|WOLFENBARGER, SIERRA - Oregon State University|
|QUESADA-OCAMPO, L - North Carolina State University|
|Gent, David - Dave|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2016
Publication Date: 6/1/2016
Citation: Wolfenbarger, S.N., Quesada-Ocampo, L.M., Gent, D.H. 2016. Powdery mildew caused by Podosphaera macularis on hop (Humulus lupulus) in North Carolina. Plant Disease. 100(6):1245. doi: 10.1094/PDIS-12-15-1525-PDN.
Interpretive Summary: In June 2015, characteristic signs and symptoms of the disease powdery mildew was observed on several hop cultivars grown in western North Carolina. Through a series of controlled inoculations and DNA-based tests, the identity of the pathogen was indeed confirmed to the hop powdery mildew fungus. To our knowledge, this is the first report of this pathogen on hop in North Carolina. Powdery mildew is a potentially damaging disease of hop and surveys are warranted to better understand the distribution of the disease in the region. That the virulence of the isolates recovered matched that of isolates only known to exist at present in the Pacific Northwestern U.S. and that only one mating type of the fungus was recovered suggests the pathogen may have been introduced from this region. Follow up studies are planned for confirmation.
Technical Abstract: In June 2015, a grower in western North Carolina detected powdery mildew in a small hop yard. Characteristic colonies of the pathogen where observed on cultivars Cashmere, Cascade, and Chinook. Leaves with powdery mildew were collected from cultivar Cashmere for confirmation of the pathogen identity. Conidia from infected leaves were transferred to detached leaves of the powdery-mildew-susceptible cv. Symphony. Within 7 days, powdery mildew colonies with barrel-shaped conidia and non-branching conidiophores typical of Podosphaera macularis were visible. To determine if these isolates were capable of overcoming the powdery mildew resistance factor R6 as observed in isolates recently found in the Pacific Northwestern U.S., conidia were transferred to cv. Nugget. Isolates also were transferred to cv. Cascade to test whether the quantitative resistance in this cultivar could be overcome, as is known to occur in Washington State. Inoculations to Nugget failed, indicating the isolates lack R6-virulence, however, inoculations to cv. Cascade resulted in a similar number of lesions as compared to Cascade-adapted isolates of P. macularis. To confirm the identity of the organism as P. macularis, two isolates from North Carolina were co-inoculated individually with mating type tester isolates of P. macularis confirmed to be either mating type MAT1-1 or MAT1-2 idiomorph. Chasmothecia only formed when isolates were paired with a MAT1-2 tester isolate, indicating both the North Carolina samples were indeed P. macularis and mating type idiomorph MAT1-1. Mating type genotypes also were determined using PCR assays, which confirmed all five isolates were MAT1-1. The PCR products were sequenced bidirectionally and compared to a sequence of P. macularis from the Pacific Northwest: the sequences were identical. To our knowledge, this is the first report of P. macularis on hop in North Carolina. The virulence of the isolates and mating type idiomorph may suggest a recent introduction from the Pacific Northwestern U.S.