Scab infection April 19-22

By Terence Bradshaw

It doesn’t take too much fancy software to tell that we are in the middle of a pretty good apple scab infection period in most every orchard in Vermont. Ascospore maturity is estimated at 4-8% for today, and up to 14% in some orchards by the end of the weekend with showers and rain expected for the next four days. Orchards that were covered with copper or another full-rate before April 12 may still have protection, but if more than one inch of rain falls, reapplication during a break in the weather would be warranted. Addition of Vangard or Scala in that spray (in combination with at least a half-rate of EBDC or Captan) will help to manage any infections that slipped though if poor coverage and/or washoff is a concern.

However, in cooler orchards with less tissue showing (and potentially low inoculum from last year), a postinfection material may not be warranted. Use your judgement here and err on covering up if in doubt. This is a good time to put on some oil (2%, good full coverage) but not if using Captan or sulfur in the next or previous 7-10 days.

Organic growers: ignore my suggestion for adding a postinfection fungicide if that means using liquid lime sulfur at this time. The scab risk isn’t great enough to offset the negative impacts that LLS has on orchard ecology and tree health. Save that ‘big gun’ for later in the season. If you feel like you need some retroactive coverage, consider Oxidate or one of the potassium bicarbonate products. Either of those must be applied shortly (<24 hours) within the onset if infection.

I’m away through the weekend, so good luck on this. In between worrying about scab (which, in the big picture, isn’t a massive threat right now), start planting trees and getting other spring activities in order. You can expect to fertilize soon when the soil warms up a bit. And of course everyone is done with pruning and pushing brush, right?


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