Did I over apply?

Asked September 25, 2018, 6:21 PM EDT

I have been treating my cucurbits with a sulfur based fungicide for powdery mildew. It seemed to be working but suddenly, the plants really deteriorated and now look like they're dying. Did I over apply the fungicide or is something else wrong? Thanks for your help. Jim

Lane County Oregon

1 Response

These symptoms (blackening only of leaves of a certain age) do make me suspect that the plants are experiencing phytotoxicity. Sulfur can cause these issues if applied during or just before hot spells (over 90F) and/or if it is mixed with or applied near the same time as horticultural oils.

Other registered products for powdery mildew control on squash include bicarbonate based products (like Milstop or Montery Bi-Carb), Bionide Fungonil (contains chlorothalonil), neem oil (be careful when applying in hot temperatures), or a biological-based product called Actinovate.

This publication from UC Davis has some good general information on powdery mildew management on vegetables. http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html

If you have further questions, feel free to reply back through this system or contact your local OSU Extension Lane County Master Gardener volunteers. They have regular office hours and can answer questions in-person, by phone, or visiting in person. https://extension.oregonstate.edu/mg/lane/have-gardening-question

Take care and thanks for using Ask An Expert!