Occurrence of bacterial spot of pumpkin (Xanthomonas cucurbitae) in Illinois, and pathogenic and genetic variations, host range, and seed borne aspect of the pathogen

Pumpkins (Cucurbitae spp.) are grown throughout the world for human food, animal feed, and decoration. Approximately 680 million kg of pumpkins are produced in the United State every year. Illinois is the leading state in pumpkin production. Bacterial spot, caused by Xanthomonas cucurbitae, has become an important threat to pumpkin production in Illinois. This study was conducted to assess occurrence of bacterial spot in Illinois, and pathogenic and genetic variation, host range, and seed transmission of X. cucurbitae. During 2009-2011, development of bacterial spot was monitored in commercial pumpkin fields. Symptoms of the disease were observed on leaves from 4-leaf growth stage until harvest. Three types of lesions were observed on leaves, which were described as Type I (1 mm, brown necrotic lesion), Type II (4 to 8 mm, translucent lesion), and Type III (1 to 4 mm, brown necrotic) lesions. Lesions on fruit were 0.5 to 1 mm in diameter, sunken with a beige center and a yellow halo. To determine incidence of bacterial spot of fruit, 17, 50, and 65 commercial fields were surveyed in 2009, 2010, and 2011, respectively. The symptomatic pumpkins were observed in 100, 80, and 88% of the fields in 2009, 2010, and 2011, respectively. A total of 1,324 bacterial isolates were collected from affected leaves and fruit, of which 964 (73%) were identified as X. cucurbitae. Sixty two isolates of X. cucurbitae were tested for pathogenic variation, and significant (P = 0.001) difference was found in virulence among the isolates. Genetic variation among 75 X. cucurbitae isolates was studied, which resulted in grouping the isolates in four distinct clusters. Fifty plant spices in the family of Amaranthaceae, Brassicaceae, Chenopodiaceae, Cucurbitaceae, Gramineae, Leguminosae, Liliaceae, Malvaceae, Purtulaceae, Solanaceae, and Umbelliferae were tested in a greenhouse for infection by X. cucurbitae. All of the plant species in Cucurbitaceae family developed symptoms iii and X. cucurbitae was isolated from the lesions. None of the species from other plant families developed symptoms. In another study, 89 USDA accessions of pumpkin and squashes (Cucurbitae pepo) were tested for their susceptibility to X. cucurbitae in a greenhouse. Bacterial lesions developed on leaves of all tested accessions, but there was significant (P = 0.001) difference in symptom severity on leaves among the accessions. In 2012, nine of the USDA accessions and 17 commercially grown cultivars of pumpkin were tested for their susceptibility to X. cucurbitae in a field. Severity of bacterial spot on leaves was significantly (P = 0.001) different among the accessions and commercial cultivars. Transmission of X. cucurbitae in pumpkin seeds was tested in the laboratory and greenhouse. X. cucurbitae was isolated from 12 of 20 seed samples collected from pumpkin fields. Also, X. cucurbitae was detected in kernels of seeds that were surface-disinfested with NaClO. In a greenhouse study, lesions developed on cotyledons of the seedlings grown from naturally-infected pumpkin seeds and X. cucurbitae was isolated from symptomatic leaves