Flower bud development in apples and stone fruits-3/19/2020


               I just wanted to update you on where we are right now in terms of flower bud development in apple and stone fruits and whether we will potentially be affected by the cold weather this weekend (Sunday). As for apples, almost all our apple cultivars in the ASH Jr. AREC’s research farm are in the silver tip, green-tip or ½ inch green now (see the table below). Pink Lady and Zestar are the only cultivars we have that show buds in tight-cluster and first-pink stages. None of the buds I investigated in Pink Lady and other commercial cultivars showed signs of frost damage so far. Based on the weather forecast for the Winchester/Frederick county area, temp-Max will range between 41-78 oF and temp-Min will range between 31-42 oF in the next 10 days. If this turns out accurate, then we should not really be worried about frost damage to our apple blossoms for the next 10 days.

Bud development stages of commercial apple cultivars (3/19/2020)
Schlect Red Delicious/M.26 ½ inch green
Honeycrisp/B.9 Green tip
Ramey York/M.9 Green tip
Pink Lady/M.9 Tight cluster – first pink
Gala/M.9 ½ inch green
Daybreak Fuji/M.9 Silver tip
Elstar/G.16 Swollen bud
Golden Delicious/B.9 Green tip
Zestar/M.9 Tight cluster – first pink
Pixie Crunch/G.935 Green tip
Gold Rush/G.11 ½ inch green
Liberty/B.9 ½ inch green
Granny Smith/M.26 ½ inch green – tight cluster

What about stone fruits? Well, based on the few apricot and plum trees we have in the lab, our plum and apricot trees are in the full-bloom and post-bloom stages now and although we had temp. of 31 oF this past Monday (Mar 16) for a couple of hours, the damage to apricot and plum blossoms that could be due to frost is less than 5%. It’s also unlikely that temps this Sunday would cause any significant damage to apricot and plum flowers unless they will drop below 25 oF. Our sweet cherries, on the other hand, are still between the green-tip and tight-cluster stages and therefore, the risk of frost damage is also minimal. Most of our peach cultivars are moving slowly toward 1-2% bloom and with temps of 78 oF tomorrow (Friday), this percentage might jump to 5-10%. At this stage, the temperature that can kill 90% of buds is 24 oF, which is not the case this Sunday, at least for most parts of the state. For more information about critical temperatures for bud developmental stages, please use this link/PDF (https://www.canr.msu.edu/uploads/396/36740/PictureTableofFruitFreezeDamageThresholds.pdf. You may also need to print a color version of this figure and keep it in your house/office as a reference.

If you are not done with apple pruning yet, for very obvious reasons, and wondering whether you should proceed with the blocks that are not pruned, I would not generally recommend doing any pruning at this stage, for two reasons: 1) trees have already lost their cold hardiness and they would be more sensitive to any low temperatures during and after pruning; 2) there is a high risk of fire blight/shoot blight if the pruned wood did not heal well enough before the full-bloom window. As for peaches and nectarines, I usually recommend pruning the trees 4-6 weeks after full bloom.

If you have any questions or comments, please send them in the “comments” window below, send me an email at: ssherif@vt.edu, or call me at my office number: 540-232-6035 and I will make sure to get back to you as soon as I can.

Stay Safe, everyone.

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