Effective disease management strategies for growers in Michigan and eastern U.S. growing regions that experience wet conditions are essential to support the upward trajectory of hops in these regions. Michigan experienced unusually wet weather during the 2015 growing season and plant losses resulting from hop downy mildew (HDM) infections caused by Pseudoperonospora humuli ranged from 20 to 100%; some producers were unable to financially recover and no longer grow the crop. We evaluated the efficacy of foliar- and drench-applied fungicides against HDM and examined P. humuli isolates for point mutations linked to carboxylic acid amide (CAA; FRAC 40) resistance. Our research shows that there are four highly effective active ingredients currently registered for use on hop in Michigan and include mandipropamid (FRAC 40), fluopicolide (FRAC 43), ametoctradin (FRAC 45), and cyazofamid (FRAC 21). The absence of resistant genotypes indicate that Michigan growers can continue to utilize CAA-containing commercial fungicides as part of an overall HDM management program. Rootstock rot complicates foliar HDM assessments, so we selected twelve cultivars to evaluate for susceptibility (2016 and 2017). Stolons from a subset of six cultivars (2018) were used to determine rootstock rot susceptibility. Wet-rot cortex discoloration in stolons of 'Tahoma', 'Newport' and 'Columbia' was comparable to cultivars with more severe foliar disease symptoms ('Nugget' and 'Cascade'). Differences in foliar disease among cultivars with a similar levels of rootstock rot suggests resistance to P. humuli. Asymptomatic HDM infections in planting material can escape rogueing since incubation can last 10 (leaves) or 21 days (shoots). Real-time quantitative polymerase chain reaction (qPCR) TaqMan assays and a recombinase polymerase amplification (RPA) assay were developed and had varying success at detecting asymptomatic infections. Molecular diagnostics applied to improve HDM management in planting material should utilize the mitochondrial marker-based qPCR assay for optimal performance. In August 2018, a Michigan hop grower reported necrosis and blighting of foliage and shattering of cones resulting in yield loss. A multilocus phylogenetic analysis identified the causal agent as Diaporthe sp. 1-MI, a novel taxon that was consistently recovered from tissue and pycnidia from both leaves and cones. Pathogenicity was demonstrated in detached leaves and whole plants. Michigan hop growers face a new disease that we proposed to name "halo blight" due to a chlorotic margin of leaf lesions and browning of cone bracts.
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