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Jim Schupp, a Penn State fruit researcher, speaks in a peach rootstock trial at Cherry Hill Orchards in New Danville, Pa., on April 25, 2023.

NEW DANVILLE, Pa. — Orchardists face tricky decisions on crop load and pest management thanks to a late spring wave of cool weather.

“It’s very difficult in a year like this to have a straightforward plan,” said Greg Krawczyk, a Penn State Extension entomologist.

He spoke at a Penn State twilight meeting April 25 at Cherry Hill Orchards.

Unusually warm early spring weather got bloom going earlier than normal, but the return to more seasonable temperatures has slowed the trees’ progress.

In Lancaster County, the temperatures have not been cold enough to produce more than spotty frosts, too close to the ground to injure apple crops.

“Fingers crossed. We’ve got a few more weeks yet,” said Tim Elkner, an Extension educator in Lancaster County.

But the temperatures forecast for the next week are low enough — highs in the 50s and 60s — to reduce the effectiveness of chemical thinners.

That’s problematic because, in Adams County at least, good bloom and pollination periods have led to a strong initial fruit set.

“That’s a happy dilemma. That means that we’ve got a crop to work with, but we’re going to have to get to work,” said Jim Schupp, a Penn State tree fruit researcher.

Chemical thinners are thought to work by inducing carbohydrate stress, causing the tree to shed fruit. But at the expected cool temperatures, trees will not be consuming carbohydrates quickly.

“It’s hard to induce a carbohydrate stress on a tree that’s only sipping from the cup,” Schupp said.

Chemical thinning works when fruits are 7 to 17 mm and performs best at 10 to 12 mm. Many fruits were 5 or 6 mm early this week.

Fruits can grow 1 mm a day in warm weather, so in an average year, farmers would be in a seven- to 10-day window for applying their thinners.

The cold will slow fruit growth, so the opportunity may last a little longer than normal. But even so, growers may want to hold out until fruits are 15 to 17 mm, looking for temperatures at least in the 70s.

“You might be waiting to the ragged, jagged end,” Schupp said.

If growers feel they must thin in cold weather, carbaryl and NAA will be the most effective options. 6-BA is ineffective below 70 degrees, Schupp said.

Once the ideal thinning window has passed, growers who still have too many fruits may want to make a rescue thinning treatment. Accede or the more powerful ethephon will be the best options, Schupp said.

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This spring’s warm-then-cool weather has also complicated insect management.

The imminent petal fall spray should target oriental fruit moth, plum curculio and European apple sawfly. Growers who have had past problems with the curculio or sawfly should spray as soon as it makes sense, Krawczyk said.

Oriental fruit moths started flying at the beginning of April. If growers are catching moths in their traps, the insects have been mating, and females could be laying eggs. When temperatures reach the low to mid-60s, the larvae will begin hatching and attacking peach shoots or fruits.

In Biglerville, Krawczyk caught eight curculios in orchard traps over the past week. These weevils will start feeding and laying eggs when temperatures reach the upper 60s. Spraying soon will take care of them unless more encroach from outside the orchard.

European apple sawflies had warm enough weather during bloom to lay eggs on the flowers. Now that the eggs are hatching, Krawczyk said, growers will want to kill the larvae before they enter the fruit and cause damage.

Applied soon, Assail will control all three pests — oriental fruit moth, plum curculio and sawfly — plus aphids. Other products, such as Imidan, will control only some of these species, Krawczyk said.

After the petal fall spray, timing for the early cover sprays could be tricky. Because of the unusual weather, normal benchmarks might not be reliable.

For example, codling moths should be controlled when the orchard accumulates 250 growing degree days. An old rule of thumb puts that at two weeks after petal fall, but that might be too early this year if the weather stays cool, Krawczyk said.

The best information on spray timing will come from the insect themselves when growers catch them in their orchards.

“Put the traps, pheromone traps, into your orchard, and they will tell you what you need to do,” Krawczyk said.

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This article originally ran on lancasterfarming.com.