Thursday morning, May 1, 2014: Rains over the past two days exceeded 2 inches and have almost certainly removed all protection from fungicides that were applied prior to the rain. Orchards that were sprayed after Saturday morning of last week (let’s call them Case-A) should have been well protected through the major scab infection period that we just experienced. In these orchards, one could presumably continue with protectant-only programs, although personally I would hedge my bets and include one of the more powerful fungicides in my next spray, both to pick up any scab that has been missed and to protect against powdery mildew.
Orchards that were last sprayed prior to Saturday morning, April 25, (Case-B orchards) almost certainly need a spray that will provide post-infection activity. In Case-B orchards where DMI fungicides are still effective, the best option will be an application of either Inspire Super-plus-mancozeb or Rally-plus-mancozeb sometime between now and sunrise on Sunday morning. I suggest that this follow-up spray with a DMI can be as late as Sunday morning because even weathered fungicide residues in Case-B orchards probably protected trees through the first 12-15 hr of the Tues-Thurs rain event. The DMI fungicides will provide 96 hr of post-infection activity, therefore cleaning up infections back to Wed morning if applied before sunrise on Sunday morning.
In Case-B orchards where Rally (or Nova, Rubigan, Vintage, etc.) have failed to provide scab control in the past, options are more complicated because other fungicides will provide only 48 to 72 hr of post-infection activity. The upper limit for reach-back may be closer to 48 than 72 hr for this event because of the warm temperatures today. If one assumes that post-infection sprays in Case-B orchards should reach back to dawn on Wed morning when the second big batch of spores were released and fungicide residues may have been depleted, then the deadline for getting that spray applied will be sometime between dawn and noon on Friday morning.
For these Case-B orchards, I would suggest that mancozeb plus one of the new SDHI fungicides may be the best option at this time. The SDHI fungicides include Fontelis, Luna Tranquility, and Merivon in New York (except for Long Island where none of these are registered). Add Luna Sensation to the list of available SDHIs in other states. The SDHI-mancozeb combination will take care of scab that may be resistant to other chemistries and will provide excellent control of both scab and mildew, along with good control of rust diseases, which are also active at this point. Other options for 48 hours of post-infection activity include Vangard (alone or as included in Inspire Super), Scala, and Syllit, but none of those products except Inspire Super will control rust or mildew. Flint and Sovran may also provide 48 hr of post-infection activity, but their reach-back activity begins to fade as the scab populations shift toward resistance to these products, and I therefore consider them unreliable for post-infection activity.
Where post-infection activity is needed, excellent coverage is extremely important because the product being applied must hit all of the leaves where scab infections have been initiated. Post-infection sprays do not have the benefit of redistribution from leaf to leaf via rain as occurs when protectant fungicides are applied ahead of infection periods. Therefore, post-infection sprays are best applied during periods of low wind. Forecasts suggest that low-wind conditions (<5 mph) will persist from 9 PM tonight until 6 AM tomorrow and again from 9 PM on Friday to 10 AM on Saturday.
Finally, I want to emphasize that, based on the RIMpro apple scab model that was developed in Europe (and that I am evaluating this year), the Tues-Thurs infection period will probably turn out to have been the most important scab infection period of the year. Significant infection periods are also predicted for next week, but they are likely to be less severe than the one we just experienced. Thus, fungicide sprays applied today through Sunday will be critical both for their reach-back effects against scab and rust, and for protecting against additional scab infection periods forecast for next week.