Distribution of canker lesions on the surface of diseased grapefruit

Abstract

The objective of this study was to describe and quantify the distribution of citrus canker lesions (caused by Xanthomonas citri subsp. citri) on the surface of grapefruits, and provide evidence for splash‐driven infection of fruit. Based on fruit diameter, each fruit was sliced in four by taking three horizontal planes across the vertical axis such that each horizontal zone (Z1–Z4, from peduncle to flower scar) had the same vertical height. Each zone had equal surface area. Lesion number was counted on each zone of the fruit. Although lesion number among fruits was variable, both lesion number and percentage of total lesions showed a decline from the uppermost zone (lesion number: 10·5, 7·0, 3·4 and 2·4; percentage: 44·6, 30·5, 14·4 and 10·5 on Z1–Z4, respectively). General linear modelling (GLM) using a Poisson distribution with a log‐link function demonstrated a significant effect of zone and cultivar on lesion number, and significant differences between all zones and both cultivars. An analysis of covariance showed no effect of lesion number on fruit size, although cultivars differed in total lesion count. Canker‐susceptible citrus fruit is susceptible for a prolonged period after fruit set and minimizing inoculum and reducing wind speed and splash, and use of copper sprays during that period, will help reduce disease on fruit, particularly on the upper surfaces that appear to be more prone to infection.