Conventional UVC (245 nm) and FarUVC (222 nm) Irradiation Affects the Growth of Colletotrichum spp. Isolates in Culture and Their Infection of Strawberry Leaves

Authors: Smith, Barbara; Sampson, Blair; Copes, Warren; Takeda, Fumiomi; Janisiewicz, Wojciech; STAFNE, ERIC - Mississippi State University; Sakhanokho, Hamidou; Carroll, Jennifer

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2024
Publication Date: 5/9/2024
Citation: Smith, B.J., Sampson, B.J., Copes, W.E., Takeda, F., Janisiewicz, W.J., Stafne, E.T., Sakhanokho, H.F., Carroll, J.B. 2024. Conventional UVC (245 nm) and FarUVC (222 nm) Irradiation Affects the Growth of Colletotrichum spp. Isolates in Culture and Their Infection of Strawberry Leaves. PhytoFrontiers. https://doi.org/10.1094/PHYTOFR-03-24-0016-R.
DOI: https://doi.org/10.1094/PHYTOFR-03-24-0016-R

Interpretive Summary: Many fungal plant pathogens have developed resistance to chemical fungicides. As an alternative to these synthetic pesticides, we evaluated the ability of UVC (254 nm) irradiation and FarUVC (222 nm) irradiation to kill spores of five species of the strawberry anthracnose pathogen and to reduce anthracnose disease symptoms on strawberry leaves without harm to the leaves. Our laboratory trials showed that exposure to FarUVC irradiation for 15 or more seconds or to UVC irradiation for 120 seconds effectively killed spores of each of the fungal species tested. For UVC irradiation to be the most effective it had to be immediately followed by a dark incubation period of 4 hours; however, FarUVC irradiation was equally effective with or without a dark incubation period. In another trial, detached leaves from strawberry plants were inoculated with spores from each of 13 isolates of the anthracnose fungus. Irradiation with FarUVC light for 30 seconds or more or with UVC light for 120 seconds reduced anthracnose symptoms on the leaves compared to symptoms on non-irradiated leaves; however, leaves receiving FarUVC irradiation for 60 seconds were harmed. The results of this study showed that FarUVC (222 nm) lights were more effective than conventional UVC (254 nm) lights for control of the strawberry anthracnose disease pathogens. These pathogens could be killed with a much shorter exposure time with the FarUVC light, and daytime FarUVC applications were as effective as night-time applications.

Technical Abstract: Various alternatives are being developed to reduce or replace chemicals for plant disease control. We conducted laboratory studies to evaluate the effect of conventional UVC (245 nm) or FarUVC (222 nm) irradiation on growth of strawberry anthracnose pathogens in culture and on the disease response following inoculation of detached strawberry leaves. Our first objective was to determine the duration of FarUVC and UVC irradiation required to kill conidia of 13 Colletotrichum isolates representing five Colletotrichum species (C. fioriniae, C. nymphaeae, C. fructicola, C. siamense, and C. theobromicola) from two Colletotrichum complexes (C. acutatum and C. gloeosporioides). In two trials, potato dextrose agar in petri dishes was inoculated with each isolate then irradiated with conventional UVC or FarUVC for various intervals ranging from 5 to 120 s. After 48 h incubation, average colony count/culture of the four isolates in the C. acutatum complex in one of the trials was significantly greater than that of the five isolates in the C. gloeosporioides complex. In both trials, FarUVC or conventional UVC irradiation for more than 5 s significantly reduced the number of colonies/culture compared to the number in the non-irradiated control petri dishes. FarUVC irradiation for 15 s or more or conventional UVC irradiation for 120 s killed conidia of isolates of each of the Colletotrichum sp. tested. Cultures exposed to conventional UVC followed by a 4 h dark incubation period had significantly fewer colonies/culture than those followed by a 4 h light incubation period; however, there were no differences due to light or dark incubation in colony counts of cultures exposed to FarUVC, thus eliminating the need to irradiate plants with FarUVC during the night that is required for UVC irradiation. Other objectives were to determine if conventional UVC and FarUVC irradiation would prevent or reduce anthracnose symptoms on detached strawberry leaves inoculated with each of the 13 Colletotrichum isolates and determine if the lethal irradiation durations of each light would injure strawberry leaves. Leaves collected from three anthracnose susceptible strawberry cultivars were inoculated with a conidial suspension of each isolate, incubated in a dew chamber for 20 h, then irradiated with conventional UVC light for 30, 60, or 120 s or with FarUVC light for 15, 30, or 60 s. Leaves were evaluated for anthracnose symptoms and any signs of injury10 days later. Irradiation with FarUV light for 30 s or with conventional UVC light for 120 s reduced anthracnose symptoms on strawberry leaves compared to symptoms on non-irradiated leaves; however, leaves receiving FarUVC irradiation for 60 s had the highest injury scores, indicating that irradiation with FarUV for 60 s or more may injure strawberry plants. FarUVC irradiation of strawberry plants is an attractive alternative to chemical disease control.