Tomato leaf spot

Septoria leaf spot infected tomato leaves

About 2 weeks ago, the lower leaves of the some of my cherry tomato plants began to show signs of Septoria leaf spot, an infection caused by the fungus Septoria lycopersici.

While I usually do not see this problem until later in summer, given all the rain we received this month, I am not surprised that it arrived this early in the season. 

Septoria leaf spot is characterize by the formation of numerous small circular lesions that are 1/16-1/8" in diameter. Spots first appear on lower leaves and have a dark brown border and a grayish-white center. 

Heavily infected leaves turn yellow and wither off the trunk of the plant. This can result in sun-scalding of fruit and ultimately a reduction in the amount of fruit produced. 

In an attempt to slow the spread of the fungus, I have had to defoliate all the lower leaves of all my cherry tomato plants. I removed infected leaves using pruning shears (cleaned with a 30% bleach solution before and after each cut). 

Sadly, it looks like I have tomato trees rather than bushy tomato plants. Luckily, none of the fruit show signs of infection. However, I have lost a few clusters of tomatoes due to my heavy pruning of the infected leaves (and not paying attention to where exactly my pruning shears were snipping...oops).

To minimize risk for disease, tomato plants should be:

• Spaced 2 feet apart
• Staked to improve circulation
• Watered early in the day using soaker hoses or drip irrigation to minimize splashing of water onto the leaves
• Grown in a new location in the garden every year

Although I followed all these recommendations, I was not able to prevent Septoria leaf spot from rearing its ugly head. Unfortunately, once this fungus appears, the only thing you can do is control its spread.

Every 7 days (or after rainfall), I will have to treat the infected plants with a copper-based fungicide or a fungicide that contains chlorothalonil. Fungicides will not cure infected leaves, but they will protect new leaves from becoming infected. I will also have to be diligent about removing infected leaves since the disease is spread by wind and water splashing up from the ground and infected leaves.