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United States Department of Agriculture

Agricultural Research Service

Research Project: DOMESTIC, EXOTIC, AND EMERGING DISEASES OF CITRUS, VEGETABLES, AND ORNAMENTALS (DEED)

Location: Subtropical Plant Pathology Research

Title: Hot Water Treatment to Reduce Angular Leaf Spot of Strawberry, Caused by Xanthomonas fragariae, in Nursery Production

Authors
item Turechek, William
item Peres, N. A. - UNIVERSITY OF FLORIDA

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2008
Publication Date: April 3, 2009
Citation: Turechek, W., Peres, N. 2009. Hot Water Treatment to Reduce Angular Leaf Spot of Strawberry, Caused by Xanthomonas fragariae, in Nursery Production. Plant Disease. 93:299-308.

Interpretive Summary: Bacterial angular leaf spot of strawberry is an important disease in strawberry nursery production. The European and Mediterranean Plant Protection Organization (EPPO) lists the pathogen as an A2 quarantine pathogen, so nurseries wishing to export plants to European countries must maintain certain phytosanitary standards. To help nurseries achieve these standards, heat treatments for killing or reducing the number of viable bacterial cells in strawberry crown tissue were investigated in laboratory, greenhouse and field studies. Bacterial populations exposed to 44 C for 4 h or 48 C for 2 h were reduced greater than 99%. The same treatments minimally affected vegetative growth of most cultivars tested if the plants were sealed in plastic bags prior to heat treatment, but flowering was adversely affected. In field trials the survival rate among cultivars exposed to these two treatments was similar to that observed in greenhouse trials and angular leaf spot developed appreciably only in non-heat-treated control plots. Heat treatment of strawberry nursery stock is feasible, but requires careful attention to the cultivars selected for treatment and can be used to supplement standard production practices for producing pathogen-free nursery stock.

Technical Abstract: Angular leaf spot is an important disease in strawberry nursery production. The European and Mediterranean Plant Protection Organization (EPPO) lists X. fragariae as an A2 quarantine pathogen. Therefore, nurseries wishing to export plants to European countries must maintain certain phytosanitary standards. To help nurseries achieve these standards, heat treatments for killing or reducing the number of viable bacterial cells in strawberry crown tissue was investigated. First, the sensitivity of bacteria to heat was determined by dispensing 1 ml aliquots of standardized cell suspensions in microcentrifuge tubes for each of four representative isolates of X. fragariae and submerging the tubes in water at 36, 40, 44, 48, 52, and 56 C for 1, 2.5, 5, 10, 15, 30, 60, 120, 240, 360 and 480 min. Bacteria were plated to determine the proportion surviving heat treatment. Two experiments were conducted in a greenhouse to determine the sensitivity of plants to heat treatment. In the first experiment, plants of the cultivars Camarosa and Diamante from two different nurseries were heat treated as follows: i) plants placed in metallic mesh cages and immersed directly into water (industry standard, direct dip); ii) plants sealed in a plastic bag and the bag immersed in water (bagged dry); or iii) plants wetted in warm water, sealed in a plastic bag, and then immersed in water (bagged wet). Plants were treated at 44 or 48 C for 0, 60, 120, 180, and 240 min. In the second experiment, plants of the cultivars Camarosa, Camino Real, Diamante, Oso Grande, Strawberry Festival, and Ventana from a single nursery were subjected to the same treatments. In both experiments, plants were potted after treatment and rated for growth characteristics. Populations of bacteria exposed to 56 and 52 C were killed completely after 15 and 60 min exposure, respectively; both treatments killed plants. Bacterial populations exposed to 44 C for 4 h or 48 C for 2 h were reduced by 5 to 6 log units. The same treatments minimally affected vegetative growth of plants bagged dry or wet, but flowering was adversely affected. These heat treatments were selected for field trials established at two locations in successive years. The survival rate amongst cultivars was similar to that observed in greenhouse trials and angular leaf spot developed appreciably only in non-heat-treated control plots. Heat treatment of strawberry nursery stock is feasible, but requires careful attention to the cultivars selected for treatment and can be used to supplement standard production practices for producing pathogen-free nursery stock.

Last Modified: 4/23/2014
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