April 19, 2014
Terence Bradshaw, UVM Tree Fruit and Viticulture Specialist
By the time I check the UVM orchards on Monday, I expect we’ll be seeing silver tip on many cultivars. By Friday 4/18, I called Empire and Macoun at silver tip, but most others were still looking tight, although buds were swelling on everything. I call silver tip when the bud scales at the tip of fruit buds first separate, but green tissue is not yet evident when looking at the bud from the side. Bud stage criteria can be viewed here: http://orchard.uvm.edu/uvmapple/hort/99budstage/BudStageCriteria.html
What does this mean for orchard management? The window between silver tip and green tip is perfect for applying copper to suppress fire blight and to act as your first scab spray of the season. Dave Rosenberger pulled together an excellent summary of the use of early season copper for scab and fire blight management in the March 25, 2013 issue of Scaffolds. But, while early season copper can be an excellent management tool, copper materials can be phytotoxic. That is why the early season spray is made before much green tissue is exposed. If applied when buds are closed, however, then cold temperatures immediately before or after spraying are not a huge concern. In fact, I have in many years had my airblast sprayer fan shroud ice up while applying copper- not an ideal situation, but it can happen at 5 AM when the temperature is 31 F and the velocity of air coming through the shroud contributes to rapid cooling, much like a snow gun on the ski slopes.
Oil, however, is a different story when it comes to applications before or after freezing weather. Delayed dormant, silver tip, and green tip are common times to apply an oil spray to help manage mites, aphids, scales, and other overwintering arthropods pests. When oil penetrates cells, it causes phtotoxicity that can affect frui development, especially when cluster leaves which supply most of the carbo0hydrates to developing fruit early in the season are damaged. Oil is often appluied at dilute rates, and the goal for a grower should be to fully saturate the tree as best possible. Application of oil just after or before freezing events (24 hours either way definitely, possibly 48 hours) can cause damage, so if you have seen or are expecting freezing temperatures, put the oil away for a couple of days.
Fortunately, oil can be applied right up to tight cluster-early pink bud stages, and in fact may be more effective then. We should be out of frost risk by then (otherwise we have bigger problems than oil on fruit cluster leaves), so maybe delaying your oil application would be prudent, so long as you can fit it around Captan sprays later in the season. Oil should not be applied within 7-1- days of a Captan or Sulfur spray. For more details on spring oil applications to manage mites and other pests, including rates and spray incompatibility issues, please refer to your 2014 New England Tree Fruit Management Guide.
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The UVM Tree Fruit and Viticulture Program is supported by the University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM Grant, and USDA Risk Management Agency Funds.