Thoughts on pest management at pink bud stage in Vermont orchards

By Terence Bradshaw

Many Vermont orchards hit the pink bud stage yesterday, which is an important time for crop management. The main concerns should be: apple scab, fire blight, and early season tree nutrition.

For apple scab, NEWA is predicting an infection period tonight and into Saturday, so orchards should be covered with a protectant fungicide. We’re in peak scab season now, so vigilance is suggested. While applying fungicide, it wouldn’t be a bad idea to apply the standard ‘blossom boost foliar nutrient mix of nitrogen, zinc, and boron. Rates are dependent on the products used, and are intended to boost blossom vigor as the trees enter the stressful bloom period Dr. Wes Autio’s (UMASS) recommendations for Prebloom Nutrient Applications for Apple Trees: 3 lbs/100 gallons (dilute equivalent) urea; 1 lb/100 gallon Solubor (or equivalent); and label rates of zinc chelate. Ground-applied or fertigated fertilizers can also start to go on any time now.

On to fire blight. Remember that the disease requires a number of factors to cause infection: sufficient buildup of the bacterial population (a function of heat in the days prior to infection, known as epiphytic infection potential (EIP)); open wounds (or open blooms); rain or heavy dew to move the bacteria into infection sites; and sufficient heat at the time of infection for disease to occur. First, most orchards in Vermont did not have a serious, or any, fire blight issue last year, so overall inoculum is likely low. If you’re one of the growers who had infection in 2016, maybe be a little more vigilant because there’s the likelihood that you have a few extra cankers in the orchard that were missed during pruning. The EIP clock starts at bloom, but sufficient heat (like yesterday’s) leading into it can increase populations. The infective EIP used in NEWA (or really, Cougarblight, which is the model NEWA uses) is 100, and orchards in the major parts of the state hovered around that number yesterday and today. However, note that the more conservative Maryblyt model showed lower EIP for South Burlington for yesterday and today, and an infection was considered unlikely.

That’s mainly because we don’t have a single open bloom in South Burlington, so the model doesn’t really apply. Where orchards have their earliest blooms (on Zestar or Gingergold maybe? What’s the bloom status in Windham and Bennington counties?) going into today’s wetting period, there is a chance of a fire blight infection period. However, the shift to cooler temperatures starting after tonight’s front passage will lower EIP and reduce likelihood of infection. I still like to keep some streptomycin on-hand during bloom, but my gut feeling is that, except in very specific circumstances of high likelihood of inoculum carryover from previous infections and open blossoms going into today’s rain, there is no need to treat at this time.

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