Revisiting Scab Risks for This Week

The scab infection risk for April 1-3 in the lower Hudson Valley may be waning, although caution is still advised. As suggested in my earlier post this morning, weather predictions that vary widely for events that are three or four days into the future often converge as the event of interest approaches.  The RIMpro output using the MeteoBlue weather input now shows a shorter predicted wetting period for April 1-2 than it did this morning, and that change has resulted in a lowering of the forecasted RIM value from over 600 to a bit under 400 as shown below. The changes will be apparent if this graph is compared to the one that I posted in a previous message earlier today.

16-03-29pm RIMpro-MB graph


A RIM value of nearly 400 is still a concern, but given that the MeteoBlue forecast is shifting toward the forecasts for shorter wetting periods that came from other weather sources (see my previous post), my bet is that further reductions in wetting periods will be forthcoming from MeteoBlue and that the predicted scab infection period for April 1-3 will mostly disappear. Most cultivars in our orchards at the Hudson Valley Lab have not progressed beyond quarter-inch green. Putting it all together, I am reversing my earlier opinion about scab risk by suggesting that, given current information, the forecasted rains on April 1-2 now seem unlikely to produce significant scab infections in orchards that were scab-free last year. However, that’s still a judgment call and the decision on whether or not to apply a fungicide tomorrow will largely depend on one’s personal approach for risk management.

Those who have not yet applied any sprays (neither copper nor a fungicide) may wish to hedge their bets by applying a spray on Wednesday to at least their higher-risk blocks (i.e., scab-susceptible cultivars, sites with advanced bud stages, orchards with slight amounts of carry-over inoculum). However, one point that I had overlooked when I made the earlier post this morning is that temperatures tonight are predicted to drop below freezing and may not rise above freezing until sometime near sunrise tomorrow. Applying sprays to frozen foliage is never a good idea, so the cold weather tonight may shorten the spray window for tomorrow.

In most blocks, it is still not too late to apply copper (in terms of the bud stage), but applying copper to green tissue shortly after a freeze may result in damage to early cluster leaves, and that damage will be even greater if oil is included with the copper. Where there is a significant freeze tonight, it may be best to stick with mancozeb as as a fungicide for any applications made tomorrow morning.

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