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Reliability of Gentisic Acid, a Fluorescent Marker, for Diagnosis of Citrus Greening Disease. M. E. Hooker, Research Associate, USDA-ARS Fruit Laboratory, Beltsville, MD 20705. R. F. Lee, E. L. Civerolo, and S. Y. Wang. Professor of Plant Pathology, Citrus Research and Education Center, University of Florida, Lake Alfred 33850; Plant Pathologist, and Plant Physiologist, USDA-ARS Fruit Laboratory, Beltsville, MD 20705. Plant Dis. 77:174-180. Accepted for publication 22 September 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/PD-77-0174.

The reliability of gentisic acid (GeA), a fluorescent marker, to predict the presence of citrus greening disease was reevaluated by a modified method for hydrolyzing gentisoyl-?-d glucose to gentisic acid. Bark tissue extracts of random samples of diseased and healthy citrus plants differed significantly in their GeA content. The severity of foliar symptoms was correlated significantly with the amount of GeA in young and old bark tissue. Older tissue had significantly more severe foliar symptoms than younger tissue and yielded more GeA in extracts. However, when dried tissue samples were used, there was a significant difference in GeA content between tender young bark and old bark but no significant difference between mature and old (woody) bark. Although plants infected with several citrus tristeza virus strains contained GeA, greening-affected plants had statistically higher mean levels of GeA. This method is reliable for diagnosing citrus greening disease in citrus when GeA levels are above 300 g/gm of tissue and when used in combination with other diagnostic criteria under glasshouse conditions.