Apple scab:

With the rain that has started today and is expected to continue through Friday, I don’t need to tell this audience that we are starting a significant apple scab infection period for orchards across the state. Please keep covered before and, if coverage was questionable, after extended wetting events. In addition to mancozeb and/or captan, which are preventative materials only that must be applied to the leaf surface prior to initiation of infection, single-site materials especially in FRAC class 7 and 11 can not only provide some kick-back activity for up to 48 hours after wetting starts, but also provide good activity against rust and powdery mildew. That said, we still strongly recommend applying all materials prior to wetting events to avoid gaps in coverage and reduce development of fungicide resistance in apple scab fungal populations. At UVM orchards, we have been rotating class 3 (Vangard) and 9 (Flint) combined with mancozeb, next spray we will add a combination group 7 & 11 material to cover our bases more broadly with the suite of diseases that are active from pink through petal fall. Even though we spray no insecticides during bloom, some fungicides can still be harmful to pollinators, this guide can help to select products with lower toxicity.

Fire blight:

The bigger news is that warm to near-hot weather starting Saturday and running through next week will push orchards into bloom while also allowing fire blight bacterial populations to build up. This bacterial disease has been on the rise in the state in the past decade or so, and blossom infection is driven by four necessary components: 1) open blooms through which bacteria can enter the plant; 2) wetting events during bloom to move bacteria into susceptible tissues; 3) sufficient heat (daily average over 60°F) during an infection event to permit bacterial reproduction in susceptible tissues; and 4) sufficient levels of bacteria present going into bloom. The first condition is met just by having open blossoms in the orchard, remember that early or late blooming cultivars can extend the window. Predictions for rain mid-next week are increasing, so we may be heading into a substantial infection event.

The final condition required for infection to occur is a sufficient population of Erwinia amylovera bacteria present in the orchard. Bacteria are present in orchards in differing amounts, and may be present in wild trees surrounding the orchard. The Cougar Blight model used by NEWA to predict infection allows for multiple settings, including “no fire blight occurred in your neighborhood last year”, “fire blight occurred in your neighborhood last year”, and “fire blight is now active in your neighborhood”. Those settings can substantially affect infection predictions, so please adjust the model as appropriate for your orchard. If you had fire blight last year or even in recent (2-3) years, consider moving the slider up into the more conservative class if using the NEWA model.

Models require entry of a date for first bloom. This is not your full bloom date, nor is it applicable only to McIntosh. Accumulation of heat units to assess the epiphytic infection potential (a measurement of the potential population level of infective bacteria) is somewhat complicated, but suffice to say that calculations begin within a certain window of bloom and end when all blossoms are gone so earlier bloom date entries may increase EIP substantially. The point of the model is to run given the best conditions and data you have in your orchard and to use that information to make a sound decision. Being too conservative in the model may overestimate potential for infection (and the reverse is true as well).

NEWA models aren’t raising alarms yet, but remember that heat activity for the fire blight model is measured in degree hours, not days, so a short hot spell can really crank populations up to a damaging level. As we approach and enter bloom, be sure to check NEWA every day, and twice a day if it gets warmer than expected.

While we may not have imminent infection conditions it may be a good idea to line up any streptomycin, prohexdione calcium (e.g., Apogee or Kudos), and/ or Actigard. The latter two materials are preventative and will stiffen plant cell walls to prevent infection (former) or boost the plant’s natura defense pathways to help fend off disease (latter), and both show good efficacy especially when applied prior to bloom and followed by streptomycin. Strep needs to be applied to open flowers, but once a flower is treated it can be considered ;protected’ in most cases until petal fall. The trick is that blossoms continually open during bloom so those that opened after you apply strep are not protected, so I recommend spraying as close to infection as possible. This is a case where you must spray during bloom—it is the open flowers that you are applying the antibiotic to.

Cropload management:

Many orchard have a very heavy bloom with potential for an excessive crop; then again, many orchards suffered substantial damage during the April 20 frost. This leaves growers in a bit of a lurch. Do you apply Promalin to potentially frost-affected blossoms to encourage fruit set, or do you apply a thinner such as NAA to get a jump start on thinning? Do nothing and see what pollination does? I recommend a fairly thorough assessment of buds using a razor blade and magnifier. If you have a heavy bloom and less than 20% damage, consider applying a blossom thinner of 4 oz per acre of NAA. If you have substantial damage, consider Promalin to help set fruit. See my April 17 post for details and consider contacting Valent representative Jim Wargo for more details.

Insects:

Things are still pretty quiet but we except more activity as temperatures warm up. Be sure to have your codling moth, dogwood borer, and oblique banded leafroller traps out now and record first trap capture to set biofix for management.

Others:

It’s a great time to get herbicides and fertilizers down.

The UVM Tree Fruit and Viticulture Program is supported by the University of Vermont Agriculture Experiment Station, UVM Extension, USDA NIFA E-IPM Program, and USDA Risk Management Agency.

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