Spraying and hot weather

By Terence Bradshaw

This is just a quick note to urge caution for anyone planning to spray grapes or apples ahead of the upcoming hot spell which is expected to start tomorrow. Meteorologists are calling already saying that this may be the most intense heat event seen in Vermont in recent history, particularly in terms of duration. Temperatures in much of the state are expected to hit the upper 90s by Sunday July 1, with a slight reprieve into the 80s mid-week, followed by another jump into upper 90s. We are expected to see 100-degree weather in some parts of Vermont over the next ten days.

Heat and crop spraying do not mix well, especially when accompanied by high humidity that limits transpiration and the cooling it brings. Many spray materials are phytotoxic under certain conditions, especially when intense heat follows application. I would be very careful spraying anything until this weather subsides. Especially dangerous materials include sulfur compounds (including lime sulfur), copper, oil, captan, foliar fertilizers, and most emulsifiable concentrates and similar liquid products that contain potentially damaging solvents.

The good news is that most fungal pathogens do not thrive in such hot conditions. Keep an eye out for fire blight in apples (which you can’t spray for now anyway, so just cut it out as soon as you see it) and keep monitoring for any insect and mite populations that you may need to treat after this heat is over.

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