Tomato leaves have dark patches on them - how to treat organically?

Q. I am looking for a way to treat dark spots on tomato leaves without harsh chemicals. Can you help?

A. The first step is to determine what the problem is on the tomato leaves. It could be either Early Blight or Septoria leaf spot - especially if leaves also have some yellowing! You can tell the difference between the two by studying the spots. If they look like they are concentric circles, you probably have Early Blight. Plain black spots are a better indicator of Septoria.

Whether it's Early Blight or Septoria leaf spot, the treatment is similar. Treat organically with a copper spray. Follow label directions. You can apply until the leaves are dripping, once a week and after each rain. Or you can treat with a biofungicide like Serenade. Apply every 7-10 days. Both these organic approaches are considered to be considerably less harsh than treating your plants with regular fungicides.

The black spots on tomato leaves could also be bacterial spot or bacterial speck. To date, there is no effective topical treatment (organic or inorganic) for these diseases. The best management for bacterial spot and bacterial speck is control. Destroy any infected crops. Rotate tomato plants next season, planting them in a different location. Sanitize your gardening tools used to handle tomatoes and the surrounding soil.

Good luck and happy gardening!
Your friends at Tomato Dirt