Outbreak of Cucurbit Powdery Mildew on Watermelon Fruit Caused by Podosphaera xanthii in Southwest Florida

Abstract

Cucurbit powdery mildew caused by the obligate parasite Podosphaera xanthii occurs commonly on foliage, petioles, and stems of most cucurbit crops grown in the United States. (3). However, in the field, fruit infection on cucurbits including watermelon (Citrullus lanatus), is rarely, if ever, observed (2). Consequently, it was atypical when severe powdery mildew-like symptoms were observed on seedless and seeded watermelon fruit on several commercial farms in southwestern Florida during November and December 2010. Severe powdery mildew was also observed on ‘Tri-X 313’ and ‘Mickey Lee’ fruit grown at SWFREC, Immokalee, FL. Infected fruit developed poorly and were not marketable. Powdery mildew symptoms were mainly observed on young immature fruit, but not on mature older fruit. Abundant powdery mildew conidia occurred on fruit surface, but not on the leaves. Conidia were produced in chains and averaged 35 × 21 μm. Observation of conidia in 3% KOH indicated the presence of fibrosin bodies commonly found in the cucurbit powdery mildew genus Podosphaera (3). Orange-to-dark brown chasmothecia (formerly cleisthothecia) containing a single ascus were detected on the surface of some fruit samples. Conidial DNA was subjected to PCR using specific primers designed to amplify the internal transcribed spacer (ITS) region of Podosphaera (4). The resulting amplicons were sequenced and found to be 100% identical to the ITS sequences of P. xanthii in the NCBI database (D84387, EU367960, AY450961, AB040322, AB040315). Sequences from the watermelon fruit isolate were also identical to several P. fusca (synonym P. xanthii), P. phaseoli (GQ927253), and P. balsaminae (AB462803) sequences. On the basis of morphological characteristics and ITS sequence analysis, the pathogen infecting watermelon fruit can be considered as P. xanthii (1,3,4). 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 with 10 μl of a conidial suspension (105conidia/ml) in water (0.02% Tween 20). Two weeks after inoculation, abundant conidia were observed on cucumber (Cucumis sativus, ‘SMR-58’) and melon (Cucumis melo) powdery mildew race differentials ‘Iran H’ and ‘Vedrantais’. However, no growth was observed on melon differentials ‘PI 414723’, ‘Edisto 47’, ‘PMR 5’, ‘PMR 45’, ‘MR 1’, and ‘WMR 29’ (2,3). The powdery mildew isolate from watermelon fruit behaved as melon race 1 (3). Mycelium and conidia were also observed on fruit surface of watermelon ‘Sugar Baby’ and a susceptible U.S. plant introduction (PI 538888) 3 weeks after inoculation. 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’. Powdery mildew on watermelon fruit in production fields can be considered as a potentially new and serious threat requiring further studies to develop management strategies. References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (2) A. R. Davis et al. J. Am. Soc. Hortic. Sci. 132:790, 2007. (3) M. T. McGrath and C. E. Thomas. In: Compendium of Cucurbit Diseases. American Phytopathological Society, St. Paul, MN, 1996. (4) S. Takamatsu and Y. Kano. Mycoscience 42:135, 2001.

Morphological and molecular characterization of powdery mildew on watermelon plants in São Paulo state

“…All samples collected and sequencing showed 100% homology with the sequence of P. xanthii (JQ728480) deposited in GenBank. Similar results was found by Kousik et al (2011), with 100% identity to the ITS sequences of P.…”

Watermelon Germplasm Lines USVL608-PMR, USVL255-PMR, USVL313-PMR, and USVL585-PMR with Broad Resistance to Powdery Mildew

“…xanthii isolate B108ML was routinely maintained in a growth chamber (22 ± 1°C) on ''Early Prolific Straightneck (EPSN)'' squash plants. For inoculation, seedlings were sprayed with a conidial suspension (10 5 condia/mL in 0.02% tween 20) as described before (Kousik et al, 2011(Kousik et al, , 2018. Disease severity was recorded on the same 0 to 10 rating scale as described previously on hypocotyl, cotyledon, and true leaves.…”

“…McGrath, 2017). Powdery mildew on watermelon has occurred with increasing frequency over the past decade (Davis et al, 2001;Keinath, 2015;Keinath and Rennberger, 2017;Kousik et al, 2011;Mercier et al, 2014). As a result, the Watermelon Research and Development Group listed PM as an important research priority .…”

Powdery Mildew–Resistant Bottle Gourd Germplasm Lines: USVL351-PMR and USVL482-PMR

“…The disease is considered a major limiting factor in cucurbit production and all cucurbit crops are known to be susceptible to PM . Podosphaera xanthii can infect hypocotyls, cotyledons, stem, leaves, and fruit (Cohen et al, 2004;Jahn et al, 2002;Kousik et al, 2008Kousik et al, , 2011. Infection by P. xanthii can also reduce seedling vigor and predispose the plant to other diseases (Kousik et al, 2008;McGrath, 2010).…”

“…Therefore, a simple pure line selection procedure was used to develop highly resistant homogenous individuals for use in rootstock breeding programs. In a second 2007 screen, seedlings of PI 271351 and PI 487482 were inoculated with a local P. xanthii race 1 isolate, classified based on Cucumis melo (melon) PM differentials (Jahn et al, 2002;Kousik et al, 2011;. Initially, data on PM development were recorded 2 weeks after inoculation on a 1 to 9 rating scale similar to a scale used by GRIN to classify PI for all diseases (Kousik et al, 2008).…”