VT Apple IPM for week of May 12

The cool weather is leading to a number of conditions in Vermont apples. Where trees are in some state of bloom, blossoms are opening very slowly. This is extending the overall bloom period. But also helping to maintain viability of pollen, styles, and stigma. It may be taking a while for pollination to happen, but we also have more time for it to happen. At our twilight meeting in South Burlington last Friday, despite the cool weather, there were abundant wild pollinators in the UVM orchards. For inland and upland orchards at pink or even earlier, the weather is delaying blossom opening. I don’t know of any orchards at petal fall but I haven’t been to the warmest corners yet. I would not expect them to be yet, but movement on bud stages is around the corner as weather is expected to warm into the 70s this week.

For growers headed into petal fall, a heavy bloom, and minimal to little cold damage from the April 25 freeze, it would be wise to consider a petal fall thinner. Petal fall means just that- no blooms in the orchard, so if you still have some varieties in bloom do not use carbaryl, an insecticide that is particularly toxic to honeybees, in this spray. I would consider NAA (Refine, Fruitone, etc.) alone or, if petals are truly off, with some carbaryl. Again, where bloom is heavy and conditions for pollination were good, plan to thin on the heavier side. Rates for many thinning materials are very confusing, and include spraying in parts per million, rate per 100 gallons dilute tree row volume, and the usual amount per acre. For most orchards, if a label (use https://www.telus.com/agcg/agribusiness to look up your latest labels) allows it, it’s easiest and totally relevant to default to the amount per acre as you usually do. Refine 3.5 WSSG, for example, gives a table and five paragraphs of information on page three about determining ppm for apple thinning, then one sentence, “Concentrate spray: Use Refine 3.5 WSG at the rate of 0.25 lb per acre (4 oz)-0.50 lb per acre (8 oz) in sufficient water to ensure good coverage at petal fall to early fruit set. Uniform and even coverage is crucial for good results.”

To summarize it, if you have a heavy bloom or hard-to-thin varieties (or both), use the high rate, if either of those is not fully true or you have freeze damage, consider backing off the rate a bit. You should also use NEWA’s apple carbohydrate thinning model to evaluate the effect of weather conditions on thinning effect, but the cool weather overall- even after it warms up to the low 70s this week- means thinning chemicals will be less effective and heavier rates should be used. The good thing is, we can try again next week as out fruit approach the ideal thinning window of 8-12 mm and the 6-BA thinners are more effective.

Cool weather means that blossom infection from fire blight does not look to be a concern this week.

Insect activity has been quiet this season, in no small part likely slowed by the cool temperatures. If you don’t have codling moth traps up yet, it would be good to get them up asap and check daily to record the biofix date from which to start calculating degree days until egg hatch. If you need traps, it would be best to order from Great Lakes IPM immediately, as my stock is out. A quick video of me setting a codling moth trap can be found at: https://www.youtube.com/watch?v=OIr_X3hLoQM.

But the warmer weather this week will encourage plum curculio to move into orchards from bordering hedgerows, and they will feed on and oviposit into fruit as they approach ~10 mm in diameter. It is standard practice to mow flowering weeds at 100% petal fall before applying pesticides to avoid attracting pollinators into sprayed areas then to apply a general insecticide to manage plum curculio, European apple sawfly, and some of the lepidopteran pests. Commonly-used materials include Actara, Avaunt, Imidan, and Verdepryn. See the New England Tree Fruit Management Guide for material suggestions.

Keep managing for scab, we’re not done yet and the less-than-promised rains have given sometimes marginal infection periods in the past week. Just stay covered for the time being.

Reach out if you have any questions. tbradsha or (802)922-2591 (cell).

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