Exacerbation of citrus canker by citrus leafminer phyllocnistis citrella in Florida

Abstract

Citrus canker (caused by Xanthomonas citri subsp. citri, Xcc) is an important bacterial disease of citrus that is spread naturally by rain and wind. Feeding damage to citrus leaves by the citrus leafminer (CLM), Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), has been shown to promote infection levels of citrus canker in a number of citrus-growing regions around the world. We conducted 2 studies to document that CLM damage exacerbates canker in Florida citrus. In 1 study, young citrus trees of 5 cultivare commonly grown in Florida were inoculated with a culture of Xcc. Two groups of trees were studied, 1 group with leaves damaged by CLM and 1 group that was treated with a pesticide to prevent CLM damage. Over all 5 cultivars, comparisons between the 2 groups of trees indicated that CLM damage resulted in 6-fold increase in the number of lesions. No difference was found between the 2 groups with respect to numbers of canker lesions on leaves without CLM damage. In the second study, a survey of commercial citrus groves was conducted to investigate incidence of canker on leaves with and without CLM injury. Low percentages of leaves infected by citrus canker were observed during the survey, with a maximum of 15% of leaves infected in 1 grove. However, during late Jul and Aug in some grapefruit and lemon groves, an average of 79% of leaves with canker had lesions directly associated with CLM damage, and an average of 36 more lesions per leaf was present on leaves with CLM damage. Exacerbation of canker by CLM during Jul and Aug coincided with the time of year when environmental conditions are usually optimal for canker in Florida and when population levels of CLM usually are most abundant. Citrus growers managing citrus canker should benefit from controlling CLM during the summer when conditions are favorable for canker infections, particularly in lemons and grapefruits.