Spot leaf diseases on tomato plants

Q: Brown spots are appearing on the leaves of my tomato plant. The tomatoes are still small and green. Last year these same spots appeared, and the leaves died, and the tomatoes stopped growing. What can I do to keep the plants alive this summer?

A: This is the time of year that tomatoes often become infected with different kinds of leaf spot, a plant disease. Septoria leaf spot is a fungal disease that produces many small, dark-brown spots. Early blight is a fungal disease that produces much larger brown spots that may have rings around them (like tree rings).

Both diseases tend to infect and kill the leaves on the bottom of the plant first and work their way upward. They start at the bottom because the fungal spores infect the plants when they’re much smaller. The spores start growing when the weather is damp, and the leaves show symptoms weeks later. If the weather is now dry, the fungus may not infect more of the plant, especially if you pluck off the lower, infected leaves.

Keeping the leaves dry and off the ground will help slow the disease spread. Water the plant on the ground rather than spraying water over the top of it. Of course, you can’t prevent rain from wetting the plant. In addition, using cages or stakes to keep the leaves off the ground is better. You also can plant the tomato plants far enough apart so air can circulate around the plants and keep the leaves dry. Use mulch in the garden to keep mud from splashing on the plants.

Next spring, look for tomato plants that are resistant to these diseases. If you think it’s worth spraying the plants with a fungicide, use one with the active ingredient chlorothalonil because it can be used until the day of harvest, meaning you don’t have to wait longer to harvest your tomatoes. Although, you could harvest before spraying the plants if it is nearly time to harvest.