Frank from Lynn, Ma writes
The Bonide Liquid Copper Fungicide could help if it is a fungus that is causing the leaves to turn yellow. Bonide Liquid Copper works best if used before diseases listed on the product label appear or when diseases first appears. You will need to mix 0.5-2.0 fl oz of Bonide Liquid Copper in a gallon of water and apply every 7-10 days. We recommend you contact your local cooperative extension office and speak to the master gardener on staff for help find out why the leaves are yellowing on your tomato plants before using any product.
Answer last updated on: 06/12/2012