Apple Disease Update: Week of May 22, 2022

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Blossom Blight on Gala

Nothing like the heat and the rain to make the pathogens start to party in the orchards this week! The photo above is from our biopesticide blossom blight trial this season. Two weeks ago, any disease pressure and now we are on a race to remove it before it reaches the scion. Two diseases I’ll focus on this week strictly because we have been observing them regularly in the orchard. The next is our foe Glomerella and bitter rot, which I have not yet observed, but as you know, infection events occur prior to symptom development….

Scouting for shoot blight should be of primary importance right now. Sometimes in a sea of fruit, minor blossom infections can go unnoticed. While many leaves in a blighted shoot may appear brown, one tell-tale sign of systemic infection by the Erwinia amylovora is a necrotic on blackening mid-vein on leaves just becoming symptomatic. Blackening midvein symptom of fire blight

If you notice an outbreak of blossom blight or shoot blight consider taking the following steps:

  • Try to avoid pruning first thing in the morning as dew and bacterial ooze might be present which can lead to spread of the pathogen causing new infection
  • Consider applying a 12 oz/100 gal rate of Kudos or Apogee to slow down systemic movement of the pathogen. This should also help to prevent additional infections. Make sure to add a water conditioner and a surfactant such as Regulaid.
  • After applying Kudos or Apogee, wait until at least the REI is up (12 hours), prune out infections at least 12 inches from the advancing symptom margin
  • Follow up with a streptomycin application. Consider the 200 ppm rate and including Regulaid or similar adjuvant. Remember strep has a 50 day PHI.

Next, let’s move onto powdery mildew which usually doesn’t get too much attention. However, in my young trellis block, primary mildew is emerging with a vengeance which could compromise photosynthesis and shoot growth. On older trees, although not too common, the powdery mildew fungus can also cause net-like russeting. Primary Powdery mildew on apple

Common summer protectant fungicides such as Captan, Mancozeb, or Ziram, really don’t have great, if any, efficacy against apple powdery mildew. In highly susceptible blocks such as Rome Beauty or Jonagold, or in young plantings, you may want to consider including sulfur (don’t apply above 85F) or less stinky options including:

  • Rally 40WSP
  • Procure
  • Topguard
  • Gatten

Products containing strobilurins (FRAC 11) are also highly efficacious, but make sure to save them for GLS/bitter rot sprays.

The last disease to mention today is our nemasis-Glomerella leaf spot and bitter rot. Although I have not observed any symptoms in our research orchard to date, in previous years, symptoms have appeared as early as Memorial Day weekend in Henderson Cty, NC. If you haven’t applied a strobilurin containing fungicide such as Merivon, Pristine, Luna Sensation, or Flint Extra, this week may be a well-timed application for you. Remember across those four products (or Sovran) you only hae 4 applications total per season. In general we have found that while the four aforementioned fungicides have similar protective efficacy against the bitter rot/GLS pathogen, Merivon and Pristine do have better kickback activity (don’t expect more than 24 hours).