Management of Xanthomonas fragariae with pre- and postharvest treatments to overcome trade barriers for California strawberries

Authors: HAACK, STACEY - University Of California; Walse, Spencer; NGUYEN, KEVIN - University Of California; ADASKAVEG, JAMES - University Of California

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/2019
Publication Date: 4/9/2019
Citation: Haack, S.E., Walse, S.S., Nguyen, K., Adaskaveg, J.E. 2019. Management of Xanthomonas fragariae with pre- and postharvest treatments to overcome trade barriers for California strawberries. Plant Disease. 103(6):1256-1263. https://doi.org/10.1094/PDIS-08-18-1395-RE.
DOI: https://doi.org/10.1094/PDIS-08-18-1395-RE

Interpretive Summary: The detection and elimination of pests is necessary to ensure the safe movement of agricultural commodities from infested to non-infested areas through marketing channels. All treatments are subject to both regulatory and market-driven concerns, including commodity value. Over the last 20 years, a “systems approach” was developed largely to support risk-assessments and mitigations that could occur in a broader based “system” of activities that cumulatively meet the quarantine requirements. We have expanded on this work, providing an example whereby both pre- and post-harvest approaches can be used sequentially to control a key quarantine microbiological pest of California strawberries, angular leaf spot.

Technical 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. Pre-harvest chemical management options are limited to copper, and there are no post-harvest treatments available that reduce the pathogen if ALS is detected at an export destination. Here, we report high pre-harvest efficacy for the experimental bactericide amino thiadiazole, alone and in mixtures with low rates of copper or the antibiotic kasugamycin, with average disease incidence reduction 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 (PPO) 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 mg/liter 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 d and at 15°C for an additional 5 d. 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.