Glomerella Leaf Spot Infections a High Risk in 2026 – Alternate Effective Fungicide Classes for Good Control

Based on the long range forecast for summer 2026, we will be expecting more rain in the following months. Glomerella leaf spot disease is a significant risk for apple production in apple orchards in Central and south Virginia. Based on literature, the strongest evidence supports that the main fungi that cause Glomerella Leaf spot are C. fructicola, C. chrysophilum, C. aenigma, C. karstii, and C. limetticola, with C. nymphaeae reported but more variable in pathogenicity. More work is needed to determine which of these species are infecting apple leaves in Virginia. These fungi can infect any areas not covered with an effective fungicide on leaves, especially when warm wetting events occur.  The oncoming extreme heat and rainy weather will favor infections by this disease during the spring end and the entire summer. This disease is favored by warm, wet and humid weather conditions. The infection pressure starts to build up at the end of May into early June in Virginia conditions and continues until the end of summer and early autumn. The infections on leaves usually occur when fungicide cover sprays are too far apart, while rains are more frequent and spray applications do not follow these rain events. This opens significant gaps in fungicide coverage allowing Colletotrichum species to infect. The frequent rains we had in the last 10 days in Virginia (Fig. 1) were favoring early Glomerella leaf spot pressure build up.

Figure 1. Historical weather conditions for Roseland, Virginia, in 2026 (Courtesy of RIMpro B.V., France)

With such favorable weather trends, as the apple leaf canopy size increases, you will need good protection with fungicides which should be effective for this disease and delivered according to the rain event frequency, not a fixed calendar spray schedule. One full fungicide cover spray will be worth: 14 days or 2 inches of rain, whichever comes first (if 2 inches of rain occurs before the 14 day mark, you should spray apply the next cover after that rain). If by any chance no rain falls for 14 days, you can extend the spray interval to 21 days, provided that no rain occurs during the 7 additional days. If rain event occurs any time during the additional 7 days, then apply fungicide close before or just after that rain event.

Infections on leaves not protected by fungicides could appear in the first week of June or July. Typical signs of Glomerella leaf spot are very small brown spots with angular margin, often having purple edges. With the leaf spot expansion, concentric rings are visible in the center of the brown grey lesion, and the leaves prematurely yellow and drop leading to canopy defoliation. The multiple concentric rings in the center of the lesion (Fig. 2) are a key telltale sign to discern this spot apart from other spots when infected leaves are observed.

Figure 2. Advanced infections of apple leaves with Glomerella Leaf spot showing concentric rings typical for this disease (Photo by Aćimović S. G., 2016).

The best cultural practices to reduce inoculum pressure from this disease are removing fruit mummies left over on the trees from last year. Mummies should be removed during dormant pruning. Decaying fruit on the orchard floor from last year should also be removed if possible or flail-mowed for faster degradation by soil microorganisms. Colletotrichum fungi can also overwinter in fruit buds, but more research is needed to prove that this occurs in the orchards of Virginia. The most effective option for control of Glomerella leaf spot are synthetic fungicides. Our trials led us to compile a list (see below) of summer cover spray combinations you could rotate, with no more than two consecutive applications of any of these combinations. In late spring we are still using not very effective fungicides for Glomerella leaf spot (e.g. Inspire Super; Prophyt), with the rationale being that there is still strong pressure from cedar apple rust, powdery mildew, and scab, to protect leaves from. Also, a good strategy is to use contact fungicides like Ziram or Ferbam late in spring and early in the summer as they leave more visible residue. Later in the summer apply effective contact fungicides with less visible residue (Captan). I list here fungicide combinations with the amounts per acre for 13 cover spray options, but your total cover number might be lower depending on your location, cultivar maturation, and number of rain events. Also the order of cover sprays can change as long as you rotate fungicides of different classes from cover to cover, e.g. group 1 like Topsin, with group 7 or SDHI like Aprovia, with group 29 like Omega, and with group 11 like Merivon, Luna Sensation, Pristine, and Flint Extra. Keep in mind that the first cover spray below is designed for time when pressure from Glomerella is not yet high. These spray recommendations take into account not only control of Glomerella leaf spot but also a good control of apple bitter rot, that is if you have any fruit left after the unfortunate two freeze events in April:

• 1C: Inspire Super 12 fl oz/A + Captan 80 3 lb/A or Mancozeb 3 lb/A**
• 2C: Topsin M 1 lb/A + Captan 80 3 lb or Mancozeb 3 lb/A
• 3C: Prophyt 64 fl oz + Captan 80 3 lb or Mancozeb 3 lb/A
• 4C: Topsin M 1 lb/A + Aprovia 5.5 fl oz (30 days PHI) + Captan 80 3 lb/A
• 5C: Aprovia 5.5 fl oz (30 days PHI) + Captan 80 3 lb/A
• 6C: Topsin M 1 lb/A + Flint Extra 2.9 oz (14 days PHI) + Captan 80 3 lb/A
• 7C: Omega 13.8 fl oz (28 days PHI) + Captan 80 3 lb/A
• 8C: Omega 13.8 fl oz (28 days PHI) + Captan 80 3 lb/A
• 9C: Topsin M 1 lb + Flint Extra 2.9 oz (14 days PHI) + Captan 80 3 lb/A
• 10C: Kenja 12.5 fl oz (20 days PHI) + Captan 80 3 lb/A
• 11C: Merivon 5.5 fl oz + Captan 80 3 lb/A
• 12C: Tidal Grow Spectra 22 fl oz/A + Captan 80 3 lb/A
• 13C: Merivon 5.5 fl oz + Captan 80 3 lb/A

**Always tank mix a single site fungicide with a multi-site fungicide (e.g. Ziram, Mancozeb or Captan). Keep in mind I am not against using the higher rate of captan. I just want it to stretch it out as much as possible to not break its per year limit of 40 lbs/A per year. Stop using mancozeb at 77 days before harvest (mancozeb has 77-day Pre-Harvest Interval or PHI when it should not be applied).

If you have fruit on trees, the following recommendations will be of use to you: Use the larger pre-harvest interval (PHI) period fungicides during June and throughout July and August so you reserve your zero days PHI Merivon for the last cover sprays just before harvest. Note that I am suggesting to you above to use very effective fungicides against Colletotrichum species, like Flint Extra, Aprovia and Omega, early in the summer due to high pre-harvest intervals of 28 and 30 days that these fungicides have. Please limit group 11 fungicides only to 4 applications per season in total (four underlined fungicides above) to avoid development of fungicide resistance in Colletotrichum population species. This does not mean you can apply 4 applications of each of these materials: Flint Extra, Luna Sensation, Pristine or Merivon. It is the opposite, since they all belong to the same group 11 fungicides which means that they have the same mode of action in the fungi, you can only use 4 applications in total of any of the fungicides in this group 11, per growing season. You can use either one group 11 active ingredient or choose different group 11 active ingredients, just as long as their total number of applications does not exceed 4 in total per growing season (whichever is cheaper for your budget). My advice is to save Merivon or Pristine for your last, i.e. fourth group 11 fungicide application due to the zero days pre-harvest interval of these materials. Captan can be replaced with ziram and/or ferbam. Do not apply Ziram within 14 days of harvest. Do not apply Ferbam within 7 days of harvest (Ferbam is usually not recommended for late season cover sprays on fresh market fruit due to the resulting dark reside spots visible at harvest). Flint Extra, Aprovia, Omega, Inspire Super, Topsin M, Merivon, Flint extra, Pristine, Luna Sensation, Ziram and Ferbam are also effective for control of sooty blotch and flyspeck. Always tank-mix these single site fungicides  with either captan, ziram or ferbam. This allows expanding the spectrum of efficacy of your tank mix and is a key anti-resistance strategy for single site fungicides.