Management of Xanthomonas fragariae with Pre- and Postharvest Treatments to Overcome Trade Barriers for California Strawberries

Abstract

Xanthomonas fragariae, the causal agent of angular leaf spot (ALS) of strawberry, is a quarantine pathogen in some export markets, causing trade restrictions and economic loss to the California fresh-market strawberry industry. Preharvest chemical management options are limited to copper, and there are no postharvest treatments available that reduce populations of the pathogen if ALS is detected at an export destination. Here, we report high preharvest efficacy for the experimental bactericide amino thiadiazole and the commercial product zinc thiadiazole, alone and in mixtures with low rates of copper or the antibiotic kasugamycin, with average disease incidence reduction of up to 92.8% compared with the control. Although effective against quarantine insect pests of strawberry, postharvest methyl bromide fumigation was ineffective against X. fragariae in diseased plant tissue at a standard commercial rate. Postharvest propylene oxide fumigation, used for decades by the California nut industries for insect and microbial disinfestation, significantly reduced X. fragariae populations in infected leaflet tissues by at least 2.5-log compared with controls at a dose of ≥142 µg/ml for 2 h at 15 to 20°C. Fumigated leaflets showed little to no phytotoxicity at effective rates, and fumigated fruit were not significantly affected in appearance or susceptibility to postharvest gray mold or Rhizopus rot following storage at 2°C for 3 days and at 15°C for an additional 5 days. Together, these new treatments offer potential strategies for establishing a systems approach with preharvest treatments significantly reducing the risk of ALS on plants and, in response to quarantine detections, a postharvest fumigation treatment that reduces viable pathogen populations in existing lesions.

An effector XopL enhances ethylene biosynthesis to promoteXanthomonas fragariaeinfection in strawberry

“…Xanthomonas fragariae (Xaf) , a bacterial pathogen highly specific to wild and cultivated strawberry, often causes angular leaf spots (ALS), and causes a huge production and economic loss in strawberry. Although strawberry ALS was reported first in the United States in Minnesota in 1960 (Kennedy, 1960), and since then it has spread worldwide, only a few is known regarding the epidemiology (Bestfleisch et al, 2015; Wang et al, 2023) and limited control options exist (Haack et al, 2019; Wang & Turechek, 2016). What’s noteworthy is that almost all strawberry germplasm was found to be either somewhat resistant or susceptible to this infection (Maas et al, 2000, 2002; Roach et al, 2016; Wei et al, 2024).…”