March 22, 2023

To Spray or Not to Spray? Apple Scab Infection Predictions for 23 March in Bristol, for 25 March in Rustburg and Tyro, for 24-25 March in Winchester

If the weather forecast for 23-25 March becomes a reality, and we do get rain events predicted, according to NEWA apple scab model we might get a weak to mild apple scab infection(s) on 23 March in Bristol, 25 March in Rustburg and Tyro, and on 24-25 March in Winchester. According to RIMpro model these rain events do not warrant a fungicide application for scab even if you had visible apple scab symptoms in your blocks last year (leaves or fruit). Unlike the NEWA scab model, RIMpro apple scab model allows you to actually determine the fate of the groups of released apple scab ascospores during these rain events and predict to you how severe the infection by them will be. And then you can gauge your response in spraying or not spraying ahead of these events, or in selection of fungicides you will use – just a contact fungicide like mancozeb if the infection is medium i.e. 100-300 RIM Infection Value (Fig. 1 example below for year 2022) or a combination of Scala or Vangard or Luna Tranquility plus mancozeb if the infection will be severe i.e. 300 RIM Infection Value or more (Fig. 1 example below for year 2022). If you had visible scab symptoms last year in your orchard, you should spray with a fungicide for predicted, early-season infection events that are at or above 100 RIM value (e.g. red curved line going to the dashed line of 100 RIM value on the RIMpro graph scale “RIM Infection Value” on the left, means that only 10% of seasonal ascospores will be infecting during that wetting event, Fig. 1 below for year 2022).

Right now, if the NWS’s weather forecast stays the same on the actual days of 23-25 March 2023, the RIMpro is not calling for an infection above 100 RIM value – you can only see the 2023 model output if you subscribe to RIMpro: https://www.rimpro.eu/ (note that the Figure 1 below is from 2022 used as an example, not 2023). Hence, as per RIMpro outputs I see as a coordinator of a group of subscribed growers for Virginia, these rain event 23-25 March are not warrantying a fungicide spray application even in the last year’s orchards that had scab, if the weather forecast stays the same like it is now. Farms that had NO visible scab symptoms in the apple orchard last year, leaves and/or fruit, should spray only for infection periods at or above 300 RIM values (High infection risk dashed line threshold in Fig. 1 below as an example). Thus, in these “clean” orchards also, if the weather forecast holds, there is no valid reason to apply fungicides ahead of rain events of 23-25 March. If rust is your major concern, take a look if you see orange galls on cedars or not, and if yes, mancozeb alone should be effective this early in the season. However, if scab is your major concern, keep watching at the forecast and models to see if the weather or scab predictions change to severe infections or not. Consider subscribing to RIMpro (one subscription to RIMpro and connecting one NEWA weather station to RIMpro costs $330 in total). If you will only use NEWA’s apple scab prediction model (see images below the Figure 1), major infection events are when there is >15% ascospore discharge predicted. So as you can see the below print screens, Rustburg is the only one showing 20% ascospore discharge predicted with rains on 25 March. NEWA scab model is available here: https://newa.cornell.edu/apple-scab