Many diseases can affect tomatoes

Septoria leaf spot is one of the most destructive tomato-leaf diseases.

A lot of people got their tomatoes off to an early start this year and are already harvesting the fruit. But there have been plenty of problems.


• We can’t do anything about tomato spotted wilt virus. The leaves at the top of the plant will begin to curl up along the midvein. There might be yellowing with coarse, brown-to-black spots on the leaves, water soaking on the leaves and petioles, and purpling on the leaf undersides. You might just see the curling and dark discoloration on top of leaves. The plant also stops growing.

Tiny insects called thrips carry the TSWV, which also can affect other vegetables in the solanaceous (nightshade) family, such as peppers, eggplant and potatoes. If your plant gets the virus, pull it up and throw it away.

• Blossom end rot is caused by a lack of calcium. By the time the tomato reaches the size of a nickel it has most of the calcium it will ever have, so blossom end rot must be prevented early. The main symptom is a dark, sunken water-soaked area at the blossom end.

The cause is not a lack of calcium in the soil, but more often, the lack of water to move calcium up to the fruit. Blossom end rot is more common when tomatoes are grown in pots because they dry out quickly. If watering is not an issue, spray the plant with calcium chloride (sold as Stop Rot). Calcium is best taken up by the roots, so sprays like this may not be that effective.

• Fusarium wilt is caused by a soil-borne fungus that can’t be controlled. Leaves turn yellow gradually from the bottom up. Wilting continues until the foliage is killed and the stem dies. The stem shows no soft decay, but if it is cut lengthwise, the woody part shows a dark-brown discoloration of the water-conducting tissue.

• Bacterial wilt causes the plant to look fine one day and be wilted the next. The stem center (pith) becomes water soaked, later turning brown and sometimes hollow. This bacteria overwinters and survives in the soil. It always waits until your plant gets big because it needs consistent air temperatures over 80 degrees.

This disease affects other solanaceous crops. The only effective control is to plant other vegetable varieties in that area for at least four or five years, sometimes for even longer. If tomatoes are planted in containers filled with potting soil, you won’t have to worry about either fusarium or bacterial wilt.

• Early blight causes small irregular, brown dead spots appear on the lower, older leaves. Spots enlarge to about half-inch in diameter in “bullseye” patterns. Leaf tissue around the brown spots begins to yellow. The greatest injury occurs as tomatoes begin to mature. Daconil 2787 or any copper-based fungicide, sprayed at seven-day intervals, will help control early blight.

• Septoria leaf spot, one of the most destructive tomato-leaf diseases, is most evident after plants begin to set fruit. Look for numerous water-soaked spots on leaves. The first infection is usually found on older leaves near the ground. The spots soon become roughly circular with gray centers surrounded by darker margins. Later, the centers show tiny dark specks in which the spores of the fungus are produced. Use Daconil and copper fungicides for control. With either septoria leaf spot or early blight, pick affected leaves off to prevent spreading.

To prevent leaf spot diseases, water tomatoes at the base to wet as little foliage as possible. If you must use overhead irrigation, water early in the morning to allow foliage to dry quickly.