2021 Apple Disease Update: Petal Fall
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!
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)