1. Apple Scab Infection Took Place During Rains 6 – 7 May; 2. Primary Apple Scab Season Not Over Yet

We are very close to the end of primary apple scab season but we are not there yet. The rains on 6 – 7 May we talked about at an in orchard Extension Meeting in Syria, VA, will likely be the last severe scab infections we had this season (Fig. 1). We call the end of the primary scab season when the apple scab fungus’, Venturia inaequalis, ascospore number in the spore bank in the leaf litter on the orchard floor drops below 5% (this can be determined easily if you click on “Venturia Table” button below the model and if you are subscribed and using RIMpro Venturia 3.0 disease model). Please see below how many scab infections we had so far in Winchester, VA (Fig. 1). Once, we reach the day when V. inaequalis ascospore number in the leaf litter on the orchard floor fall’s below 5% I will let you know via this blog. Finally, what is my recommendation for fungicide selection for this time of the year? Think about what was it that you applied the few last two covers sprays before? If you applied DMI fungicides the last cover spray, the next cover spray could be an SDHI (group 7) fungicide plus mancozeb. Some SDHI-s you can consider are Excalia, Sercadis, Miravis, Fontelis or any other stand alone SDHI material. Manocozeb will be effective for scab and any rust infections which are still taking place (infections 6 – 7 May) or eill take place with new rain events. SDHI will be effective for scab and powdery mildew only. If, on the other hand you did not apply a DMI in the previous two cover sprays, I would recommend a DMI fungicide now, plus mancozed. Good materials would be either: Cevya, Inspiter Super, Procure, Sonoma, Rally or Rhyme. They will be effective for apple scab, apple powdery mildew and cedar-apple rust. Always add into tank a contact multi-site fungicide with your single site fungicides like DMI or an SDHI so as to broaden the specter of efficacy of your tank mix and achieve a thorough kill of the germinating pathogen spores. Finally, use the apple scab infection prediction models – they are the only tool that can indicate to you when the infection will happen with RIMpro providing the added benefit of telling you the actual severity of the infection for each rain event – with it’s RIM value, which stands for Relative Infection Measure. When RIM value is divided by 100 it tells you the percentage of ascospores that successfully infected in each rein event. That is critical for selecting which fungicide(s) you will apply. You can read more about that in the article linked here: S. G. Aćimović, A. E. Wallis, M. R. Basedow (2018): Two Years of Experience with RIMpro Apple Scab Prediction Model on Commercial Apple Farms in Eastern New York. Fruit Quarterly, Winter issue, Vol 26 (4), pg. 21-27.

Figure 1. RIMpro apple scab model output for 2026 in Winchester, VA (historical data from an on site NEWA weather station). 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 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 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. Courtesy of RIMpro B.V., France.