squash the previous year when the grower saw similar symptoms on a few plants. In 2004, this field was planted to wheat. There are 1,350 ha of pumpkins produced in Texas, mostly in High Plains counties, and 730-ha squash produced throughout the state. To my knowledge, this is the first report of Phytophthora blight on pumpkin and winter squash in Texas. Phytophthora blight has been a long-standing problem of peppers grown in the Lower Rio Grande Valley of Texas (2), which is 900 km from Yoakum County.