I’ve heard some confusion about apple scab this week, and chatter that the apple scab model in NEWA isn’t working correctly. Please let me reiterate what I said a few weeks ago. Models are just tools that helps understand what’s going on in the orchard, based on data (from weather stations, generally) and assumptions (about biology). NEWA is great, because it outputs a clear, green/red indicator of a disease infection event. But neither the data going into those models nor the biology in the orchard are that cut and dry.

TLDR: Keep your orchard covered with fungicide this week. Period.

The small, intermittent rains and wetting periods can add up to a large spore release and infection if you’re not covered. Because of the intermittent nature of this week’s rains, NEWA models appear to be correcting (maybe changing is a better word) almost daily, and orchards within short distances of one another show different results. None of these events so far this week has added up to one major infection event, but cumulatively, you could have a decent infection on your hands, and if you’re waiting for NEWA to scream red, you could be too late. Also, you run the risk of facing poor spraying conditions or just not having buildup from prior applications should a decent wetting event come. It’s one thing if you can spray your orchard in an hour, another if it takes a day. Keep the coverage on.

The NEWA scab models require a number of assumptions. First, ascospores, which are released from overwintering inoculum from last tear’s infections, mature according to a fairly reliable growing degree day accumulation. If that incoming data gets off (as happened on some stations in the region, since fixed), or a sufficient dry period occurs, the model may not accumulate development in-line with what’s happening in the field. Once we have mature spores, they need a wetting event, typically considered between 0.01-0.1” of rain, to be released from the leaf litter. More release in the daylight than at night, but a small percent of ‘night spores’ from a large inoculum can be significant. Then we need the leaves to be wet for a certain number of hours depending on the temperature. And those wetting hours can be cumulative across a number of days if the dry periods in between are less than 24 hours. This makes for a pretty complicated model.

This also makes for some potentially bad decisions if we’re waiting for this model to give us an overly simplistic change from green to red, when in reality, there are gradations within that model determination, and fuzzy distinctions and limits among all of those assumptions mentioned.

That said, the NEWA scab model is indeed running as it should. The earlier quirks have been dealt with, and I expect that any delayed ascospore maturation from dry spells has caught up to the statistical confidence interval built into the model.

We’re in the peak of primary ascospore season, everywhere in the state. There are chances of showers most days this week. While I applaud good IPM, this is not the time to cut corners. If you have a chance to stay covered with even a basic rate of captan / manzozeb (or sulfur if organic), then do so. Once a week is good, but don’t get caught off-guard. Warmer weather this weekend and early next week also brings potential fire blight infection. Keep an eye on that model daily if you have anything still in bloom.