2021 Apple Disease Update: Petal Fall

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Petal Fall on Gala

On most cultivars in Western NC, petal fall is quickly approaching or has come and gone without too much disease concerns to this point. In the south, petal fall is normally the time we begin the transition from intensive “early season” disease management to intensive “summer disease”  management programs. However, given the cool and relatively dry weather throughout much of April, I’d strongly encourage you to continue to focus on early season diseases-in particular apple scab. Similarly to this time last year, there have only been 7 primary scab infection events so far in 2021. Unlike recent years in which ascospores (overwintering spores that cause the primary or initial infections) have fully matured and have been released by the petal fall stage, there are still ascospores that will be released during the next daytime rain (rainfall greater than 0.1 inch). This next rainfall, is predicted for April 23, so you still have a few days to take action!apple scab on fruit

There are several fungicide options for managing apple scab, including, but not limited to mancozeb + captan (applied at half rates), the FRAC 3 (DMI) fungicides such as Inspire Super or Cevya, single active ingredient FRAC 7 (SDHI) fungicides such as Aprovia, Miravis, Excalia, and Fontelis, FRAC 11 (strobilurin) fungicides such as Flint Extra and Sovran, and several pre-mix options that contain two active ingredients (e.g. Merivon, Luna Sensation, Luna Tranquility). Certainly what you decide to apply depends on several factors including resistance history, budget, and pressure from other diseases. Here are some of my tips based on observations from our research:

  • Always tank mix a single site fungicide with a half rate of captan or mancozeb. In North Carolina, I’d advise using mancozeb at the 3 lb/A rate until the 77 day PHI or until you’ve applied the seasonal application limit of 21 lbs at this rate (whichever comes first)
  • Inspire Super and Cevya have higher intrinsic activity against apple scab compared to other FRAC 3 fungicides with the same mode of action such as Rally 40WSP, Procure, Topguard, and Indar 2F. In orchards with resistance to the latter fungicides, Cevya and Inspire Super can still provide a high level of control, particularly when tank mixed with a half rate of mancozeb or captan.
  • Although the strobilurins (FRAC 11) and premixed products containing a strobilurin (e.g. Merivon; Luna Sensation) also have high efficacy against scab, I’d save them for Glomerella leaf spot and bitter rot control.
  • In general, SDHI fungicides are highly efficacious against apple scab. In our field trials, Aprovia and Miravis have provided the greatest reduction of leaf and fruit scab. (see below)
Performance of SDHI Fungicides against apple scab on apple
Since the weather has been relatively cool, I’m reluctant to suggest adding any GLS/bitter rot fungicides (i.e. the strobilurins) to the mix just yet. If there is not a history of resistance to DMI fungicides in your orchard, I’d be inclined to rotate a DMI with an SDHI fungicide (+ mancozeb) from now until 2nd cover.
shoot blight on apple
Management for the shoot blight stage of fire blight should also begin around petal fall. In order for shoot blight to occur, their must be inoculum present within or near an orchard (i.e. there must be a local inoculum source). Thus, even just a few blossom blight infections can serve as a strong catalyst for shoot blight infection. Shoot blight is the result of infection to young, emerging leaf tissue. 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. Prohexadione calcium 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). 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.