Small brown spots are often visible on tomato leaves at this time of year. The spots, usually surrounded by bright yellow halos, start on the lower leaves and gradually appear on upper leaves over time. Spots often coalesce together, leading to browning and eventual death of entire leaves.
A close look at the leaves reveals several details. The spots show a dark border that surrounds leaf tissue that has turned light gray. Tiny pepper-like spots are scattered within the gray tissue. The black, pepper-like spots are structures of the fungus that causes the leaf spot disease, Septoria lycopersici.
Because Septoria leaf spot is favored by wet leaf conditions, any cultural practices that minimize leaf wetness will discourage disease development. Promote good air flow among leaves to assist drying by proper spacing and staking of plants. Also, avoid wetting the leaves when watering.
Fungicides are available from garden centers to protect tomato plants from infection. Sprays should begin before disease begins to develop, or at the very first signs of disease. Sprays are typically repeated every 7-10 days. Read and follow the label for specific instructions.
The Septoria fungus is able to overwinter on diseased plant parts. Good sanitation practices and rotation will help minimize the presence of the fungus in your garden. Unfortunately, no resistant cultivars for leaf diseases of tomato are available at this time.