Please remember that it is the blossoms that are most susceptible to the bacteria; assuming that there are no rat-tail or autumn blossoms, and in the absence of a hail event, the probability of secondary infections in orchards is minimal. In the event that rat-tail blossoms are apparent and extensive, it is critical to prevent their infection. If fewer than 3 applications of streptomycin have been made during bloom, it can be used to protect these rat-tail blossoms as long as it is within 50 days to harvest (PHI) for apples and 30 days (PHI) for pears. Better still, if time and labor are available, removing the rattail blossoms by hand may be more sustainable.
What to do next:
“Based upon current information, growers need to distinguish between situations in which the disease is detected on blossom clusters, succulent shoots, or lateral branches, versus first detected on main branches and limbs. In the first situation, growers are advised to make a distinction between spring and autumn infections. In the spring, recommendations are not to touch trees with limited growth vigor. On these trees, fire blight infections are likely to be restricted to the spurs and not to invade the main branches of the trees. Cutting these infections off, if it did not successfully eradicate the bacteria from the trees, could make the situation worse. If trees with vigorous growth (also called way too much fertilizer!!!) are infected, growers need to differentiate between those bearing few and those bearing numerous infections. With limited infections, growers should eradicate the infections by cutting back to a healthy section of the plant, about 20” from the site of visual symptoms. If numerous infections are observed on vigorous trees, the experience from this study suggests that the eradication efforts will likely be unsuccessful and may even make situation worse. Thus, to minimize unsuccessful pruning efforts, growers should postpone the pruning until winter. In these cases, the infections limbs and branches should be marked with colored paint so growers know what to prune in the late winter when the plants are dormant.
Trees that repeatedly show symptoms of fire blight (more than three years in a row) should probably be removed and burned. Three cans of spray paint can help: Green for year 1; yellow for year two, and red for year three. Or forget the red spray paint and grab a chainsaw. I say this for three reasons: First, continuous pruning of the same infected trees year after year is not sustainable; Second, the repeating infection indicates that the tree is systemically infected, and third, this tree now serves as a reservoir for additional infections all throughout the orchard. In this scenario, you are best advised to literally “cut your losses” and prune at ground level.
Finally, unless you are still at bloom (or extensive rat-tail bloom), streptomycin use is not recommended, despite what the label says (“After petal fall, continue applications at 10- to 14-day intervals to control twig blight (this could mean an additional 6 – 8 applications after blossom sprays”). There is simply no evidence to show that this is effective, and a lot of evidence (MI) to show that it drives antibiotic resistance. Its efficacy (along with the plant growth regulator, Apogee), followed up by one or two applications through petal fall. No antibiotic is recommended for trauma blight. In Indiana, where there are no reports of streptomycin resistant fire blight bacteria, there is no need to use any other antibiotic. And if we want to keep it that way, we need to be mindful of how and when we use this product.