First report of Colletotrichum fioriniae causing anthracnose on common hop in Maine

Common hop (Humulus lupulus) is cultivated in the Northeastern United States as an ornamental and for regional brewing. Colletotrichum fioriniae, a member of the C. acutatum species complex, is the causal agent of anthracnose in Humulus spp. (Hatlen et al., 2023; Frederick et al., 2022). It is also present in deciduous forest ecosystems in the Northeast United States (Martin et al., 2021). Leaf samples exhibiting anthracnose-like symptoms were collected from a H. lupulus cv. Tettnanger’ plant at a brewery in Ellsworth, Hancock County, ME on June 9, 2025. The symptomatic plant was in front of a bay door through which the brewery has received orders from the Maine hopyard Aroostook Hops. Symptoms consisted of small lesions (<1 cm diameter) with chlorotic halos. Disease incidence on the sampled plant was 100%, with the highest severity observed on leaves subjected to mechanical damage and water runoff. Three other hop plants, cultivars indeterminate, located elsewhere on the property were asymptomatic. Leaf lesions from the symptomatic plant (n = 6) were excised, surface-disinfected in 0.5% NaOCl, and plated onto acidified potato dextrose agar (APDA: 85% lactic acid, 19.5 g PDA, 1 L DI water). Emerging colonies were subcultured on PDA (19.5 g PDA, 1 L DI water) for morphological characterization. Mycelia appeared bleach white, becoming tan or grey with tangerine conidiomata. Conidia were cylindrical, mostly rounded on both ends, with some tapering at one end. Mean conidial dimensions (n = 35) were 12.3 ± 1.6 µm in length and 4.0 ± 0.6 µm in width, consistent with C. fioriniae (Damm et al., 2012). Pathogenicity assays were conducted using 4 cv ‘Tettnanger’ plants and 2 feral plants collected in Wales, MA. Two leaves per plant were selected; each leaf was wounded 3 times with a toothpick. One plant served as a negative control (sterile water), while the other was inoculated with a suspension of mycelia and conidia. Plants were maintained in a humidity chamber at 18–23°C under natural light. Lesions consistent with field observations developed at 2 weeks post-inoculation (wpi). The pathogen was reisolated on PDA as described above. At 3 wpi, 5 of 6 inoculated ‘Tettnanger’ samples showed mycelia growth and conidia morphologically consistent with C. fioriniae, 0 of the 3 Wales samples showed growth and all controls were negative (0/9). At 5 wpi, 5 of the 6 ‘Tettnanger’ samples were positive, none of the 3 Wales samples were positive and all controls were negative (0/9). These findings satisfy Koch’s postulate. Genomic DNA was extracted from 1 isolate, and PCR amplification was performed targeting the calmodulin (CAL), ITS and β-tubulin (TUB-2) regions using primer pairs CAL-228F/CAL-737R, ITS1/ITS4 and Bt2a/Bt2b (Carbone & Kohn, 1999; Glass & Donaldson, 1995; White et al. 1990). Amplicons were sequenced using Oxford Nanopore technology (Psomagen, MA) and deposited in the European Nucleotide Archive (ENA accession numbers ERR17024322, ERR17024319, ERR17024324). BLAST analysis revealed that the calmodulin region shared 100% identity (473/473 bp; 99% coverage) with C.fiorniae (type sequence MN895514.1), ITS 100% identity (585/585 bp; 98% coverage) (type sequence MH865005.1), and β-tubulin region 98% identity (502/513 bp 99% coverage) to C. acutatum sensu lato (AB273715.1). This represents the first report of C. fiorniae causing anthracnose on common hop in Maine. Further investigation is needed to assess cultivar susceptibility and if management strategies are warranted.