Early apple season considerations

By Terence Bradshaw

As has been the case in the past several even-numbered years, growers are increasingly concerned about early bud break and potential for cold weather to damage developing apple buds. At the UVM Hort Farm, ‘McIntosh’ buds were just entering silver tip yesterday March 31. Growers are reporting silver tip across most of the major growing regions of the state, and a tiny bit of green tip has been mentioned. That may push out a bit today with warm temperatures, so some orchards may be in green tip by April 2.

This is a good time to look over and be familiar with the critical temperatures for bud damage in tree fruit. Michigan State University has published a guide at http://agbioresearch.msu.edu/uploads/396/36740/TreeFruitCriticalTemperatures.pdf

Based on this chart, orchards in silver tip can withstand 15°F with 10% fruit bud kill, and 2°F with 90% bud death. An orchard in green tip would have critical temperatures, respectively, of 18 and 10°F for 10% and 90% kill. It’s a pretty good bet that any bud development will slow through tomorrow, Saturday April 2 and halt after that through next week until warmer weather comes. So where you are at tomorrow is where your critical temperatures will be through the next expected cold spell. Orchard with no bud development (pre-silver tip) should be fine through this event.

Depending on where you are or what models you are looking at, low temperatures are expected to be in the teens on Monday and Tuesday mornings next week, and low spots will may be colder. There isn’t anything to do now but sit tight and see what comes. Get your pruning done and prepare your sprayers because the growing season will be here any time. I wouldn’t worry too much about early season scab right now unless significant green tip occurs, and even at that, it’s going to be tough to run sprayers around freezing weather (and definitively do not spray oil within 48 hours before or after a freeze event).

When the time comes, copper should be applied any time after green tip but before the half inch green ‘mouse ears’ stage. 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 applied at green tip will also give about seven days’ protection against apple scab. Applications should be made to all trees in the orchard, not just susceptible varieties. 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. I’ve attached a presentation by Kari Peter and Brian Lehman of the Plant Pathology Dept at Penn State that should provide good information on use of copper in early season apple sprays to manage fire blight inoculum and potentially that first scab infection period.

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.