|Kropf, Suzanne -|
|Twomey, Megan -|
|Woods, J -|
|Putnam, M -|
|Serdani, M -|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 21, 2011
Publication Date: April 30, 2012
Citation: Kropf, S.M., Twomey, M.C., Woods, J.L., Gent, D.H., Putnam, M.L., Serdani, M. 2012. Sclerotinia wilt of Hop (Humulus lupulus) caused by Sclerotinia sclerotiorum in the Pacific Northwest U.S. Plant Disease. 96: 583. Interpretive Summary: Sclerotinia sclerotiorum is a widespread, destructive pathogen that causes a disease called white mold or Sclerotinia wilt on a number of host plants. Hop plants with disease symptoms characteristic of Sclerotinia wilt were observed in a hop yard in Oregon during June 2011. A fungus was recovered from symptomatic plants and the fungus was confirmed as Sclerotinia sclerotiorum based on several tests. To our knowledge this is the first report of Sclerotinia wilt on hop in the U.S. The disease appears to be localized to a limited number of hop yards in western Oregon, although given the widespread distribution of the pathogen in Oregon and elsewhere it seems likely that the disease may occur in other hop yards. Further investigation is needed to determine the prevalence of Sclerotinia wilt, its impact on hop, and aspects of the disease biology to development sound management recommendations.
Technical Abstract: Sclerotinia sclerotiorum is a widespread, destructive pathogen with an exceptionally broad host range. During June 2011, wilted hop plants (Humulus lupulus cv. Nugget) were observed in a hop yard in Marion County, Oregon. Some affected plants had upward curled leaves with necrotic margins, whereas other had bines that were severed at the soil line and completely wilted. The wilted bines tended to have a smaller diameter than bines that displayed some foliar symptoms but were not wilted. Yield loss was estimated at 10 to 20% due to bine wilting before cone development. Water soaked lesions covered in white-cottony mycelium were apparent on affected stems approximately 2.5 to 5 cm below the soil surface, some bearing large, irregularly shaped sclerotia. Isolations from stem lesions made onto potato dextrose agar (PDA) yielded isolates with rapid growth rates and morphological characters consistent with S. sclerotiorum. The pathogen identity also was confirmed by PCR amplification and sequencing of the internal transcribed space regions and 5.8S rDNA from two isolates (SS001 and SS002) as described before (3). The amplicons were sequenced bidirectionally and the consensus sequences were 100% similar to S. sclerotiorum (GenBank accession JN013184.1). Two nucleotide polymorphisms were present that differentiated the sequences from those of S. trifoliorum accession EU082465.1 (2). Greenhouse pathogenicity assays utilizing a toothpick inoculation procedure (1) were conducted to fulfill Koch’s postulates. Segments of sterile toothpicks were dipped into molten PDA and placed on Petri dishes containing solidified PDA. After the PDA congealed, a mycelial plug of a given isolate was placed in the middle of the Petri dish and incubated 7 days at 20°C to allow the fungus to colonize the toothpicks. Five 4-week old hop plants of cv. Agate were inoculated with a single toothpick by piercing the stem near the soil line and burying the inoculation site and toothpick with wetted potting soil. Control plants were similarly inoculated with PDA-coated toothpicks without the fungus. The plants were placed in a greenhouse where mist was applied 10 sec every 10 min for 4 days following inoculation. The plants were then placed in a greenhouse in a tray with water to maintain soil moisture near saturation for an additional 7 days. All inoculated plants developed symptoms similar to those observed in field; 4 of the 5 plants inoculated with isolate SS001 completely wilted, and 2 of 5 plants inoculated with isolate SS002 completely wilted. Isolations from the basal portions of inoculated stems yielded cultures with morphological characters identical to the S. sclerotiorum isolates used for the inoculations: S. sclerotiorum was not recovered from non-inoculated control plants. To our knowledge this is the first report of Sclerotinia wilt on hop in the U.S. The disease appears to be localized to a limited number of hop yards in western Oregon, although given the widespread distribution of S. sclerotiorum in Oregon and elsewhere it seems likely that the disease may occur in other hop yards. Further investigation is needed to determine the prevalence of Sclerotinia wilt, its impact on hop, and elucidate aspects of the disease epidemiology to inform growers’ management strategies.