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Poster: Biology & Disease Mgmt: Chemical Control


Effects of Penicillin Injection for Citrus HLB Control on Culturable Bacteria in Petioles and Rhizosphere, and Penicillin Resistance
K. SHIN (1), M. Ascunce (1), H. Narouei-Khandan (1), X. Sun (2), D. Jones (2), E. Goss (1), A. van Bruggen (1) (1) University of Florida, U.S.A.; (2) Division of Plant Industry, U.S.A.

Trunk injection with penicillin has been tested to control citrus huanglongbing (HLB), but environmental safety must be assured before approval of penicillin injection in groves. We investigated potential effects of penicillin injection on rhizospheric and endophytic bacterial populations using grapefruit trees in field and greenhouse experiments. Trees were injected with penicillin G, using application rates of 0, 1000, or 6000 µg/ml. Bacteria were isolated on a low carbon medium from roots plus rhizosphere and surface-sterilized petioles at various times after penicillin injection. Selected isolates were tested for penicillin resistance (20 µg/ml) and glyphosate resistance (7000 µg/ml), because glyphosate is widely used and cross-resistance against antibiotics had been documented. Bacterial populations in petioles and root-rhizospheres initially increased after penicillin injections, possibly due to release of nutrients from dead bacteria, and then returned to control levels after one week. Penicillin resistance was common in penicillin-injected and control trees (30 to 94 %). A few isolates (0 to 2%) were only resistant to glyphosate, while more isolates (29 to 30%) were only resistant to penicillin. Most isolates (54 to 71%) were resistant to both glyphosate and penicillin. Considering the widespread penicillin resistance in control soil, potential side effects of penicillin injection in citrus are small.