Apple Scab –– Weather or Not

The first scab infection period of the year may occur later this week in the Hudson Valley, but  the probabilities of that happening depend on which weather forecast you prefer to believe.  The weather variable is the critical factor regardless of what model or predictive system you might be using, but I will illustrate the problem with the output from the RIMpro program.

This year I am evaluating RIMpro using two different data input systems, both of which are providing outputs for the orchard at the Hudson Valley Lab. One data input system uses our NEWA weather station at the Hudson Valley Lab (Highland) to log weather data as it measured by the on-the-ground equipment. This system uses the Norwegian “” weather forecast system for weather predictions.  I call this model system “Highland-N”

The other input system uses only the MeteoBlue data for both past weather and predicted weather; I call this Highland-MB.  This is a virtual station that never accesses any real data from our orchard. MeteoBlue is a Swiss weather forecasting system that provides data for all locations around the world.

In addition, I subscribe to Accuweather for weather forecasts and occasionally check the US Weather Service, which I can access through the NEWA Network. Thus, I can access four different forecasting systems when I want to know what to expect over the next few days.

Last night (Monday, 28 March) the output for Highland-MB indicated that a major scab infection period with a RIM value over 600 will occur April 1-2-3 whereas the Highland-N output indicated that the wetting periods April 1-2-3 will be too short to allow any significant infection, generating a RIM value of only about 40. (On the RIMpro graphs, rains are shown in the bottom bar-graph in dark blue with associated wetting periods in light blue, and anytime the RIM value shown by the red line exceeds 300, one can anticipate a significant scab infection period.)  Accuweather agrees with the forecast whereas the US Weather Service forecast agrees with MeteoBlue.

Below is a screen shot that compares the two NEWA forecasts alligned so that the dates correspond vertically, with the Highland-MB output uppermost in the photo. Note that MeteoBlue provides only a 7-day forecast whereas provides a 10-day forecast. As a result, the RIMpro output must be alligned by date, not by the edges of the model output graphs.

16-03-29pm RIMpro comparison

Here is the interesting part of the story: The spray window, based on wind conditions as predicted by US Weather Service will be Wednesday, starting about 1 AM and continuing until rains begin on Thursday morning.  Accuweather was providing the same wind forecast.

So, should Hudson Valley growers spray on Wednesday (as suggested by the RIMpro with MeteoBlue forecast), or can we expect to ride through the rains this week-end without getting a scab infection period?  The predicted infection periods are still too far away to get an accurate read (i.e., weather forecasts have a lot of error when they are for more than 48 hr in advance), but we should have a better idea by Tuesday evening.  Often the various weather models begin to converge as we get closer to the predicted event.

I pass along this info primarily to help everyone understand how I find value in RIMpro, but also to illustrate that the RIMpro output is no better than the weather forecast that it is using. If you are using RIMpro, it can give a valuable heads-up about the potential for major infection periods as they approach. However, because weather forecasts are often inaccurate, expecially beyond 48 hours, RIMpro will predict false-positives at times (as may be the case right now with the MeteoBlue forecast), or it may give false negatives as may be the case right now with the forecast.

For critical spray decisions such as many will be facing this week, it would always be wise to look at the weather data going into RIMpro  and then compare that with at least one other source of weather forecasts.  If you can wait to start spraying until 48 to 24 hours before the predicted infection event, then you will have more accurate predictions because the accuracy of weather forecasts is better when the prediction period is short.  However, I know that waiting until 24 to 48 hours before an event to make decisions often is not possible, either because of wind conditions or because of the time required to cover all acreage.

Right now, if I had my orchard at risk (i.e., without recent fungicide coverage), I’d be planning to spray on Wednesday given the risk levels as I see them, especially considering that the forecast includes warm temperatures (high 60’s Thursday and Friday) that will allow rapid bud growth, and given that the four weather forecasts that I have checked are evenly split on whether or not we will be getting enough rain and wetting for a major scab infection period.

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