How Delayed Dormant Copper Impacts Apple Scab and Fire Blight?
In terms of delayed dormant copper, so far we have always operated under the assumption that the delayed dormant copper application is effective as one mancozeb application for apple scab. Before thinking about starting a scab spray program, one must determine if they have cultivars at Green Tip or not and did they see apple scab symptoms last year in the orchard or not? We define the Green Tip (GT) as the date at which on your earliest apple cultivar of interest, that is susceptible to scab, the trees reached 50% of fruit buds at green tip. In southern Virginia, Carrol county, I have a report we are beyond green tip on early cultivars. Green Tip was on 2nd March there. In Roseland, VA, green tip was reported to me on 9 March, so if you are there likely you are already spraying copper.
Infection from apple scab can occur on any exposed green tissue, starting from GT, if the scab ascoposres in apple leaf litter on the on the orchard floor are mature, and a rain event(s) occurs. The best way to determine if your trees are at green tip is by randomly looking at 100 floral buds (not vegetative or terminal buds) on at least 10 or 15 trees, and tallying up in tabs of five on a piece of paper the buds in dormant, silver tip, and green tip stage categories. If you get 50 out of 100 of all buds you look are at green tip, you record that day as Green Tip stage date. The best way to predict apple scab infection using the weather forecast are the models like NEWA scab model is available here: https://newa.cornell.edu/apple-scab (free) and RIMpro model available here: https://www.rimpro.eu/ (subscription based, and not that expensive, since to connect one weather station the cost is ~$299 which is equivalent to 250 EUR). I would warmly recommend RIMpro model as it offers opportunity to reduce fungicide applications for scab control based on my experience.
If you had visible scab symptoms on apple leaves and/or fruit in 2025, both early or late in the season, you will need to apply fungicides starting at green tip (GT) and not skip the following applications at 5 to 7 days spray interval during the following stages: quarter inch green (QIG), half-inch green (HIG), tight cluster (TC), pink bud (PK), bloom (BL), petal fall (PF) and first cover (FC). The reason why such strict schedule is necessary is because you are starting the 2026 growing season with a high apple scab fungus spore dose in leaf litter on the orchard floor which means that the carryover infection pressure is high from the previous season. Your delayed dormant copper applied from GT to QIG is equal in efficacy to one mancozeb spray application, so no point in doing both, one right after another.
Delayed dormant copper can reduce fire blight bacterium inoculum which reaches bark surface from cankers during warm weather in spring in the form of exuded orange ooze of the bacterium or as ooze-less bacterial colonies. Copper will also affect any apple scab fungal mycelium overwintering in buds. The usual rate is 6 to 8 lbs of some of the popular products like Cuprofix, COCS, NuCop (fixed coppers, 40 to 50% copper equivalent). However, if you had fire blight last year or heavy rains are predicted in spring, it is wise to use the highest labelled rate of copper for delayed-dormant use, so that some residue lasts on the tree through the rain. If fire blight was not an issue in your or any nearby orchards and you do not have fire blight cankers on your trees, you can apply mancozeb instead of copper. Keep in mind that maximum rate of copper allowed between silver-tip and green-tip stage is that of Basic Copper 53, limited to no more than 11.3 lbs of product/Acre (6.0 lbs copper/A), which might be something you can consider if you had fire blight and/or bitter rot in your orchard last year (if can help with reducing overwintering inoculum). Each label is different in terms of when the cut off growth stage is for delayed dormant application, so read the label, it is the law.
In 2025 (last year), the first major scab infections were recorded based on the RIMpro’s advanced apple scab prediction model on 3 April in Winchester (GT: 14 March 2025 on ‘Gala’) (Fig. 1). You have to monitor prediction models daily to determine if early scab infections are possible, especially in orchards which had visible scab symptoms last year.
Figure 1. RIMpro apple scab model infections reported in 2025 for Winchester, VA (actual historical data from a weather station used in this view). White camel hump-like areas labelled “Germinating spores” show cumulative number of Venturia inaequalis ascospores that germinate over time and are read using the right-side vertical Y-axis scale that is labelled “Discharge”. The vertical red curved lines are scab infections. Read each red curve’s peak RIM infection value(s) using the vertical Y-axis scale on the left-hand side of the graph labelled “RIM Infection Value”. Peak RIM value divided by 100 gives you the percentage of the total season’s ascospores that will cause infection for a given infection period (wetting event). Orange area labeled “Primary stroma” represents scab lesions that were initiated by infection from germinating spores and that are incubating in the leaf after which scab lesions will become visible. This is worth knowing because if no fungicide was applied before the infection started, some or all of the incubating infections can be eliminated by using fungicides with post-infection activity. The light red area in the middle graph labeled “Maturation” is the proportion of mature ascospores that are ready for discharge with wetting events whereas the dark red area shows the proportion of immature ascospores still remaining in leaf litter on the orchard floor. The dark blue bars in the bottom graph showing wetting and dates, are the actual rain periods. The light blue bars are actual wetting periods when no rain is falling but trees are still wet after rain. Used by permission of RIMpro B.V., France.
In 2026, apple scab RIMpro model for Virginia has not yet predicted that an infection is possible. We cannot report RIMpro’s prediction(s) for first scab infection publicly as this model is subscription based and the data shown on the RIMpro personal account is the ownership of the subscriber and not for public sharing. RIMpro has the best apple scab infection prediction model on the market and is well worth the subscription paid. It follows the destiny of each group of released ascospores and uniquely gives you severity of infection for each rain event (aka RIM value, where RIM stands for Relative Infection Measure).
However, the NEWA’s freely available apple scab model, has a prediction that a very weak scab infection can occur on 12 March in Roseland VA. If you will use only NEWA’s apple scab prediction model, major scab infection events are when there is >15% ascospore discharge taking place or predicted (so, if you ask me, and I use RIMpro, I do not think the 12 March rain in Roseland VA will be an actual infection worth being worried about. Copper you already applied or will apply will take care of it).
NEWA’s scab model is available here: https://newa.cornell.edu/apple-scab Please get yourself accustomed to at least using this model, it is FREE. All you need is to type in the green tip date on your earliest apple cultivar on farm susceptible to scab. Here is the screen shots of the weak infection on 12 March in Roseland, VA, I mention above in NEWA scab model:
