Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 26, 2011
Publication Date: September 7, 2011
Citation: Kousik, C.S., Donahoo, R.S., Webster, C.G., Turechek, W., Adkins, S.T., Roberts, P.D. 2011. Outbreak of powdery mildew on watermelon fruit caused by podosphera xanthii in southwest Florida. Plant Disease. 95:1586. Interpretive Summary: Watermelon is an important crop grown in forty-four states in the US. Every year, Florida grows about 26000 acres of watermelons, valued at over $112 million. In late 2010 (November-December), we observed that the skins of many watermelons from several farms in southwest Florida were severely afflicted by an unknown disease. Surprisingly, tests showed that this disease was powdery mildew, which normally only infects leaves. This disease can lead to unsightly and thus unmarketable fruit, resulting in a loss of revenue for watermelon growers. Identification of the disease can be considered a first step towards conducting further studies to determine why the fruit were affected, and how this type of powdery mildew can be managed.
Technical Abstract: Powdery mildew caused by the obligate parasite Podosphera xanthii occurs commonly on foliage, petioles and stems of most cucurbit vegetable crops grown in the U.S. (1). However, in the field, fruit infection on cucurbits including watermelon (Citrullus lanatus), is rarely, if ever, observed. Consequently it was atypical when powdery mildew-like symptoms were observed on seedless and seeded watermelon fruit on several farms in southwestern Florida during November and December 2010. Severe powdery mildew was also observed on seedless watermelon Tri-X 313 fruit grown at SWFREC, Immokalee, FL. Powdery mildew symptoms were mainly observed on young immature fruit, but not on mature older fruit. Abundant powdery mildew conidia (28 µm length), and conidial chains typical of cucurbit powdery mildew were detected on the fruit surface, although not on the leaves. Observation of conidia in 3% KOH indicated the presence of fibrosin bodies commonly found in the cucurbit powdery mildew genus Podosphera (1). Orange to dark brown cleisthothecia with a single ascus were also detected on the surface of some fruit samples. Conidia were subjected to PCR reactions using specific primers designed to amplify the ITS region of Podosphera (2). The resulting amplicons from the fruit powdery mildew isolates were sequenced and were found to be 100% identical to the ITS sequences of P. xanthii in the NCBI data base (D84387, EU367960, AY450961, AB040322, AB040315, AB026146). The sequences from the watermelon fruit isolate were also identical to several P. fusca (synonym of P. xanthii), P. phaseoli (GQ927253) and P. balsaminae (AB462803) sequences. Based on morphological characteristics and ITS sequence data, the pathogen infecting watermelon fruit can be considered as P. xanthii (1,3). The powdery mildew isolate from watermelon fruit was maintained on cotyledons of squash (Cucurbita pepo, ‘Early Prolific Straight Neck’). Cotyledons and leaves of five plants each of various cucurbits and beans were inoculated with10 µl of a conidial suspension (105conidia/ml). Two weeks after inoculation, abundant conidia were observed on cucumber (Cucumis sativus, SMR-58) and the melon (Cucumis melo) powdery mildew race differentials Iran H and Vedrantis. However, no growth was observed on the melon differentials PI 414723, Edisto 47, PMR-5, PMR-45 MR-1 and WMR-29 (1). The powdery mildew isolate from watermelon fruit behaved as melon race 1 (1). Development of conidia was also observed on fruit of watermelon cultivar “Sugar Baby” and a susceptible U.S. plant introduction (PI 538888) three weeks after inoculation with a conidial suspension. However, the disease was not as severe as what was observed in the fields in fall 2010. The pathogen did not grow on plants of Impatiens balsamina or on select bean (Phaseolus vulgaris) cultivars (Red Kidney, Kentucky Blue and Derby Bush), but did grow and produce abundant conidia on Pinto bush bean as has been observed before (4). Further studies to understand the epidemiology of the pathogen and development of management strategies will be needed in the near future.