It didn’t take long after I posted my last recommendations for someone to point out to me that it’s unlikely that apple scab ascospores can be released through two inches of snow. Point taken. This cool weather has put things into that slow, on-hold pattern a bit, but activity will pick up today and tomorrow as temperatures get into the 50s. In warmer southern Vermont predicted ascospore maturity is getting up around 10%, which is when we should be taking scab more seriously. In the Champlain Valley, where ascospore maturity is estimated around 5-7%, buds appear to be hovering around the half-inch green stage. Upland and inland, still at silver tip. Rains Sunday and Monday are expected to bring an infection period, so orchards with significant tissue exposed should be covered with a fungicide in the next two days.
Weather is looking good for oil application today and tomorrow, assuming that your site isn’t expecting freezing weather in the next couple of days. I am a proponent for putting oil on as late as possible, up to tight cluster or even pink. The rate should be adjusted down as buds open more: 2-3 gallons per 100 gallons water (straight % in tank, not adjusted for tree for volume or per acre) is good from dormant through green tip; 2 % GT-tight cluster; and 1% as you approach pink. Oil should be put on dilute- slow down and open up your nozzles if you can. For most orchards, 100 gallons of water per acre should be the minimum for applying oil. That means recalibrating your sprayer in many cases.
For growers with substantial bud development and concern about cold temperatures the past week, you may want to pinch some buds and look for browning in the developing ovaries…or not. There’s not much you’ll do about it now, anyway, but come bloom, it would be wise to observe flowers and be ready to adjust thinning if more than 20-30% damage is observed. I’m pretty hopeful that most sites have been fine, the critical temperature for 90% bud kill at tight cluster is 21°F, although some damage may be seen at 28. For half inch green, we can expect buds to be all right down as low as 23 or even colder. Trees at silver or green tip should be fine. Remember, if you suspect bud damage from cold temperatures, it is important to contact you crop insurance agent ASAP to get the claims process started.
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