First Report of Colletotrichum fioriniae Infecting Hop (Humulus lupulus) in Michigan.

Abstract

In July of 2020, a hop (Humulus lupulus L.) grower in Berrien County, Michigan submitted 'Chinook' leaf samples to MSU Plant & Pest Diagnostics. The leaves were covered in small, tan colored lesions, with a small chlorotic halo with an approximate diameter of 5 mm. The grower reported that foliar lesions were in the lower 2 m of the fully developed hop canopy. Disease incidence and severity were estimated at approximately 20% and 5 to 10%, respectively. After incubation at 100% relative humidity, acervuli with orange spore masses and a few setae were present. A pure culture was obtained from these sporulating lesions using water agar. The isolate was hyphal tipped onto potato dextrose agar (PDA) and stored in a glycerol-salt solution at -80o C (isolate CL001) (Miles et al. 2011). On PDA, cultures displayed gray growth on the top of the colony and a red color on the underside of the Petri dish. After 14 days, acervuli with no setae appeared exuding orange conidial masses on the surface of the culture. Conidia were hyaline, aseptate, smooth-walled and rounded at the ends and measured on average 15.89 μm (13.81 to 16.91 μm) × 7.26 (6.82 to 8.41 μm) (n = 20). The color and size of the conidia matched other descriptions of C. acutatum sensu lato (Damm et al. 2012). Four loci (ITS/515 bp - OQ026167, GAPDH/238 bp - OQ230832, CHS1/228 bp -OQ230830, and TUB2/491 bp - OQ230831) were amplified from isolate CL001 (using the primers ITS1/ITS4, GDF1/GDR1, CSH-79f/CHS-354R, and T1/Bt-2b, respectively) and had 100% pairwise identity with C. fioriniae 125396 (JQ948299, JQ948629, JQ948960, JQ949950, respectively, Damm et al. 2012). The GAPDH, CSH1, and TUB2 sequences from isolate CL001 were trimmed, concatenated and aligned with 31 different members of Colletotrichum acutatum sensu lato and C. gloesporioides 356878 (Damm et al. 2012; Kennedy et al. 2022). The alignment was then used to produce a maximum likehood phylogenetic tree using Geneious Prime (Biomatters Ltd.) with the PHYML add on using the HKY + G model (G = 0.34) (Guindon et al. 2010). Isolate CL001 had the closest similarity to C. fioriniae with a bootstrap value of 100. Pathogenicity tests were performed on 2 month-old 'Chinook' hop plants. Twelve plants were inoculated with 50 ml of a conidial suspension (7.95 x 106 conidia/ml) of isolate CL001 (n = 6) or water (n = 6) using a spray bottle until runoff. Inoculated plants were sealed in clear plastic bags and grown in a greenhouse at 21o C with a photoperiod of 14 h. After 7 days, lesions appeared on the hop plants inoculated with CL001, but no symptoms appeared on the water inoculated hop plants. Lesions with a chlorotic halo were observed but they were smaller than field lesions and no setae were present (approx. 1 mm in diam.). Leaves were surface sterilized (0.3% sodium hypochlorite solution for 15 s and then rinsed three times) and the leading margin of the lesions or healthy tissue (water control) were placed on 1% ampicillin amended PDA. Fungal isolates on PDA morphologically matched C. fioriniae were recovered from all CL001-inoculated plants. No C. fioriniae isolates were recovered from the water-inoculated plants. Based on conidial morphology, the four loci, and the phylogenetic tree, isolate CL001 was identified as C. fioriniae. This is the first report of Colletotrichum fioriniae (syn = Glomerella acutata var. fioriniae Marcelino & Gouli) infecting common hop and further investigation is needed to determine if management is needed for this pathogen.