Apple scab, what would we do without you?

By Terence Bradshaw

Here’s a quick note to remind everyone that we’re in the middle of an extended wetting and apple scab infection period that is likely to peak on Monday. Every orchard in the state still has mature ascospores in the leaf litter from last year, so stay protected. We’re also still in rust season, so it’s not a bad idea to make sure your fungicide products have some efficacy against that disease. If you’ve had more than an inch or so of rain between spray events, and especially if more than 1.5-2”, you should really think about applying a DMI (FRAC 3, e.g., Indar, Rally, etc., high efficacy against rust), strobilurin (FRAC 11, e.g., Flint high efficacy against rust), or SDHI (FRAC 7 e.g., Merivon, moderate efficacy against rust) with your protectant material (captan is good, no rust efficacy; or mancozeb, good rust efficacy but only allowed four applications through bloom at 6 lb/acre rate so you may have used that up). Remember to rotate between FRAC codes to avoid resistance development, and the mix products like Inspire Super (3+9) or Luna Tranquility (7+9) or Sensation (7+11) count as both when planning your rotations. That is, if you use two back-to-back application of Luna Sensation (7+11), your next spray including one of the single-site products should be a DMI (3) or anilinopyrimidine (9, which are less effective as temperatures warm) material. More information on fungicide resistance management is available in the New England Tree Fruit Management Guide.

For organic growers, the standard is still sulfur, and possibly judicious use of lime sulfur if you’ve missed some coverage and need some protection to reach backwards. Remember though, lime sulfur is nasty stuff, so use judiciously and take all necessary precautions. For rust, sulfur isn’t very effective. I have heard reports that Regalia, a biological fungicide, has some efficacy against rust so this is as good a year as any to try it out.

One great bit of news on the disease front- fire blight should be a complete non-issue this year. That could change for late-blooming cultivars or if active cankers are leading to shoot blight later on, but for now, it’s just too cool for any activity from the bacteria that causes the disease. NEWA’s Cougarblight model shows “high” risk for Sunday and Monday, but that’s to be taken with a grain of salt. There are four conditions necessary for blossom blight to occur: open blossoms (check); wetting (check); sufficient heat, measured in degree hours, over 60°F during the infection event (maybe); and sufficient heat leading up to infection to allow the bacterial population to increase to a level high enough to cause disease (nope). Using NEWA’s fire blight model, keep an eye on the “Infection Potential EIP Value”, that should reach 100 before you need to be concerned about blossom blight. Cougarblight assumes “High” risk if three of the four conditions are met, even if the fourth necessary condition (EIP in this case) is not likely to be met. We would need temperatures a good ten degrees higher than predicted tomorrow through Monday to move that EIP into any range to worry about.

Figure 1 NEWA fire blight model for Putney, VT. Most sites in the state will have even lower risk than this. Take-home: don’t worry about blossom blight now.

Orchards are in or entering bloom. That means a few things. First, get those blossoms and the ovules they’re attached to in good shape by applying a foliar fertilizer if you haven’t yet, I’ copying my own text from 5/5 here for that: 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. Organic growers may want to apply fish and/or seaweed products at this time.

Remember, bloom means bees, bees pollinate, and pollination = ovule fertilization = fruit. Protect them, both managed and on-managed (wild). That means avoid spraying anything, if you can, during bloom. Even some fungicides, DMIs in particular, have shown adverse effects on bees when applied to blooming orchard crops. Mow your dandelions to both force bees into the trees to work and to reduce their (the bees’) presence after bloom when you think about applying a petal fall insecticide. And for heaven’s sake, do not apply insecticides when there are flowers (apple blossoms or dandelions) in the orchard. The exception to that is Bt or other lepidopteran-specific products. On to insects…

Insect activity is really slow this season. The only insect captures of note have been European apple sawflies. If you’re at pink (i.e., ZERO blooms open) and have a population above threshold (five per white sticky trap, average), this may be a warranted application. But, application of insecticides so close to bloom is still a dangerous endeavor. Many wild bees are moving now, and since they often fly in cooler, wetter, windier conditions than managed honeybees, they may be especially important this year given the crappy weather we’re having. If you can hold off until post-bloom, I strongly recommend it.

Get your codling moth and obliquebanded leafroller traps up ASAP and check daily to set the biofix you’ll use to manage the larvae in a few weeks.

On a final note, herbicides, if used, can be applied any time, and application to weeds 6” or shorter is always more effective than waiting. Be careful with contact to trees, including mature bark, with glyphosate or glufosinate. Extra time spent is well-worth it to avoid trunk damage.

Are we having fun yet?

-Terry

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