Colletotrichum fioriniae infecting invasive Japanese hop (Humulus scandens) in the United States

Japanese hop (Humulus scandens) is a non-native, invasive plant that colonizes disturbed riparian areas throughout the eastern United States and Canada, forming dense, monocultural stands that displace native plant communities due to a high reproductive rate, rapid growth, climbing bines, and dense shading (Balogh and Dancza 2008). It is capable of serving as a reservoir for agronomically important plant pathogens, such as the Tomato spotted wilt virus and powdery mildew species that infect commercial hemp and hop fields (Yoon et al. 2018; Weldon et al. 2020). In the spring of 2016, diseased populations of H. scandens were observed along the Monocacy River in Frederick County, Maryland with severe chlorotic and necrotic leaf lesions. Symptomatic leaves were surface sterilized and placed in moist chambers at 25°C for sporulation. Sporulating acervuli, lacking setae, developed on irregular, tan necrotic leaf lesions following 7 to 12 days in a moist chamber (Figure 1). Conidia were hyaline, aseptate, smooth-walled, fusiform to cylindrical with both ends acute (Figure 1B). Conidia measured (n = 100) [L x W; Average (+ Std. Err), range]: 12.42 µm (± 0.10), 8.41 – 14.48 µm; x 3.91 µm (±0.03), 3.03 – 4.91 µm. Monoconidial fungal cultures were obtained by transferring conidia with a sterile glass needle to acidified potato dextrose agar and incubated at 25°C for 2 to 3 days. Based on phenotypic characteristics and conidial morphology and size, the pathogen appeared to belong to the Colletotrichum acutatum complex (Damm et al. 2012). Therefore, six loci (ITS, GADPH, CHS1, HIS3, ACT, and TUB2) were amplified and sequenced from a representative isolate, 16-008, for species characterization (GenBank accessions MW023070 to MW023075) (Damm et al. 2012). For the ITS region and ACT, GADPH, and CHS1 loci, isolate 16-008 was 100% identical to C. fioriniae and shared 99% similarity to TUB2 and HIS3 for multiple accessions of C. fioriniae in GenBank. Gene sequences were aligned, trimmed, concatenated, and analyzed against 32 reference strains, within the C. acutatum complex (Damm et al. 2012). Concatenated loci were used to generate a maximum likelihood phylogeny using W-IQ-TREE (Trifinopoulos et al. 2016). Results from the phylogenetic analysis demonstrated that isolate 16-008 was most genetically similar to C. fioriniae with a bootstrap support of 100% (Figure 2). Based on phenotypic and sequence analyses, isolate 16-008 was identified as C. fioriniae. Humulus scandens seedlings from Maryland (n = 3) were inoculated with a conidia suspension (107 conidia mL-1) with 0.125% Tween 20® and applied with an atomizer until runoff. Inoculated plants were placed in a dew chamber at 25°C for 2 days. Experimental plants were distributed in a mist tent at 25°C with 14 h of light and monitored for 2 weeks. Negative control plants (n = 2) were sprayed with a sterile 0.125% Tween 20® water solution. All inoculated plants were symptomatic by 12 days post inoculation. No symptoms were observed on the mock-inoculated plants. Symptoms were identical to disease field samples. Inoculations were repeated with the same results. Colletotrichum fioriniae was reisolated and confirmed from excised leaf lesions via ITS and ACT sequencing. To our knowledge, this is the first report of C. fioriniae naturally infecting H. scandens within the United States (Farr and Rossman 2020). Future studies will evaluate the host range of this isolate due to the species broad host range and the weed’s extensive distribution.