First report of powdery mildew on cucumis zambianus, cucurbita digitata and zehneria scabraCaused by podosphaera xanthii

Authors: RENNBERGER, GABRIEL - Clemson University; Kousik, Chandrasekar - Shaker; KEINATH, ANTHONY - Clemson University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2017
Publication Date: 9/6/2017
Citation: Rennberger, G., Kousik, C.S., Keinath, A.P. 2017. First report of powdery mildew on cucumis zambianus, cucurbita digitata and zehneria scabraCaused by podosphaera xanthii. Plant Disease. 102(1):246. https://doi.org/10.1094/PDIS-06-17-0916-PDN.
DOI: https://doi.org/10.1094/PDIS-06-17-0916-PDN

Interpretive Summary: Powdery mildew is a serious disease of cucurbit crops including cucumber, watermelon, pumpkin and squash all across the United States. This disease was observed for the first time on two wild cucurbit species that are known to grow in the United states. The information will be useful to extension agents and University, and government researchers when advising growers of potential additional hosts of powdery mildew that maybe responsible for spreading the disease.

Technical Abstract: Powdery mildew is a serious disease of cucurbit crops worldwide. In the fall of 2016, symptoms of powdery mildew were observed on 2-month old plants of Cucumis zambianus, Cucurbita digitata and Zehneria scabra in research plots in Charleston, SC. Incidence on 28 plants of C. zambianus was 64.3%. On both C. digitata and Z. scabra incidence was <1%, as only a few symptomatic leaves were observed. The symptoms were typical for powdery mildew with white powdery spots on both leaf surfaces of all species and on stems of C. zambianus. Isolates were recovered and maintained as described by Kousik et al. (2011). Four isolates were obtained from C. zambianus and one from each of C. digitata and Z. scabra. Presence of fibrosin bodies in conidia of all isolates was confirmed by microscopic examination of conidia treated with 3% aqueous potassium hydroxide (Kable and Ballantyne, 1963). Conidia were produced in chains and measured 34.2 µm (SE = 0.4 µm) in length and 22.1 µm (SE = 0.3 µm) in width (n = 30). The isolates from C. digitata and Z. scabra and one isolate from C. zambianus were used for further identification. Pathogenicity and race tests for each isolate-cucurbit pair were carried out in separate incubators at 20°±1C. Four seedlings of each cucurbit were inoculated with a conidial suspension of a isolate obtained from the same species as described by Kousik et al. (2011). For race determination, four seedlings of each powdery mildew melon differentials (Vedrantais, Iran-H, WMR-29, MR-1, PI-414723, PMR-5, PMR-45) were also inoculated. After 14 days, plants were examined for symptoms of powdery mildew. The tests were repeated once. In both tests abundant sporulation of the pathogen was observed on leaves, petioles and stems of all three species. In both tests, all three isolates produced symptoms on differentials Vedrantais and Iran-H but not on any other differential and were thus characterized as race 1 (Pitrat et al. 1998). For molecular identification, conidia of isolates from C. digitata and Z. scabra and two isolates from C. zambianus were scraped from cotyledons and suspended in 50 µl of a 0.02% solution of Tween 20 in nuclease free water, vortexed and stored at -20°C until further use. Conidia suspensions were thawed and heated for 35 min at 95°C for cell lysis. Primers ITS4 and PxT (5’-TTTGGCGGGCCGGGCTCGACC-3’), which is specific for Podosphaera xanthii (Takamatsu and Kano 2001), were used to amplify a 465-bp fragment of ribosomal DNA as described Takamatso and Kano (2001) with the exception that 1 µl of heat-lysed conidial suspension was used as DNA template. The PCR fragments were sequenced from both directions. The sequences of all isolates were identical, and showed 100% identity to ITS sequences of P. xanthii in the NCBI database (KX369541, KP980563, KM260742, KR822240, AB774158). The sequence was deposited in GenBank under accession number MF153392. This is the first report of powdery mildew caused by P. xanthii on C. zambianus, C. digitata and Z. scabra in the U.S.A. Due to their distribution in North America C. digitata and Z. scabra could serve as potential inoculum sources for powdery mildew epidemics in commercial cucurbit crops.