Apple Disease Update: Week of May 10, 2021

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Fire Blight Ooze on AppleThe destruction of the two cold snaps on fruit production and quality is beginning to come to light for many apple growers throughout the region. For the remainder of this year my plan is to send out weekly updates/management options for different production scenarios based on cultivar susceptibility to disease, % of crop remaining, and targeted markets for remaining apples. I will also include my usual “diseases to watch out for this week” segments. Unfortunately, I will likely be unable to account for all financial considerations or production questions in these short posts so please feel free to email me (sara_villani@ncsu.edu) for any follow-up questions/concerns!

Diseases to Be on the Lookout For…

Disease Concern 1: Shoot BlightShoot blight on apple

Regardless if you have an apple crop this year, I’ll put money on it that your trees still have leaves and shoots remaining. This young, succulent tissue is prime location for infection by the fire blight pathogen, Erwinia amylovora. Injury to this susceptible leaf tissue provides the means for bacteria to invade and progress down young shoots. Insects with sucking or piercing mouth parts, or other types of wounds created from environmental conditions (e.g. hail, wind, soil abrasion, etc). Application of a plant growth regulator, prohexadione calcium (marketed as Apogee or Kudos) has demonstrated the greatest efficacy against shoot blight in commercial plantings and in research trials. The chemical retards shoot growth, thereby reducing the amount of susceptible tissue available, and also thickens xylem cell walls acting as a barrier to the bacterium. At least two applications of prohexadione calcium should be applied during the season for shoot blight management. For mature trees a 6-12 oz/100 gal rate is recommended at 1-3″ of shoot growth and then 14-21 days later (considering the late season vigor in NC, I’d lean towards the 12 oz/100 gal rate). For trees less than 5 years old in which you are trying to fill the canopy, make applications at the same timings but at a reduced rate of 3-6 oz/100 gallons (again, I’d lean towards the higher rate, especially in trees 3rd leaf or older). Even if you have not yet made an application, I would still encourage starting now. Make sure to follow label guidelines on the addition of water softeners and penetrating adjuvant when making an application. Also, do not apply calcium in tank mixture with ProCa. Again, all trees regardless if there is a crop or not should be protected against shoot blight.

Disease Concern 2: Powdery MildewPrimary powdery mildew on apple

I am beginning to see signs and symptoms of powdery mildew popping up in my ‘Rome Beauty’ block. Often when I visit apple growers, ‘Jonagold’ is also one of the common cultivars in which I notice mildew symptoms. The powdery mildew pathogen overwinters in dormant buds that were infected during the previous growing season. Infected shoots with primary mildew (shoots in which dormant bud was infected) will be stunted, malformed, and have a white/silver color (see above). Secondary powdery mildew infections on foliage will appear as white, fuzzy tufts on either side of the leaf. Older lesions look burnt out and can take on a red/purple color. Leaves with severe infection curl inward. As leaves age they become increasingly resistant to infection.

Disease Concern 3: Bitter Rot and Glomerella Leaf Spot

GLS on Gala apple

To date, my level of concern about spraying fungicides specifically for Glomerella leaf spot and bitter rot has been fairly low. HOWEVER, this week will remain cool, and temperatures not likely to exceed 59 deg. F during wetting events (rain or dew). However, in warmer regions of the state, rain should be able to dislodge any mature spores that may be overwintering on leaf debris on the ground.

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/a) 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. However, spray program 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 and weather remains cool and dry, you may be able to get by with 14-21 day spray intervals. 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 weather remains cool and dry, you should be able to get by with 10-day spray intervals. 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-carry over 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. 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. Application should be made on 10 to 14-day intervals.
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.