Development of Xanthomonas fragariae populations and disease progression in strawberry plants after spray-inoculation of leaves

Xanthomonas fragariae is the causative agent of angular leaf spot disease of strawberry. Greenhouse experiments were conducted using a X. fragariae isolate tagged with a green fluorescent protein (GFP) for detailed population dynamic studies in and on leaves after spray-inoculation. The GFP-tagged bacteria were monitored with dilution plating of leaf washings and leaf extracts, and analysis of intact leaves using a non-invasive monitoring system called PathoScreen, based on laser radiation of fluorescent cells in plant tissues and signal recording with a sensitive camera. PathoScreen was also used to monitor bacteria grown on an agar medium after leaf printing. During the first 3 days after inoculation, bacterial populations washed off leaves rapidly decreased by at least a factor of 1000, after which populations remained stable until 14 days post-inoculation (dpi), when symptoms first started to appear. Thereafter, populations increased to a level of 1012 colony-forming units (CFU) g-1 of leaf material or higher. Similarly, densities in leaf extracts were low during the first 3 days after inoculation, at a level of 100–1000 CFU g-1 of leaf tissue. Gradually populations increased to a level of 109–1012 CFU g-1 at 28 dpi. Higher densities of epiphytic populations were found on the abaxial side than on the adaxial leaf side during the first 2 weeks after inoculation. After spray-inoculation of leaves, bacterial populations released from infected plants remained low until symptoms appeared, after which plants became highly infectious, in particular under high humidit