Strategies to minimize bacterial canker in high density sweet cherry orchards

Production of fresh market sweet cherries (Prunus avium L.) using high density canopy training systems can improve labor efficiencies and early returns on investment. However, some systems, such as the "Upright Fruiting Offshoots" (UFO), require a support trellis that may increase the potential for infection by Pseudomonas syringae (the causal agent of bacterial canker) due to plant tissue wounds caused by rubbing against trellis wires. Bacterial canker can cause death of spurs, loss of limbs, decreased yields, and tree mortality. Once the bacteria enter the tree, the infection may become systemic, making treatment difficult. Three types of trellis wires were examined over two years for simulated rubbing and infection potential following inoculation of 'Early Robin' and 'Ulster' sweet cherry trees with lab cultures of P. syringae pv. syringae (PSS). High tensile plastic or plastic-coated steel wires reduced infection by 50 to 75% compared to traditional high tensile steel wire. Canker bacteria also can gain entry through natural openings, such as leaf scars in the fall, and natural wounds such as spring frost damage to flowers. Research was conducted to examine whether prophylactic application of a range of potential control treatments, including antibiotics (such as oxytetracycline), plant defense inducers (such as Actigard), or microbial bio-controls (such as Optiva), can reduce flower infections. Antibiotics were most effective, reducing infection 48 to 90% compared to the inoculated control. The bio-controls and plant defense inducers were less effective and more variable, ranging from a 45% reduction (Blossom Protect bio-control) to little or no apparent effect. Further research on application parameters (e.g., timing) may improve the efficacy of these materials.