The Perfect Fungal Storm – Septoria Leaf Spot on Tomatoes

A few weeks ago the determinate tomatoes started looking very bad. More than half of the leaves had died on some of the plants; a couple of plants had lost all of their leaves. Tomatoes usually look a little beat up by late summer with some of the bottom leaves dying but it’s never been this bad.

There’s been a lot of talk about late blight but when I looked at the tomatoes I found that only the leaves were affected. I’ve seen late blight and when plants have it there are lesions on the fruit and stems. The stems and fruit of these tomatoes were fine. The only problem was that the leaves were dying.

Tomato Plants with Septoria Leaf Spot

With a little research I found out that the problem was septoria leaf spot, a fungal disease caused by the fungus Septoria lycopersici. The initial symptoms of this disease are small spots on the lower leaves of tomatoes. If the conditions are wet, the fungus can form fruiting bodies which produce spores. The spores are spread by rain and can defoliate the plant if the conditions are right. It’s obvious that the conditions were right for a bad case of septoria this year so what were those conditions?

The first condition was that the tomato plants this year were huge. I planted Pony Express, which is a determinate tomato, and the plants were the biggest I’ve ever seen and each was full of fruit. As an habitual “underfertilizer,” I had made the decision to fertilize a little more this year. It’s obvious that the tomatoes liked the additional fertilizing.

Septoria Leaf Spot in All Stages – unaffected leaves, leaves beginning to show spots and leaves that have died from the fungus

The second problem was that the plants were so heavy with green tomatoes that the tomato cages I got at the local big box store weren’t able to stand up to the weight of the plants. A couple of wind storms right before the fruit started to ripen knocked over most of the cages. As a result, the plants were jumbled together and no longer elevated above the ground.

The final issue was that after a dry June and early July, the rain started to fall. During the middle of July it rained a little every few days and when it wasn’t raining, it was hot and humid.

Put these three conditions together and you have the perfect storm for an outbreak of septoria. The large, collapsed plants no longer had good air circulation. Also, the septoria spores, which are found in the soil and on the infected leaves, could now splash on most of the leaves of the plants. Add to these conditions a little rain, heat and humidity and it’s no wonder that the tomatoes look so bad.

While I doubt I can prevent septoria completely, I am already making some plans for next year to limit its damage. While I have no control over the weather, I can make sure that the plants remain above the ground and have good air circulation. I plan to make sure that the tomato plants are spaced properly and have some sturdy support. I read about a way of supporting determinate tomatoes called the Florida weave that’s used by professional growers – it could work well. I might also make my own cages out of some sturdier material. I have the winter to think about this and plan for the coming year.

While the septoria has done a good job of damaging the tomatoes, I’ve still had a good harvest. There have been more than enough tomatoes for freezing and canning. So while no one wants diseases in the garden, septoria is one that isn’t too bad. It only affects the leaves and, with proper growing conditions, it can be managed.

So here’s to next year with tomatoes more widely spaced, more strongly supported and, hopefully, less affected by Septoria lycopersici.


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