First report of resistance to boscalid in podosphaera xanthii, cucurbit powdery mildew, in South Carolina

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

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2018
Publication Date: 8/30/2018
Citation: Keinath, A.P., Gabriel, R., Kousik, C.S. 2018. First report of resistance to boscalid in podosphaera xanthii, cucurbit powdery mildew, in South Carolina. Plant Health Progress. 19:220-221. https://doi.org/10.1094/PHP-03-18-0009-BR.
DOI: https://doi.org/10.1094/PHP-03-18-0009-BR

Interpretive Summary: Various cucurbit crops including pumpkin, watermelon, squash and cucumbers are grown in the United States. Many diseases and pests attack these crops and reduce their yield resulting in monetary loss for growers. One such disease, known as powdery mildew can infect most of the cucurbit crops grown in South Carolina. Growers manage powdery mildew by spraying various chemicals known as fungicides. One such fungicide called "boscalid" has been used to manage powdery mildew of cucurbits for many years. Recently it was found that the fungicide was not very effective in managing powdery mildew in South Carolina. This was because powdery mildew in South Carolina had developed resistance to boscalid. This is the first report of resistance in powdery mildew to boscalid in South Carolina. The results of this study will be useful to watermelon growers, extension workers, seed industry and University researchers who work with cucurbit crops in the state.

Technical Abstract: Resistance to boscalid, one of the older succinate-dehydrogenase inhibitors (SHDI) in Fungicide Resistance Action Committee (FRAC) Code 7, was detected in Podosphaera xanthii, the most widespread cucurbit powdery mildew fungus, in New York and New Jersey in 2009. In 2013, boscalid was the most effective of four fungicides representing four FRAC codes used in a bioassay with potted yellow summer squash plants exposed to natural inoculum in Charleston, SC. In July 2017, four potted pumpkin (Cucurbita pepo cv. Gold Challenger) plants were treated with 0.28 g/liter (682 ppm) boscalid (Endura 70WP, BASF Corporation, Research Triangle Park, NC) or water. One day later, the eight plants were placed among pumpkins cv. Jack-o-Lantern with symptoms of powdery mildew in a research plot in Charleston, SC. The field pumpkins had been treated from mid-April to the end of June with alternating applications of metrafenone (three times), cyflufenamid (two times), triflumizole (two times), and a single application of the SDHI penthiopyrad on 2 June. After 24 h exposure to naturally occurring inoculum, potted pumpkins were moved to a greenhouse. After 10 days incubation, powdery mildew colonies were counted on the upper and lower surfaces of the first and second leaves. The total number of colonies on plants treated with boscalid (mean 6.5) was not significantly different from the number of colonies on the water control plants (mean 8.7) (P = 0.58). The powdery mildew fungus on Gold Challenger was identified as P. xanthii by observing fibrosin bodies in conidia mounted in 3% KOH and by sequencing a portion of the internal transcribed spacer (ITS) region in the ribosomal DNA with primers ITS 4 and PxT. The nucleotide sequence of the 465-bp product (GenBank accession number MH021603) was 100% identical to sequences of P. xanthii in the NCBI database (e.g. GQ902939, KX369541, and AB774158). Gold Challenger plants were kept in the greenhouse to serve as a source of inoculum for two additional experiments, each done twice. In two greenhouse experiments, six yellow summer squash (C. pepo cv. Early Prolific Straightneck) plants were each sprayed with 682, 133, 26, 5, or 0 ppm boscalid and inoculated 1 day later with a conidial suspension (2 x 105 conidia/ml) prepared as described by Kousik et al. (2011), except that the inoculum concentration was lower (2 x 104 conidia/ml) in the second experiment. Two sets of plants were inoculated with conidial suspensions h from leaves on boscalid- or water-treated Gold Challenger plants. Severity of powdery mildew was rated visually on the upper surfaces of the second- and third-oldest leaves 9 and 12 days after inoculation in the first and second experiments, respectively. There was no significant effect of boscalid concentration on severity of powdery mildew (P = 0.33), and no difference between isolates (P = 0.17). Mean severity across all concentrations of boscalid and both isolates was 30.5% and 7.0% in experiments one and two, respectively. In two laboratory bioassays, cotyledons on 7- to 10-day-old squash cv. Early Prolific Straight neck plants were sprayed with 682, 341, 170, or 0 ppm boscalid. One day later, four detached cotyledons, one treated with each boscalid concentration, were placed on six replicate plates of culture medium supplemented with 400 µl/liter pimaricin and inoculated with P. xanthii conidial suspensions prepared from leaves taken from boscalid- or water-treated Gold Challenger plants. After 11 days incubation at 20°C and 12 h/day fluorescent lighting, P. xanthii from both types of source plants completely colonized all boscalid-treated and water-treated cotyledons. A boscalid-sensitive isolate (B108ML) collected from watermelon (Citrullus lanatus cv. Mickey Lee) in 2008 in a research greenhouse in Charleston, SC, was suppressed completely by all concentrations of boscalid in both repetit