Brown spots on tomato leaves, turning necrotic

by Jon Waller
(Secret Harbour, Western Australia, Australia)

Q. I've noticed over the last few weeks that the leaves on my tomatoes, which are mostly heirloom varieties, have developed brown spots which eventually turn necrotic. One plant is also yellowing on the leaves, almost potassium deficiency symptoms. I give them about 16-18 liters of water a day, sometimes twice (morning and evening) if it's a hot day. They also receive a little liquid nitrogen fertilizer about once a week. Can you help?

A. We are so sorry about your plants! We know how discouraging tomato diseases can be. But there is hope.

It can be hard to diagnose what ails your tomato plant without knowing the progression of the disease - that is, how old is the plant? Does it have flowers or fruit? Are their any symptoms on the fruit? Did the symptoms begin on the fruit or the leaves?

Given the information you've shared,our best diagnosis is early blight, especially the browning of leaves, occasional yellowing, and leaf loss. Septoria leaf spot is another distinct possibility, as it and early blight are often confused.

In either case, both are caused by fungi and are treated in the same way: with a fungicide.

Treat organically with a biofungicide like Serenade to lessen symptoms. Follow label instructions. Or apply a fungicide such as chlorothalonil (sold as Fungonil), Mancozeb, or Daconil at the first sign of disease or when conditions indicate a strong potential for it to develop. Follow label directions. Re-apply every 7-10 days or after rain. Other diseases (such as late blight and gray leaf spot) can be controlled by these biofungicides and fungicides, so application is multi-purpose.

Good luck and happy gardening!
Your friends at Tomato Dirt