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Etiology and epidemiology of bacterial blight of red raspberry in British Columbia

University of British Columbia

Pseudomonas syringae van Hall was recovered from 31 of 32 samples of red raspberry tissue showing typical symptoms of bacterial blight. Of the 99 isolates recovered, 85 were physiologically-typical, P. syringae isolates, three did not produce the fluorescent pigment, six did not utilize lactate and five did not produce toxin as determined by the Geotrichum candidum bioassay. When a suspension of 10⁷ CFU/ml was sprayed on the leaves of 6-week-old raspberry plants, 42 of 48 isolates caused necrosis within 4 days. No other bacterium recovered from the diseased raspberry tissue was pathogenic to raspberry in greenhouse tests. P. syringae isolates remained viable and retained their toxin-producing ability when stored for one year on nutrient glycerol agar at 5°C. Identical isolates stored on nutrient agar at 5°C or in sterile distilled water at room temperature either did not survive or lost their toxin-producing ability. There seemed to be a relationship between toxin-producing ability and an isolate's virulence as about 50% of toxin-producing isolates were rated pathogenic and 35% weakly pathogenic while none of the non-toxin-producing isolates were rated pathogenic and 60% were rated weakly pathogenic. A scheme was devised for rapid identification of P. syringae from raspberry tissue. An isolate was determined to be P. syringae if it produced a distinctive raised mucoid colony on nutrient sucrose agar, produced a fluorescent pigment, was oxidase negative and reacted in drop agglutination tests with an antiserum prepared against syringae. These tests could be done within three days. This scheme was used to study the overwintering site of P. syringae on raspberry. P. syringae was found to naturally populate 25-75% of raspberry buds during the winter months. Populations of 10⁴ CFU/six bud sample were most common in the buds that contained P. syringae. During the spring and summer months, P. syringae survived as an epiphyte on raspberry leaves both in the field and in the greenhouse trials. P. syringae was also shown to cause brownish-red spots surrounded by yellow halos on the leaves of raspberry during the summer months. About 90% of the P. syringae isolates from raspberry were ice nucleation active. Raspberry plants that had been sprayed with a suspension of 10⁷ CFU/ml and then held at -2°C for 4 hours developed symptoms similar to those of bacterial blight within 12 hours of the freeze treatment. In preliminary tests, raspberry cultivars showed varying degrees of resistance to P. syringae infection. Raspberry cultivar Chilcotin showed greatest resistance and cultivar Mailing Leo showed greatest susceptibility to Pseudomonas syringae infection. Three different species of bacteria were found in the normal microflora of the raspberry that were antagonistic to P. syringae in vitro.

Text

2010-03-03

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http://hdl.handle.net/2429/21462

Plant Science

University of British Columbia

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