Spring disease management in apples

I think it’s safe to say that spring is officially here, and the business or managing our orchards is about to pick up pace real fast. In walking the orchards at the UVM Hort Farm yesterday, I saw a lot of swollen, delayed-dormant buds, but only one cultivar, Zestar, which had reached silver tip. Given the warm weather expected all week, I would expect to see at least some cultivars reach green tip by early next week if not this weekend. Thus begins the disease management season.

I know many growers are still catching up on pruning after a difficult winter, and at this point, you should wrap up what you can and get your brush pushed to make room for sprayer access. I am a believer in using copper at green tip for disease management. The timing of this spray is very important- too soon (no green tissue showing) and you risk ‘wasting’ some of the effectiveness of the material against apple scab when susceptible tissue isn’t present. Too late (beyond ½” green tip) and you risk fruit russeting, which can be quite severe. Given that, it’s better to err a bit on the early side, but you should have green tissue showing in the orchard before applying. Copper applied at green tip will give about seven days’ protection against apple scab.

Copper’s primary benefit is in reducing overwintering populations of fire blight bacteria. Fire blight has become a regular disease to manage in Vermont, and a multi-pronged approach will be needed to keep it at bay. Copper should be applied to all trees in the orchard, not just susceptible varieties, and in as dilute a spray as possible. The specific copper material is less critical than the amount of metallic copper that is applied in the spray, and copper sulfate, copper hydroxide, copper oxychloride sulfate products all will be effective when used at label rates. A good primer on spring copper applications to pome fruit by Dr. David Rosenberger can be found in the March 28, 2011 issue of Scaffolds. Addition of one quart oil per 100 gallons can help improve penetration into bark crevasses where fire blight may reside. However, this could be a good time to apply a full oil spray to manage overwintering mites, and a 2% solution is recommended at this timing. Oil and copper products are compatible for tank mixing at this time of the year, but likelihood of phytotoxicity increases as more green tissue emerges. One benefit of applying oil in your first spray is that it allows more time for it to degrade or wash off before incompatible fungicides such as Captan and sulfur may be used as primary scab season ramps up.

Now that the ground is clear and firming up, it also would be a good idea to perform spring orchard sanitation to reduce overwintering scab inoculum. Leaf shredding with a flail mower is an effective practice that also may be used to reduce small pruning wood to mulch, but the mower must be kept low in order to lift and grind leaves that harbor overwintering inoculum. Alternatively, there is still time to apply urea (40 lbs/100 gal water/acre) to leaf litter which aids in decomposition and breakdown of inoculum. Leaves should be wetted thoroughly and the majority of material directed into the tree row. This application would add 18 lb actual nitrogen per acre which should be accounted for in your fertilizer applications later in the season.

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The UVM Apple Program is supported by the University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM Grant, and USDA Risk Management Agency Funds.