Apple scab management this week

By Terence Bradshaw

Well, so much for spring. Things sure cooled down this week, with apple trees pretty much parked between green tip and half-inch green bud stage, with a little more advancement in southern parts of the state. The primary scab ascospores that drive the whole disease cycle for the year and overwinter on the orchard leaf litter are maturing a decent pace, with about 15-18% of potential spores mature in the south, 10% in the cooler northern Champlain Valley, and only 2-5% in inland Vermont. Relatively warmer weather tomorrow should push things ahead a bit. Key take-home: Those warmer areas with any appreciable green tissue showing and a history of scab in the orchard last year are entering the accelerated phase of ascospore maturity and should be prepared to maintain protective fungicide coverage before rain periods until the ascospores are all released. That means that orchards should apply between rains this week, after 1-2 inches of rain, and when new growth emerges (say a move from 1-2 bud stages and increasing shoot leaf expansion, when that starts anyway). That means that in weather like we’re seeing now, maintaining a roughly 7-day schedule. Please use NEWA to track weather and disease infection periods to best plan your spraying events. It’s getting very muddy out there, so try to minimize your trips by applying tank-mixed fungicides including a protectant (e.g., captan, mancozeb, sulfur if organic) and a kick-back material (I recommend Vangard or Scala, still, at this time of year).

Insects are still quiet, but expect tarnished plant bug and European apple sawfly to start moving with the first warm day or two.

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