Keep an eye out to catch common tomato problems early

By now you are enjoying the fresh, homegrown taste of tomatoes. To continue savoring them all summer, be on the lookout for common diseases or other problems. Caught early, a solution can often be found.

Blossom end rot is caused by calcium deficiency during fruit set. There may not be enough calcium in the soil, or it is unavailable due to uneven watering practices. The foliage is healthy, but tomatoes develop leathery, dark brown or black spots at the blossom end. The spots grow larger as tomatoes rot.

A soil test will indicate whether amendments are required. Foliar sprays of calcium chloride can also prevent blossom end rot. Watering plants deeply and regularly will ensure an even water supply. If any tomatoes develop blossom end rot, remove them immediately.

Sunscald often begins on green fruit on the side facing the sun. The damaged area turns yellow and then white and papery. Black mold may also grow. Protect fruit from sunscald by resisting the urge to prune too aggressively. Leave some foliage to provide shade.

Cracking usually occurs when the inside of a tomato grows faster than its skin. Hot, rainy weather and overfertilization are common causes of cracking. Keeping plants well watered prevents them from taking up too much moisture from a summer storm.

Anthracnose is a common problem in hot, rainy summers or when plants are watered by overhead sprinklers. Small, sunken spots develop on tomatoes and increase in size, resembling a bull's-eye. Eventually the fruit rots.

Fungicides with copper may help. Prevent anthracnose by removing the lower branches to prevent the fungus from splashing up to foliage. Avoid overhead watering and harvest ripe tomatoes. Overripe tomatoes are more apt to contract the disease.

Early blight is caused by a fungus that lives in the soil or overwinters on diseased plant debris. It begins on older leaves and appears as dark spots with concentric rings. Leaves turn yellow and then brown before falling off. Tomatoes can also be infected at any stage in their development.

Fungicides are available to treat infected plants. At the end of the season, remove all plant material from the garden and refrain from planting tomatoes, eggplants or peppers in the same spot every year.

Late blight is also caused by a fungus that spreads rapidly. Dark, greasy-looking spots appear on leaves before turning papery. Tomatoes also develop large, “greasy” spots. Cool, wet weather is ideal for disease infection and progression.

Late blight is difficult to control. It also spreads to potato plants and is the fungus responsible for the Great Potato Famine. Remove plant debris from the garden — from tomato and potato plants — in the fall and practice crop rotation.

Fusarium wilt affects the vascular system of the plant, destroying its ability to move water and nutrients. It may appear healthy one day, wilt the next and die the day after. Like many other fungi, it overwinters in the soil and plant debris. Prevent fusarium wilt by growing resistant varieties and rotate crops.

Verticillium wilt is also caused by a soil-borne fungus and can affect many different types of plants. The fungus is moved with water throughout the plant and then prevents the normal flow of water, causing plants to wilt during the hottest part of the day. Although plants recover in the evening, leaves eventually yellow and then brown, beginning at the base of the plant and moving upward.

There isn't a control for verticillium wilt. Affected plants should be removed from the garden. To prevent verticillium wilt, choose resistant varieties and rotate crops.

While we can't control the weather, gardeners can take actions to prevent disease in the garden. Get a soil test to ensure the soil offers appropriate nutrients. Plant tomatoes in warm, well-drained soil in a site that gets a daily dose of six hours of sunlight. Water regularly and deeply at the base of plants and fertilize appropriately. Do a thorough fall clean up and practice crop rotation.

• Diana Stoll is a horticulturist, garden writer and speaker. She blogs at gardenwithdiana.com.