Location: Subtropical Plant Pathology Research
2012 Annual Report
The risk of introduction of Asiatic citrus canker (Xanthomonas citri subsp. citri (Xcc)) to new, unaffected citrus producing areas both within the U.S. and internationally is a major concern for those citrus producing areas attempting to remain free of citrus canker. Prior research by an international group, which included the proposing investigators, resulted in a publication that served as the justification for USDA’s APHIS to promulgate a new regulation (7 CFR Part 301, FR Doc E915508 “Citrus Canker; Movement of Fruit from Quarantined Areas). This rule eliminated the requirement that fruit lots be inspected at the packinghouse and found to be free of canker symptoms and reduces industry costs by an estimated $15 million annually. It also removed the prohibition imposed on the movement of fruit from a quarantined area to commercial citrus producing states. While relieving restrictions on interstate movement of fresh citrus fruit within the U.S., the new ruling still protects canker-free areas from citrus canker. However, international trading partners remain unwilling to import fresh fruit from canker infected areas and seek further evidence that fruit is not a pathway for Xcc introduction to new areas, and that Xcc does not survive on fruit in sufficient populations to establish new infections. This project is devised to address this barrier to international trade by providing additional objective data to support the hypothesis (in previous work) that indicated lesions on harvested, packingline-processed fruit are of no risk to citrus industries in canker-free areas overseas. This report describes the results from multiple years of experiments investigating preharvest activity of canker lesions, and the likelihood of dispersal of bacteria of Xcc from these infected plants and fruit. Lesion activity was quantified on samples of fruit, stems and leaves. Results show that the proportion of active lesions on fruit of grapefruit decline during the season, although many lesions do remain active at the time of harvest, and the quantity of bacteria produced from individual lesions is highly variable throughout the season. This underscores the need for post-harvest treatment to neutralize the perceived risk of infected material. However, whereas lesions on fruit declined in activity, lesions on leaves were the most active throughout both seasons, suggesting the lesions on mature fruit pose a lower risk. Stem lesions had consistently the lowest activity, and also declined as the season progressed. The simulated wind dispersal events showed bacteria were dispersed downwind at wind speeds up to 20 m sec-1 (the highest tested) from both cankered fruit and foliage, but infected foliage invariably resulted in greater quantities of bacteria collected, with more severe disease on trap plants.