Apple Disease Update: Week of May 17, 2021

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Fire blighted blossoms on apple

A brief update for this week since hopefully most attended the Petal Fall Twilight Meeting last week in Henderson County. There are a few diseases to keep in the back of the brain as week-especially as temperatures are rising and falling only into the mid to upper 50s during the night hours. Although there is only a slight chance of rain in the forecast for Western NC over the next few days, leaf wetting hours can and will accumulate as a result of dew formation. Coupled with these warmer night temperatures, this can be a pathogen’s paradise.

The first disease I believe is worth highlighting is fire blight-both on the blossoms and shoots. Funny story-we conducted a blossom blight trial in a block of ‘Gala’ this season and went out to rate the trial this past week. Sure, this was a trial in which we evaluated biological products, but it was unusual that for nearly every treatment we were only seeing a 10% reduction in blossom blight incidence at best in the majority of the treatments. Usually this number is around 50%. We then realized, that our numbers were skewed because only blighted flowers were left on the tree as all of the non-infected clusters had fallen off due to the double freeze! I mention this because a few blossom infection can really start a local fire blight epidemic in your block and haunt you for years to come. Especially if you aren’t visiting/scouting these blocks as frequently because there is no crop in them! If you do find blossom infections, remove and destroy wood no less than 12 inches from the visible symptoms. I’d also suggest applying a 12 oz/100 gal rate of prohexadione-calcium (Kudos, Apogee) prior to and 10 to 14 days following infection removal.

Of course with the warming weather, my other concern is certainly Glomerella leaf spot and bitter rot. In many trees without a crop, foliage is looking quite dense, increasing the leaf drying times. In our block where we conducted fire blight trials this year I observed the other day what appeared to be early Glomerella leaf spot: Purple specks distributed throughout the upper leaf surface.

Early Symptoms of Glomerella Leaf Spot on Apple Leaf

I went back out today and observed more advanced lesions:

More advance GLS lesion

This ‘Gala’ block has only had a few fungicide applications this season, but I’ll continue to stress that even if you do not have a crop, make sure to continue fungicide applications at slightly extended intervals throughout the spring and summer! You’ll thank yourself next year.

Disease Management Plan for: “I have a crop that’s going to fresh market”

If this is your situation, from a disease management standpoint my suggestion would be to continue with a normal disease management program as you would in during any standard growing season. I realize that financially, if this a reduced crop, then perhaps including some of the more expensive chemicals such as Merivon or Inspire Super may not be feasible. You will likely able to get by with a protectant fungicide program of captan + mancozeb (stay within seasonal limits and PHI) on a seven to ten-day interval. Consider incorporating a DMI fungicide (Procure or Rally) or sulfur to help with mildew control over the next few weeks. Also, consider adding Topsin or a generic thiophanate methyl fungicide for flyspeck and sooty blotch management (every other spray).

Primary Diseases to Manage: Apple scab (secondary), powdery mildew, frogeye leaf spot, Glomerella leaf spot, bitter rot, shoot blight, flyspeck, sooty blotch
Fungicide Options: Captan 4L (2 qt/Aa) OR Manzate Max (2.4 qt/A) OR Manzate Max (2.4 qt/A) + Captan 4L (2 qt/a) OR 1/2 rate of captan or mancozeb + Rally 40WSP (10oz/A) or Procure 480SC (12-16 floz/A) OR 1/2 rate of captan or mancozeb + Topsin M (0.75 lb to 1 lb/A)
Disease Management Plan for: “I have a crop that’s headed for processing”
If this is your situation, from a disease management standpoint you still need to grow rot-free fruit. However, it may not make sense financially to apply single-site fungicides nor should a top concern be fruit finish. Spray programs will depend on cultivar in this situation.
Not Susceptible to Glomerella leaf spot and not highly susceptible to bitter or black rot: If most of the fruit remaining are from cultivars that are not highly susceptible to GLS, bitter rot, or black rot given the warming temperatures, I’d suggest a fungicide application this week if you DID NOT apply one last week. A protectant fungicide program of Captan 4L (2 qt/A)+ Manzate Max (2.4 qt/A) (stay within seasonal limits and PHI) or simply Captan 4L (2 to 4 qts/A) may be sufficient. On cultivars highly susceptible to powdery mildew, consider tank-mixing sulfur (Microthiol Disperss 10-20 lb/A).
Susceptible to Glomerella leaf spot and/or Bitter Rot: If most of the fruit remaining are from cultivars that are highly susceptible to bitter rot or black rot and given the warming temperatures this week, my suggestion would be to apply a fungicide application this week-especially if you bypassed it last week. In general you should be able to get by with 10-day spray intervals throughout this season. A protectant fungicide program of Captan 4L (2 qt/A)+ Manzate Max (2.4 qt/A) (stay within seasonal limits and PHI) or simply Captan 4L (2 to 4 qts/A) may be sufficient. On cultivars highly susceptible to powdery mildew, consider tank-mixing sulfur (Microthiol Disperss 10-20 lb/A). If the weather remains cool and dry, consider increasing the application interval to 14 days, but keep in mind, there may be some break-through infection.
Disease Management Plan for: “I don’t have much of a crop and I don’t plan on harvesting the limited fruit I have”
If this is your situation, there will be no fruit diseases to manage, and the inoculum carryover to any remaining fruit that may become infected will likely be negligible. Your game plan should be management of foliar diseases: Alternaria leaf blotch on ‘Red Delicious’, powdery mildew, Glomerella leaf spot, and frogeye leaf spot. Again, given the risk of carry-over inoculum (in 2022) with Glomerella my suggestion is the following:
Cultivars Susceptible to Glomerella leaf spot: A protectant fungicide program of Captan 4L (2 qt/A)+ Manzate Max (2.4 qt/A) (stay within seasonal limits and PHI) or simply Captan 4L (2 to 4 qts/A) is sufficient. If you sprayed last week, I’d skip this week and plan on spraying next week. If we do get a surprise rain, you may want to consider an application after the wetting event. On cultivars highly susceptible to powdery mildew, consider tank-mixing sulfur (Microthiol Disperss 10-20 lb/A) every other spray until new foliar growth is no longer observed.
Cultivars NOT Susceptible to Glomerella leaf spot: A protectant fungicide program of Captan 4L (2 qt/A)+ Manzate Max (2.4 qt/A) (stay within seasonal limits and PHI) or simply Captan 4L (2 to 4 qts/A) is sufficient. On cultivars highly susceptible to powdery mildew, consider tank-mixing sulfur (Microthiol Disperss 10-20 lb/A) every spray or every other spray until new foliar growth is no longer observed. Application should be made on 21 to 28-day intervals.

Written By

Sara Villani, N.C. Cooperative ExtensionDr. Sara VillaniExtension Specialist (Apple and Ornamental Plant Pathology) Call Dr. Sara Email Dr. Sara Entomology & Plant Pathology
NC State Extension, NC State University
Posted on May 18, 2021
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