Copper for Bacterial Disease Control in Tomatoes: Is the Plant Protected?

One of the main components of a bacterial disease management strategy in field tomatoes is the application of fixed copper to protect the plant. Growers need to ensure that the plant is continuously protected with adequate levels of copper, throughout its early growth.

How much copper are we getting on the leaves with our greenhouse and field spray programs? How long does it remain on the leaves? In order to learn more about this, we have been working with Dr. Ron Pitblado and his staff at Ridgetown College to measure copper levels on tomato plants in commercial transplant greenhouses and fields. Twenty leaves were randomly selected from treated plants. The leaves were washed in 50 mL of water and analyzed for copper ions using a photometer.

Transplant greenhouses:
Sampled immediately after copper application (0) to eight days after application (8).

Field - Copper from greenhouse applications, not sprayed by field grower yet:
Sampled immediately after transplanting (0) to 8 days after transplanting (8).

Field - After field grower has applied copper:
Sampled immediately after copper application (0) to 18 days after application (18).

In some cases there are higher readings in the field (from greenhouse copper applications) than we found in the greenhouses. This is because the plants we sampled in the field were not necessarily from the same greenhouses where we were sampling. There is a range in copper residues among greenhouses and plant lots due to various factors, including last spray date, watering schedule, copper rate, and spray water volumes.

How much copper is enough? I can't say for sure, but our samples indicate that by four to six days after spraying, copper levels averaged about half of the residues found in the first day after spraying. By seven days after spraying, copper residues averaged around 1/3 of the initial copper levels. In some fields, copper levels were already near zero just 2 days after spraying. Many of these samples were taken during rainy periods, so we would expect quite a bit of loss of copper.

Since copper is only moderately effective in controlling bacterial disease (and we have no better product to use, yet) it looks to be risky to extend spray intervals past 7 days. Most fixed copper labels specify a 7-10 day re-spray interval for field tomatoes, although some allow application "more frequently when disease is severe". Check product labels for details. Always read and follow label directions.