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Past Articles Library | Plant Diseases & Control | Tomato Blight

Every summer unexpected gardeners will find their tomato plants suffering from an unknown infection.  This infection affects the leaves and fruit of their tomato plants and seems to appear year after year when tomatoes are planted in the same spot.  This infection is referred to as the “tomato blight.”

This blight can occur as an early blight or late blight.  The Alternaria solani fungus brings on the early blight and appears late in the tomato season.  Foliage that is suffering from this fungus will have brown lesions that are in concentric rings.  These rings move from the bottom, older leaves up to the new growth.  As this disease progresses, the tissue around the lesion turns yellow and then turns brown and dies. 

Tomato fruit also suffers from early blight and symptoms appear on the stem attachment.  These symptoms include darkening of the stem and fruit.

Late blight is caused by Phytophthora infestans and appears as brown spots on stems.  The plant can even look like it had been injured by frost.  Brown spots on stems will appear along with a white fungus that shows up underneath the leaves.  A severe case of late blight will cause the leaves of the plant to drop off.

The fruit of a tomato plant with late blight will have a spotted appearance and will be hard.  The fruit will rot before ripening.

Following a few simple rules can reduce the chance of developing tomato blight.  First use only seeds and plants that are immune to tomato blight or use the organic control described later in this article.  Next, only water in the morning and only water the soil.  Wet leaves help create the perfect environment for both fungi.  Plant plants the correct spacing to allow for airflow.  Develop a fertilizing regime to keep nitrogen and phosphorus levels in the correct range for tomatoes.  Tomato plants that do not receive enough of these nutrients are more susceptibility to the blight.  Rotate any crop that is in the nightshade family every three to four years.  This includes tomatoes, peppers, potatoes, and eggplant.

Another approach to preventing tomato blight includes following the simple rules but also treating the tomato plant with an organic technique to reduce the chance of developing this problem. 

This technique begins with assembling the materials.  The gardener will need 16 gauge copper wire, wire cutters, new spray bottle, and 3 percent hydrogen peroxide.  To begin the process, cut the copper wire into 3-inch lengths.  Once this is done, begin to apply the copper wire to each tomato plant.  The plant stem needs to be about the size of a pencil when this treatment begins.

Once you have tomato plants the correct size, measure ½-inch up from the soil level and push one of the 3-inch copper wires through the stem.  Adjust the length of the wire so that each side of the wire is equal in length.  Then fold down each side into the soil or leave straight.

After each plant has been treated with the copper wire, then fill a spray bottle with 3 percent undiluted hydrogen peroxide.  Mist each plant with this solution once a week.

This method works so well in the prevention of tomato blight that it can be found in spray form.  Many organically based fungicides contain copper sulfate.

This technique is great as a preventative measure for those plants resistant to the blight but also for heirloom plants.  Many of these plants have not been bred to be resistant to the blight, which can be a problem.  But using this technique will allow the gardener to welcome them into the garden space without the worry of transferring the disease.

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