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Ask a Gardener: Tips for keeping your tomato plants healthy

Home-grown tomatoes are a summer jewel, vine-ripened and eaten fresh.
Home-grown tomatoes are a summer jewel, vine-ripened and eaten fresh. Tribune News Service

Question: I want a good tomato harvest. What do I need to do to ensure that?

Answer: Many of us hunger for that drippingly ripe, explode-in-your-mouth taste of a perfectly grown summer tomato straight off the vine.

Imagine the disappointment when one day while checking your tomato plants you notice something seriously amiss. Help! How did it happen so fast? What can I do?

There are three diseases that afflict tomato plants. They are found throughout the U.S. Each does its damage in different ways, but the result from all three diseases is the same: death and dying, of both plant and fruit.

First of all, remember that in spite of the sunny days we’re experiencing of late, there’s still that “fungus amongus” lurking, ever ready to cause heartbreak.

Here are the fearsome three:

Early Blight

Caused by a fungus (Alternaria solani) and present all over the world, it overwinters on the tomato plant debris and some perennial plants. Spores will spread to tomatoes in the spring via the wind or splashing rain. The fungus needs wet leaves to germinate and grow, and the early morning dew is the perfect medium for this nasty fungus.

Early blight causes spot with outer rings around a bull’s eye center. Damage starts with the bottom leaves, and usually spreads to the lower third of the plant, so the plant may still be able to produce a good crop. In rainy years, however, early blight can take out the entire plant.

Prevention: Rotate tomato plants to a new area each year. Keep foliage as dry as you can by mulching heavily around each plant. Do not crowd the plants; leave plenty of air space. Keep the foliage off the ground, and water early in the day, preferably with a drip system.

Fusarium Wilt

Present in soils around warm to moderate climates around the world, this nasty soil fungus enters the plant through the roots, and then multiplies in the plant’s vascular system, causing the leaves on individual stems to show first signs of the disease.

Though infected plants grow normally for a while, at the point where they set fruit, leaves will start to yellow and wilt severely in the sun. The damage will continue until the plant inevitably dies. No other tomato disease causes such distinctive yellowing.

Prevention: I hate to be the bearer of bad news, but it can’t be avoided. Once the Fusarium spore is in the ground, it will forever stay in the ground. If you want to plant tomatoes, you will have to rotate tomato planting so plants are not in the same place for four years.

When you are buying tomato plants, look for the many varieties that are Fusarium-resistant. And don’t overlook an easy way to avoid this disease: plant tomatoes in large pots filled with packaged potting soil.

Late Blight

With a name like Phytophthora infestans you can’t expect anything good. Unfortunately, that’s sadly the case with this tomato disease, all too common in our humid climate.

So you plant your tomato plant, it takes off right away, with lots of dark green foliage that artistically sets off the abundance of bright yellow flowers. You notice lots of pollinators around. Life’s good.

Then, almost overnight, there are dozens of marble-sized tomatoes. In a relatively short time your plant is loaded with reddening fruit, and you think….just a few days until I taste the best tomato ever!

But instead, you find that your tomato plant’s leaves show tan, wet patches, and are drooping. It starts in only a few places, then it covers almost the entire plant, fruit included. The plant is toast.

The culprit is the algae-like micro-organism that loves wet conditions. The kind of conditions the Northwest experiences most often. It is most often seen in years that are mild and wet. (This might be a thing of the past?!)

Prevention: Do whatever you need to do to keep your tomato plants dry. Don’t water late in the day or at night. Use a drip system; never sprinkle overhead. Find late-blight resistant plants. Think encouraging thoughts.

Most Northwest gardeners have encountered these diseases. Some years it’s bad, others years not so much. Don’t give up growing tomatoes, however. Knowing what you’re up against is the best way to keep all your plants healthy and happy.

Kathleen Bander of Bellingham is a life-long gardener. Her column will appear in The Bellingham Herald weekly through the summer growing season. If you have a gardening question you’d like answered in the column, please email it to For more gardening information online, go to