Post-rain pest management

June 2, 2015

by Terence Bradshaw

We are coming out of the biggest apple scab infection period of the season in Vermont orchards, and despite model predictions, I and many others in the region are assuming that there were plenty of mature ascospores that were released since rains started on Sunday. For orchards that were recently covered going into the infection, you can assume that fungicides were washed off after the first 1.5-2″ of rain, and that there was no protection for the latter half of this rain event, nor for showers that are expected this weekend. Spray conditions look good to excellent from this afternoon through Friday. I strongly recommend coming in with a fungicide with kick-back activity (DMIs, SDHIs, or Strobilurins, check your spray guide for specifics) as soon as possible. As always, these materials should be combined with a protectant fungicide to increase efficacy and reduce the development of resistance.

Plum curculio is still active, so a final application of an insecticide may be appropriate in problem blocks. Codling moth egg hatch is still about a week away, so specific materials against this pest should be held out until 250 degree days have passed since the first moth capture, which we expect in another couple of weeks.

Fire blight strikes have been observed at the UVM orchards, likely from the blossom infection that occurred May 9-11. Scout your blocks carefully and remove any strikes in dry weather.

The window for chemical thinning id rapidly closing as king fruit approach 15 mm in size. Cool weather for the remainder of the week and cool, cloudy weather the last few days have allowed the trees to build up significant carbohydrate reserves that will making chemical thinners less active. If you need more thinning, apply carbaryl + NAA as soon as possible. Higher rates (1 pint cabaryl 4L, 4-8 oz/acre NAA) would be prudent in blocks which had heavy bloom and good fruit set.

I will be away at a conference next week, so likely will not send out updates. Next week may be the first that we’ll take a break in the spray schedule anyway, assuming that we get good spray coverage on this week and the rains that come are not the downpours we just experienced.

Good luck with it.

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