Submitted to: Workshop Proceedings
Publication Type: Proceedings
Publication Acceptance Date: October 26, 2011
Publication Date: November 8, 2011
Citation: Bock, C.H., Gottwald, T.R., Graham, J.H. 2011. A survey of survival and activity of citrus canker lesion populations on foliage, fruit and shoots in a Florida grapefruit orchard in 2009 and 2010. In: Proceedings of the Workshop on Xanthomonas citri/Citrus canker, November, 17-18, 2011, Ribeirao Preto, Brazil. p. 24-26. Technical Abstract: Citrus canker (Xanthomonas citri subsp. citri (Xcc)) can infect several species of citrus. The disease can develop on the leaves, shoots and fruit, causing erumpent lesions, that on fruit precludes sale to the fresh market. We assessed lesion activity in orchard-grown grapefruit to provide information on the population dynamics of fruit lesions in a commercial situation to help gauge risk associated with infected fruit entering fresh markets. Every month from June 2009 to January 2010, and from June 2010 to January 2011, a grapefruit orchard in east-central Florida was sampled for 50 leaves, 20 shoots and 54 fruit. Lesions counts were made on each sample unit, and a random selection of 80 lesions from fruit, 50 lesions from leaves and 20 lesions from shoots were taken using a cork-borer. Bacteria flux density (BFD) was calculated for each lesion (bacteria/mm2/min). Between June 2009 and January 2010 the analysis indicated a slight decline in the proportion of active lesions (R2 = 0.44) from 88% in June to 69% in January, at the time of harvest. On leaves there was no apparent reduction in the proportion of active lesions (R2 =0.06) with 94% active in July, and 88% active by January. On stems there was a decline (R2 = 0.44), with 95% active in June and 20% active in January. From June 2010 to January 2011, linear regression analysis indicated a decline in the proportion of active lesions for fruit from 98% to 6% (R2 = 0.80), for leaves from 100% to 66% (R2 = 0.44) and for stems from 45% to 10% (R2 = 0.41). In 2009 the mean BFD (bacterial flux density) from lesions on fruit, leaves and stems was variable and erratic. On fruit, mean BFD was greatest in November (3.9x104 Xcc/mm2/min, Figure 2A), and least in August (2.8x103 Xcc/mm2/min). On leaves, mean BFD ranged from 2.1x103Xcc/mm2/min in November to 7.9x03Xcc/mm2/min in December. Lesion activity was most erratic for stems. From June 2010 to January 2011 the mean BFD on fruit was variable and ranged 2.7x104 in June to 0.8 bacteria/mm2/ min in December (Figure 2B). From June 2010 to January 2011, the mean BFD was variable and erratic on leaves (3.5x102 to 2.4 x 105 bacteria/mm2/ min) and stems (4.0 to 1.4x104 bacteria/mm2/ min). Although the 2009-10 season was warmer and wetter on average (particularly later in the season), there was no discernable effect of these conditions on lesion activity using regression analysis. Despite the decline in proportion of active lesions, the fact the lesions on grapefruit can maintain such output of bacteria even in the presence of copper sprays reinforces the need to focus on post harvest approaches for deactivating lesions of citrus canker on fruit being sold for the fresh market. Postharvest disinfection treatments might mitigate the risk of viable Xcc on fresh fruit, and if possible approaches to deactivate fruit lesions should be developed.