Spotless strawberry plants: how to keep them free from Xanthomonas?
Jan van der Wolf
Xanthomonas fragariae (Xfr) is the causative agent of angular leaf spot of strawberry, a quarantine organism in Europe in plant propagation material (EPPO A2). Protected cultivation of strawberry plants in general resuIts in a pathogen free crop. However, if plants are grown in the field, infections occasionally occur which can result in high economic damage. Motile Xfr cells can enter the plants via wound or natural openings, in particular via stomata. The development of symptoms is favored by a high relative humidity and when temperatures are between 18-24 degrees Celsius. Under high-moisture conditions, bacteria can exude from lesions and provide secondary inoculum which can be dispersed in different ways.
In this study, risks were assessed on the dissemination of the pathogen in symptomatic strawberry fields by machineries, splash water and aerosols. In addition, inoculum thresholds were determined for infection of plants, both under glasshouse and field conditions. Findings are translated in practical advices to growers of strawberry plant material.
A rotary mower was used to clip leaves of symptomatic plants. The cutter blades of the machine became heavily contaminated. Use of these contaminated machine resulted in (symptomless) infections of plants initially free of Xfr, at least at 10 m distance from the symptomatic plants. Cleaning of the mower with a water hose was not sufficient to remove the pathogen. In similar experiments with a runner cutting machine, only low densities were found deposited on plants initially free of the pathogen (10^3 colony forming units (cfu) g-1) and no infections had established.
Spread of Xfr from symptomatic plants by splash water during overhead irrigation at moderate wind, resulted in a spread over a distance of at least four meter. At windless weather conditions and light rain a similar minimum distance of spread was found but bacterial densities recovered were lower. An exponential decrease of the Xfr density was found with distance from the inoculum source.
In artificially created aerosols, Xfr could be collected by an air sampler up to a distance of 100 m from the source and those created by mowing a symptomatic crop up to a distance of minimally 25 m. Aerosols released by a sprayer, carrying in total ca. 10^11 cfu of Xfr was able to cause infections of strawberry plants up to a distance of at least 10 m from the sprayer. Inoculation of wetted plants resulted in a higher infection prevalence than inoculation of dry plants. In supplementary glasshouse studies in which strawberry plants were spray-inoculated on leaves, an inoculum level as low as 10 cfu per plant enabled development of symptoms both in cv Elsanta and cv Sonata.
It is concluded that there is a considerable risk of spread of Xfr from symptomatic plants by machines and wind-driven aerosols within and between nearby fields. Splash dispersal during rain or overhead irrigation in general will result only in a spread Xfr within the field. Growers are therefore recommended not to grow strawberry field in close proximity, and in particular to keep a distance between high- and low grade crops. Mowing should be done preferably at sunny and dry weather conditions to avoid secondary infections. A protective shield around mower blades may reduce the spread of Xfr during mowing. Washing and disinfection of machines will also contribute to a spotless cultivation of strawberry propagation material.
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