At this time of the year, we should always be keeping an eye on the weather in the week ahead, and the shift toward warmer temperatures this week suggests that we may be seeing the first signs of green tissue on trees in many sites in Vermont. Green tip indicates the beginning of the growing season, and is an important biofix for apple scab models. Keep checking your trees daily and note the date when 50% of buds on ‘McIntosh’ have opened enough to see green tissue from the side. That date should be used in NEWA to mark your biofix for apple scab ascospore maturity.
Potential for apple scab infection increases with temperatures as more ascospores mature to be released in rain events. In the early season. Not many spores are mature, and if you had good scab control last year, i.e., no visible lesions all season, then the small percentage of mature spores in a low inoculum situation may give you some early season breathing room. But if you did have a problem with scab last year, or a neighboring orchard harbors substantial inoculum, you should be ready early in the season to protect your trees. Copper is the standard, first spray of the year in virtually all orchard, as it helps to reduce fire blight inoculum and is a moderate fungicide against scab that can cover the first infection event for that disease.
But copper is a tricky material- it must be on the plant and not washed-off by the time fire blight cankers are oozing with the onset of warm weather, but if applied too late, copper ions on developing buds can cause fruit russeting. I would plan on applying copper to any orchard that had any amount of fire blight last year and which is showing green tissue or at least solid silver tip as soon as you have a suitable spray window. If possible, I would plan on applying copper to any orchard, period, that is between silver tip and half-inch green in the next 7-10 days. There is a pile of materials out there and for all intents and purposes for this delayed dormant spray any of them are effective as long as you are applying a good full rate of copper ions. The standard dry materials like Champ, C-O-C-S, Cuprofix, Kocide, etc. will give you the best bang for the buck here, and I would apply the full label rate for any of them and thoroughly spray the whole orchard. The only caveat I offer is if phenology advances rapidly before you can get out there and the trees are at 1/2” green tip, in that case, I would apply a low to middle rate. After 1/2” green tip, unless you don’t care about fruit finish (e.g., cider fruit), I would avoid copper.
This isn’t a bad time to get oil on, either, but the rate should be 2% by volume and coverage absolutely thorough to soak overwintering mite eggs, scale, and aphids. If time is of the essence, focus on copper first.
There is still time to do some sanitation in the orchard by flail mowing leaves and fine brush, and/or applying a coarse urea spray (44 lb feed grade urea in 100 gal water applied per acre, directed at the leaf litter) to speed decomposition and reduce apple scab inoculum.
Keep an eye on NEWA regularly as we enter into the 2023 season. Up-to-date spray tables may be found in the New England Tree Fruit Management Guide available online at netreefruit.org and shortly in hard copy format.