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

Authors: Frederick, Reid; Cavin, Craig; Thomas, Jami; Bruckart, William; Tancos, Matthew

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2021
Publication Date: 7/19/2021
Citation: Frederick, R.D., Cavin, C.A., Thomas, J.L., Bruckart, W.L., Tancos, M.A. 2021. Colletotrichum fioriniae infecting invasive Japanese hop (Humulus scandens) in the United States. Plant Disease. 106:330-330. https://doi.org/10.1094/PDIS-06-21-1191-PDN.
DOI: https://doi.org/10.1094/PDIS-06-21-1191-PDN

Interpretive Summary: Japanese hop is an invasive weed that is found in much of the Eastern United States forming dense, monocultural stands primarily along disturbed riparian areas. While chemical and cultural controls can be effective on Japanese hop, the inaccessibility of many areas infested with Japanese hop renders these strategies challenging and cost prohibitive. The use of microorganisms for biological control may present an ecologically safe alternative to address these limitations and concerns. A fungal pathogen, Colletotrichum fioriniae, isolated from infected Japanese hop has the potential to be developed as a microbial biopesticide. Future studies will evaluate the host range of this novel fungal isolate since the species is known to have a broad host range.

Technical Abstract: Japanese hop (Humulus japonicus) 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. 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. In the spring of 2016, diseased populations of H. japonicus 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 7-12 days in a moist chamber. Conidia were hyaline, aseptate, smooth-walled, fusiform to cylindrical with both ends acute. 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 2-3 days. Based on phenotypic characteristics and conidial morphology and size, the pathogen appeared to belong to the Colletotrichum acutatum complex. Therefore, six loci (ITS, GADPH, CHS1, HIS3, ACT, and TUB2) were amplified and sequenced from the representative isolate, 16-008, for species characterization. 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. Gene sequences were aligned, trimmed, concatenated, and analyzed against 32 reference strains, within the C. acutatum complex. Concatenated loci were used to generate a maximum likelihood phylogeny using W-IQ-TREE. Results from the phylogenetic analysis demonstrated that isolate 16-008 was most genetically similar to C. fioriniae with a bootstrap support of 100%. Based on phenotypic and sequence analyses, isolate 16-008 was identified as C. fioriniae. Humulus japonicus seedlings (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. japonicus within the United States. Future studies will evaluate the host range of this isolate due to the species broad host range and the weeds extensive distribution.