Exacerbation of citrus canker by citrus leafminer, Phyllocnistis citrella in Florida

Authors: Hall, David; Gottwald, Timothy; Bock, Clive

Submitted to: Florida Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2010
Publication Date: 9/27/2010
Citation: Hall, D.G., Gottwald, T.R., Bock, C.H. 2010. Exacerbation of citrus canker by citrus leafminer, Phyllocnistis citrella in Florida. Florida Entomologist. 93:558-566.

Interpretive Summary: Citrus canker is an important bacterial disease of citrus that is spread naturally by rain and wind. Damage to citrus leaves by the citrus leafminer (CLM) has been shown to promote infection levels of citrus canker in a number of citrus-growing regions around the world. We conducted two studies to document that CLM damage exacerbates canker in Florida citrus. In one study comparing five different citrus cultivars, CLM damage resulted in an overall five-fold increase in the number of canker lesions. In the second study, a survey of commercial citrus groves during late July and August in some grapefruit and lemon groves indicated CLM caused an average increase of 30 percent of leaves with canker, and an average of 36 more lesions per leaf was present on leaves with CLM damage. Exacerbation of canker by CLM during July and August coincided with the time of year when environmental conditions are usually optimal for canker in Florida and when population levels of CLM are usually 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.

Technical 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. 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 two studies to document that CLM damage exacerbates canker in Florida citrus. In one study, young citrus trees of five cultivars commonly grown in Florida were inoculated with a culture of Xcc. Two groups of trees were studied, one group with leaves damaged by CLM and one group which was treated with a pesticide to prevent CLM damage. Over all five cultivars, comparisons between the two groups of trees indicated that CLM damage resulted in a five-fold increase in the number of lesions. No difference was found between the two groups with respect to numbers of 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 one grove. However, during late July and August in some grapefruit and lemon groves, an average of 79 percent 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 July and August coincided with the time of year when environmental conditions are usually optimal for canker in Florida and when population levels of CLM are usually 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.