|TWOMEY, MEGAN - Oregon State University|
|STONE, JEFF - Oregon State University|
|Gent, David - Dave|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2015
Publication Date: 4/30/2016
Citation: Twomey, M.C., Stone, J.K., Gent, D.H. 2016. Black wilt of hop (Humulus lupulus) caused by Diplodia seriata in New York State. Plant Disease. 100(4):861. doi: 10.1094/PDIS-10-15-1140-PDN.
Interpretive Summary: In August 2012, wilted hop bines were observed in a yard near Seneca Castle, New York, affecting 10 to 20% of the plants. Affected bines had a dark stem discoloration and wilted leaves, which remained attached after bines were killed. A fungus was recovered and identified as Diplodia seriata based on morphological characters and DNA testing. Disease symptoms were reproduced on inoculated hop plants, confirming that the cause of the disease was indeed Diplodia seriata. To our knowledge, this is the first report of infection of hop by D. seriata in the U.S. At present the disease appears to be only a minor issue on hop in New York State. D. seriata is a common and broadly distributed pathogen on grapevine and other woody hosts. The long incubation period of this fungus may limit symptom development on the annually produced shoots of hop.
Technical Abstract: In August 2012, wilted hop bines were observed in a yard near Seneca Castle, New York, affecting 10 to 20% of the plants. Affected bines had a dark stem discoloration and wilted leaves, which remained attached after bines were killed. Dark brown to black erumpent pycnidia were aggregated in the cortical tissue on portions of affected stems. Isolations were made from surface-sterilized stem pieces onto potato dextrose agar (PDA) and 1% water agar amended with shredded sterilized hop stem pieces (HSA). A pycnidial fungus was consistently isolated that produced dark-brown to black colonies within two weeks on HSA. Conidia measuring (19.5–) 24.4 (–31.7) µm × (9.7–) 11 (–12.2) µm were dark brown when mature, thick-walled, aseptate, and ovoid, with a truncate to rounded base. These characteristics suggested Diplodia seriata. Conidial measurements and other morphological characters were consistent with those of an ex-type specimen of D. seriata (CBS 112555). ITS, elongation factor 1-a, and beta-tubulin gene sequences were obtained from two isolates obtained from hop plants with standard primers (Phillips et al. 2007; Úrbez-Torres et al. 2008) and deposited in GenBank (accession numbers KT447248 & KT447249, KT460188 & KT460189, KT460190 & KT460191). The sequences for each isolate and locus were 100%, 99%, and 100% homologous to corresponding sequences of Diplodia seriata isolate CBS 112555 (GenBank accession numbers NR_111151, AY573220, and DQ458856, respectively). Pathogenicity assays were conducted with isolates DS-01 and DS-02 utilizing a toothpick inoculation procedure (Putnam 2004) to wound and deliver inoculum into the lower portions of stems near the soil. Twenty-nine percent of plants inoculated with isolate DS-01 and 38% of plants inoculated with DS-02 had stunted growth and wilting, and developed diffuse lesions radiating outward from the inoculation site. Non-inoculated stems and plants appeared normal. The fungus was re-isolated from all symptomatic stems and confirmed as D. seriata based on morphological characters and resequencing of the three loci.