First Report of Xanthomonas cucurbitae Causing Bacterial Leaf Spot of Pumpkin on Réunion Island
Authors: Pruvost O. , Robène-Soustrade I. , Ah-You N. , Jouen E. , Boyer C. , Waller F. , Hostachy B. 
Published: October 2008

Bacterial leaf spot of cucurbits caused by Xanthomonas cucurbitae (3) can be of economic importance in tropical and subtropical production areas, most often within the Cucumis, Cucurbita, and Citrullus genera. The bacterium induces angular, water-soaked leaf spots, which sometimes turn necrotic with a chlorotic halo. Scab-like lesions on fruit may also be observed (1). During 2000, water-soaked, angular leaf lesions were collected from pumpkin (Cucurbita pepo) in a production field located at Petit Serré, Réunion Island. Yellow-pigmented Xanthomonas-like bacterial colonies were isolated on yeast peptone glucose agar. Amplified fragment length polymorphism analysis was performed on four pumpkin isolates together with reference strains of X. cucurbitae (LMG 690 [type strain] and LMG 8663) and the type strain of all other valid Xanthomonas species using SacI/MspI and four primer pairs (unlabeled MspI + 1 [A, C, T, or G] primers and 5′-labeled – SacI + C primer for the selective amplification step) (N. Ah-You, L. Gagnevin, P. A. D. Grimont, S. Brisse, X. Nesme, F. Chiroleu, L. Bui Thi Ngoc, E. Jouen, P. Lefeuvre, C. Verniére, and O. Pruvost, personal communication). The four isolates from pumpkin showed identical fingerprints and were most closely related to X. cucurbitae, with evolutionary genome divergences ≤0.05 (N. Ah-You et al., personal communication). One strain from diseased pumpkin (JW210-1) was further analyzed by multilocus sequence analysis using three housekeeping gene portions (atpD, dnaK, and gyrB) as described previously (N. Ah-You et al., personal communication). Although not fully identical, this strain displayed a similarity of >99% to the two reference strains of X. cucurbitae. Pumpkin and bottle-gourd (C. maxima), squash cv. aurore (C. pepo), cucumber cv. L-04 (Cucumis sativus), melon cv. cezanne (Cucumis melo), and watermelon cv. fou-nan (Citrullus lanatus) leaves were infiltrated (10 inoculation sites per leaf; three replicates) with bacterial suspensions prepared from strains JW210-1, LMG 690, and LMG 8663 and containing approximately 1 × 105 CFU ml–1. Negative controls consisted of leaves infiltrated with sterile tris buffer. Typical, water-soaked lesions that developed into necrotic spots were observed 6 to 8 days after inoculation for all inoculated plant species-strain combinations, but not for negative controls. One month after inoculation, mean Xanthomonas population sizes recovered from leaf lesions on KC semiselective medium (2) ranged from 1 × 107 to 1 × 108 CFU per lesion, typical of a compatible interaction. The reported outbreak was circumscribed to a single field and did not affect the local industry. No major outbreak of bacterial leaf spot of cucurbits has been reported on Réunion Island since 2000 on any host species of X. cucurbitae.

References: (1) J. F. Bradbury. Page 309 in: Guide to Plant Pathogenic Bacteria. CAB International, Slough, UK, 1986. (2) O. Pruvost et al. J. Appl. Microbiol. 99:803, 2005. (3) L. Vauterin et al. Int. J. Syst. Bacteriol. 45:472, 1995.