Apple scab ascospore maturity models for essentially every weather station in the state indicates that we are at the end of the primary scab infection season. That means spores that could cause apple scab have matured and either has or likely will release after the next good wetting event. However, growers should maintain protective fungicide coverage on all trees for the next 1 to 2 weeks and should inspect their orchard carefully to ensure that infections have not occurred before backing off from coverage for the season. This is a good time to use group 7 or 11 fungicides to also provide protection against powdery mildew and apple rusts.
The warm weather we are expecting this week could cause fire blight infections in susceptible varieties with lingering blossoms such as newly planted trees. Consider protection if your orchard is in that situation. This is also a good time to maintain a program of 3-4 sprays of prohexdione-calcium (e.g., Kudos, Apogee) at 6 oz per acre or 2-3 oz per acre with Actigard at 2 oz per acre to stiffen up cell walls in developing shoots to reduce shoot blight susceptibility.
Plum curculio is active in orchards. If full block insecticide coverage was applied at petal fall, only border row sprays may be warranted until you have reached 308 degree days base 50F after petal fall, after which the insect is no longer actively laying eggs and causing fruit damage.
Codling moth are slow to take flight around the region. Growers should check pheromone baited traps daily and set the biofix for the management model as the first date of trap capture. The NEWA model will calculate accumulated degree days (base 50F) after sustained flight to predict egg hatch, most insecticides should be applied 100-200 DD later to be most effective. We don’t expect that to occur until next week, anyway.
The big management activity for this week should focus on thinning applications. The warm weather we are expecting today through Friday coupled with low wind conditions mean that thinners will be particularly active this week. In most orchards that I have seen in the state that were not affected by the April 20 freeze, I am recommending an aggressive thinning strategy this year to prevent a large crop of small fruit and subsequent biennual conditions next year and the years following. The weather conditions in the next few days are especially conducive to using the 6-BA thinning materials, which are especially useful on small fruited cultivars such as Empire, Macoun, and Gala. Larger fruited cultivars may benefit from NAA applications. Either of those would be enhanced with the inclusion of 1 pint of carbaryl per 100 gallons. This is the most important window for you to manage this year’s crop, so take advantage of it.
Terence Bradshaw (he/him)
Associate Professor, Specialty Crops
Chair, Dept of Agriculture, Landscape, and Environment
(formerly Plant and Soil Science)
College of Agriculture and Life Sciences
University of Vermont
117/210 – Jeffords Hall | 63 Carrigan Dr
Burlington, VT 05405
(802) 922-2591 | tbradsha
https://go.uvm.edu/alebradshaw
UVM Commercial Horticulture | UVM Fruit Blog
Horticulture Research and Education Center
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