Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 8, 2011
Publication Date: March 16, 2011
Citation: Bock, C.H., Parker, P.E., Gottwald, T.R. 2011. Distribution of canker lesions on the surface of diseased grapefruit. Plant Pathology. 60:986-991. Interpretive Summary: Citrus canker is an important disease of grapefruit in Florida. The distribution of disease on fruit surfaces was investigated on 336 infected grapefruit cankered grapefruit from groves in East Florida. Lesions were enumerated on each quarter section from the top down (by slicing three planes through the peduncle-flower scar axis). Sequentially on the four quarters, 44.6% of all lesions were found on the upper quarter of the fruit, and 30.5%, 14.4% and 10.5% of lesions were found on the lower three quarters, respectively. The highest count was consistently on the upper portions of the fruit. Presumably the upper surfaces of the fruit are more prone to infection as they have greater exposure to splash born inoculum.
Technical Abstract: Citrus canker (caused by Xanthomonas citri subsp. citri) can cause direct yield loss of citrus, and infection of fruit can result in trade restriction being imposed on canker endemic areas. Developing fruit become infected through splash dispersed inoculum. The objective of this study was to describe and quantify the distribution of lesions on the fruit surface, also providing further evidence that fruit infection occurs through splash dispersal. Based on the diameter of each fruit, the fruit was sliced in four by taking three horizontal planes across the vertical axis such that each horizontal zone had the same vertical height (Z1-Z4 = peduncle-flower scar). Each zone had equal surface areas. Lesions were counted on each zone of the fruit. Fruit surface area ranged from 92 to 343 cm2. Although the lesion counts on individual fruit was variable, the percent of total lesions showed a decline from the uppermost zone, with 44.6, 30.5, 14.4 and 10.5 % counted on Z1to Z4, respectively. In only 33.6% of occasions was lesion count on Z2>Z1, and only in 15.2% of occasions was Z4>Z1. Most often lesions counts were highest in the upper two zones. The relationship between the percent lesions on each zone and zone position was described by an exponential model. Canker-susceptible citrus fruit can be susceptible for a prolonged period after fruit set and minimizing inoculum and reducing wind speed and splash, and use of copper sprays (or new management alternatives) during that period can help reduced disease on fruit, particularly on the upper surfaces that appear to be more prone to infection.