RELIABILITY OF GENTISIC ACID, A FLUORESCENT MARKER, FOR DIAGNOSIS OF CITRUS GREENING DISEASE

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-beta-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 mug/gm of tissue and when used in combination with other diagnostic criteria under glasshouse conditions.