Bacterial Spot (Xanthomonas cucurbitae): An Emerging Disease of Pumpkins in the Midwest

During 2009–11, bacterial spot, caused by Xanthomonas cucurbitae, occurred widely in pumpkin fields in the Midwest. Yield losses up to 90% forced some growers to abandon pumpkin production. A study in 2009 showed that the disease occurred in all 17 commercial pumpkin fields surveyed in Illinois. Another Illinois survey in 2010 showed that the disease occurred in 40 of 50 commercial pumpkin fields with bacterial spot symptoms on 34% of fruit. During a survey in 2011, X. cucurbitae infected fruit were observed in 95 of 111 pumpkin fields in the Midwest (Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, Ohio, and Wisconsin). In all surveys, the severity of the disease on fruit (percentage of surface area of the fruit with bacterial lesions) ranged from 1% to 20%. X. cucurbitae was isolated from infected fruit on nutrient agar medium. The identity of the X. cucurbitae isolates was confirmed by conducting Gram staining; colony growth on yeast extract-dextrose –CaCo3 agar (YDC) at 33 °C and morphology on YDC; and oxidative and fermentative (O/F), oxidase, catalase, nitrate reduction, starch hydrolysis, sculin hydrolysis, and motility tests. Further confirmation of the identity of the isolates was achieved by using primers RST2 and RST3. Koch’s postulates for representative isolates of X. cucurbitae were carried out on 'Howden' pumpkin in greenhouse and field studies. Bacterial inoculum was cultured on Laura Broth agar medium and suspension of 10⁸ cfu/ml of sterile-distilled water (SDW) was prepared. Leaves and fruit of ‘Howden’ pumpkin were spray-inoculated. A positive control was inoculating pumpkin leaves and fruit with a known X. cucurbitae isolate. Also, a negative control using SDW was included. The severity of bacterial lesion on the leaves and fruit was assessed 3, 10, and 21 days after inoculation. X. cucurbitae was isolated from all leaves and fruit inoculated with the bacterial isolates. X. cucurbitae infection also was observed and confirmed on all winter squashes grown near pumpkin fields visited.