In the two weeks following bloom, scout for, and prune out, fire blight strikes promptly. Destroy pruned strikes by burning or leaving them out to dry. It is best to prune well back into healthy wood, at least 12 inches behind the water-soaked margin. Take care as summer pruning may stimulate active shoot growth leading to new susceptible tissues that could later become infected. If fire blight reaches the central leader, the tree should be removed. An effective removal practice is quickly cut the tree off below the graft union hang on the wire and dry out until you have time to remove it. Dead trees will not continue to ooze, but if trees are oozing, they should be promptly removed to avoid inoculum spread. Fortunately, the location may be safely replanted in later years.
Post-bloom applications of prohexadione-calcium (Apogee or Kudos) for shoot blight management should be considered, especially on vigorous blocks of highly-susceptible apple varieties during shoot elongation which begins in late bloom.
For maximum effectiveness, prohexadione-calcium (Kudos, Apogee, etc.) should be applied at 6-12 oz/100 gal (3-6 oz/100 gal for tree <5 years) when trees have 1-2” of shoot growth. A second application should be made 14-21 days later.
Preventative applications of copper can be used post-bloom and during the summer to protect against the spread of shoot blight infections. Copper must be applied before infection occurs as it will only reduce bacteria on the surface of tissues. It will have no effect on existing shoot blight infections and may cause fruit russet in young developing fruit. Apply with adequate drying time and use hydrated lime to reduce the potential of phytotoxicity from copper. Remember terminal shoots can outgrow protective residues of copper. A low-rate fixed copper program consists of applications on a 7–10-day schedule during high-risk weather until terminal bud set.
It may be possible to save plantings on resistant rootstocks that have a moderate amount of shoot blight. Apply prohexadione-calcium (Kudos, Apogee, etc.) at the highest rate for the planting (6-12 oz/100 gal) and allow 5 days for the product to take effect. Afterwards, prune out existing and newly developing shoot blight every two weeks for the rest of the season, but remove any trees where fire blight has reached the central leader. If pruning stimulates additional shoot growth, a second application of prohexadione-calcium could be warranted.
If you need to interplant apple trees in existing orchards where trees were killed by fire blight and removed, replant these missing trees ‘skips’ in late fall to better synchronize next season’s bloom with established trees.
