Adult Citrus Leafminers (Phyllocnistis citrella) Are Not Efficient Vectors for Xanthomonas axonopodis pv. citri

Interest in the interaction between the citrus leafminer (Phyllocnistis citrella) and citrus bacterial canker, caused by Xanthomonas axonopodis pv. citri, has increased as a greater incidence and severity of canker-diseased plants was observed in groves infested with the citrus leafminer. To determine whether adults of the citrus leafminer could act as vectors of citrus canker, we investigated two potential mechanisms for direct spread by leafminer adults using experimental microcosms. First, adult leafminers were raised on canker-infected foliage and were allowed to mate and lay eggs on healthy plants. These plants then were observed for development of citrus canker symptoms. In a second set of experiments, adults raised on healthy plants were given free access to canker-diseased plants during the period in which they mated and laid eggs on healthy plants. In all, 3,119 mines were produced by developing larvae on a total of 2,384 leaves examined for citrus canker symptoms. No symptoms of citrus bacterial canker disease were observed on any of the healthy test plants in 37 independent experimental trials conducted to test these two potential mechanisms of spread of citrus canker, and the pathogen was not recovered from insects exposed to symptomatic Rangpur lime plants. The upper limit on the rate of transmission was estimated to be less than 0.2% per oviposition event based on the binomial probability distribution. However, when adult P. citrella insects were artificially contaminated with high levels of X. axonopodis pv. citri, transmission to Rangpur lime plants with the induction of citrus canker was observed. This suggests that the ability of P. citrella to transmit X. axonopodis pv. citri is limited by the rate at which it can acquire inoculum from infected plants. The results support the conclusion that adult citrus leafminers are not efficient vectors for citrus canker bacteria, and the disease is unlikely to be spread this way.