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Fungi can plague garden tomato plants

By Doug Hartman
Extension Brown County Horticulture Assistant

BROWN COUNTY – Have you ever had brown spots on the lower leaves of your tomato plants causing the leaves to turn yellow and die? Well, you are not the only one.

There are two common diseases tomatoes can get which cause concern for gardeners, resulting in many calls to the Horticulture Help Desk.

The culprits responsible for these problems are early blight and Septoria leaf spot.

While these diseases are caused by different fungi, the symptoms created are very similar and management recommendations are the same.

The best management practices begin before the disease hits since with fungal issues it is always better to try and prevent rather than cure.

Below are steps you can take in an attempt to stay ahead of these problems.

1. Use varieties that are known to be resistant to early blight and Septoria leaf spot. Note “resistant” does not mean immune so you can’t always solve the problem this way.

2. Fungi thrive with humidity and leaf wetness. Decrease both by spacing out plants, thinning outside branches and watering at the base of the plant rather than overhead (to avoid wetting leaves). The more air flow you maintain within and between plants will help dry the leaves out more quickly.

3. At the base of your plants, place about an inch of mulch like straw, grass clippings (not chemically treated), shredded leaves, etc. This can help prevent fungal spores in the soil from splashing onto lower leaves during heavy rains.

4. Rotate your tomato crops each year. This can be difficult in small gardens or raised beds but moving plants as much as possible to separate them from the fungal source can be helpful.

5. Prune out and destroy (burn or bury) infected leaves as soon as you see signs of infection. At the end of the season, dispose of any affected plants. If you compost, make sure the pile gets to at least 140 degrees to kill the fungal spores.

6. If necessary, apply fungicides labeled for use on vegetables. Products containing copper and chlorothalonil can be effective if applied before the fungus gets too established. Remember – fungicides prevent, not cure.

Black bottomed tomatoes

Another common issue we see each year is a physiological disorder known as blossom end rot.

Blossom end rot typically occurs on the first fruits formed by the plant while fruit produced later is not affected.

This disorder is evidenced by the rotting of the bottom of the tomato (blossom end) which, interestingly enough, is caused by lack of calcium in the plant.

In much of Northeastern Wisconsin, there is limestone bedrock which results in sufficient calcium in the soil.

But the problem lies in plants unable to take up the calcium that is present, primarily due to not enough water, too much water or inconsistent water.

To control blossom end rot, water your plants evenly throughout the season at the base (not overhead to avoid fungal issues) and do not overdo fertilizer since excessive nitrogen can compound the problem.

If your soil is low in calcium (determined by a soil test), add calcium sources, like bone meal, to boost levels.

Popular tomato varieties like Big Boy are more prone to end rot so a little research to determine varieties less prone to this condition while still meeting your culinary parameters is helpful.

As always, the Horticulture Help Desk at Extension Brown County is here to help you with any of your horticulture issues.

Contact us at [email protected] or 920-391-4615 and let us know how we can help.

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