Harvesting Tomatoes After Spraying with Copper

by Beth Loxley
(Tauranga, Bay of Plenty, New Zealand)

Q. I get tomato blight regularly every year. This year I was determined to spray with copper oxide to prevent it. However, the season has got away early, and I have fruit on very healthy plants already.

• Can I spray the plants bearing fruit?
  
• Will washing remove the spray from picked fruit?
  
• Is there a withholding period?
  

I would appreciate any information you can give me.

A. Short answer: yes, you can safely spray fruit-bearing tomatoes with copper fungicides. Washing will remove residue and there is no waiting period after treating or harvesting.

Long answer: Copper is one of the most commonly-used fungicides for treating tomatoes organically. The U.S. government’s Environmental Protection Agency (EPA) routinely reviews fungicides and their safety. Currently, in the U.S. there are no human toxicity concerns associated with tomatoes treated with copper spray. The EPA recommends a defined concentration range for fungicide application. There are no restrictions on the total amount that can be applied to a crop over a season.

According to officials at Cornell University’s Department of Plant Pathology, copper is strictly a contact fungicide (versus systemic fungicide), meaning it impacts plant and fruit surfaces only, not the tomato’s interior. It often leaves a blush residue. But residue can be washed to reduce or eliminate copper fungicide traces.

Copper is a natural element that is an essential trace mineral for humans. The U.S. National Research Council recommends 1.5 to 3 mg of copper per day for adults to avoid copper deficiency. Copper has few toxic effects. Absorbed in excess (more than 20 milligrams a day), it can lead to vomiting. One cautionary note: people with Wilson's disease, an inherited genetic disorder, cannot properly excrete copper. Its intake in Wilson’s disease patients can accumulate to levels that lead to liver disease and mental retardation.

Good luck and happy gardening!
Your friends at Tomato Dirt