NDSU Extension - Ramsey County


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July 20, 2015 Horticulture Column

Healthy Tomato: Do as I Say, Not as I Do

By Roxanne Hawley, Master Gardener Intern


There is nothing better than a garden grown ripe tomato. However, tomatoes are susceptible to many diseases. Good practices that prevent disease are easier to put in place than trying to treat disease once it is present.

Unfortunately, our tomatoes get a disease called Septoria leaf spot each year. Small (1 /16 to 1 /8 in diameter) gray or tan circular spots with a brown margin brown, first appear on the undersides of older leaves.  These spots enlarge and in the center of the spots there are dark brown, pimple like structures. The spotted leaves turn yellow, then brown and drop off.

The reasons our tomatoes get leaf spot have to do with our gardening practices, Mother Nature, and the garden layout. The vegetable garden is my husband’s territory. The 20 plus flower gardens are mine. My husband likes using overhead sprinkling. Drip irrigation is a much better choice to prevent diseases on plants. We also had a long cool spring. In fact our tomatoes are lucky to be alive at all.

I grew our tomato plants from seed and they were a foot tall early in May. They reached a point where they had to go out. They were thriving and happily blooming in the garden until the forecast came for 30 degrees. We got creative and covered by creating a “garden greenhouse”,  putting plastic over tall stakes and setting a space heater at both ends. This environment left the leaves continuously damp and prone to disease. A week later, the forecast was for temps in the 20’s and 5 inches of snow. We unplanted our 36 tomato plants along with the peppers and eggplant. After the garden was tillable again, we replanted.

When planting tomatoes, one should not plant in the same spot more than once every three years. Members of the same family which include potato, pepper, and eggplant should not be in the same area as they are susceptible to many of the same diseases as tomatoes. Plants that are beneficial to plant with tomatoes to repel pests are basil, chives, garlic, marigolds, mint or parsley. Our garden is low, lake bottom ground and often only the front part is tillable in the spring. Therefore, our tomatoes haven’t a lot of choice where they need to go.

Tomatoes should be kept on a regular watering schedule to keep them healthy and strong. Over watering provides the perfect environment for many bacteria, fungi, and viruses to multiply. Our tomatoes were “over watered” as in Mother Nature had the ground soggy enough that we practically had to “mud” in our plants. Then we got busy with the 100 other projects that I needed done around the yard and the garden was often ignored.

If you have had diseased tomatoes in the past, then you should monitor your plants daily for symptoms. Look at the leaves for spots or yellowed or mottled foliage. Look for curled leaves. Check the stems for mushiness, mold, or streaks. Once you notice symptoms compare these to specific diseases.

Septoria leaf spot and late blight are both treated with a copper-based fungicide mixture or Chlorothalonil. Apply every 5-7 days during cool, wet weather. Remove and destroy infected leaves as soon as you observe them. Make sure you clean up the garden at the end of the season remove and destroy all plant parts. Don’t compost infected plant material.

Fortunately most diseases are usually not fatal. However, a severe loss of leaves can lead to sunscald of the fruit. We do get plenty of good tomatoes each year despite the ravage of Septoria Leaf spot disease.


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