Tomatoes are one of the most popular vegetables grown in backyard gardens because they can be grown almost anywhere. They flourish in pots, raised beds and truck patches. All they need is moderately well-drained soil.
Although tomatoes aren’t particular about their growing needs, they are still susceptible to disease. Gardeners need to be aware of common tomato diseases because they set in quickly and can be difficult to diagnose.
Reducing risk
An ounce of prevention is worth a pound of cure when it comes to growing healthy tomatoes. Your first line of defense should be using gardening practices that prevent disease.
- Mix in compost. Before you plant your tomatoes in the spring, mix in compost and organic matter to encourage healthy plant growth from day one.
- Rotate crops. Tomatoes and related vegetables — potatoes, peppers and eggplant — should not be planted in the same area within your garden more than once every three years. Any crop planted the year after your tomato crop should be a member of the grass family. Corn is an ideal crop to rotate with tomatoes because it supplies plenty of organic matter and doesn’t promote the growth of disease organisms that attack tomatoes.
- Plant disease-resistant cultivars.
- Purchase certified seed.
- Maintain air circulation. Make sure to plant your tomato plants with enough space around them as seedlings, so that they don’t touch when they are mature.
- Follow proper fertilization and irrigation methods.
- Never compost diseased plant material.
- Clean up your garden. From one season to the next, remove plant parts and overgrowth.
Common tomato diseases
Recognizing the symptoms of common tomato diseases earlier can help you diagnose and remedy the problem quicker. Every tomato gardener should be familiar with these diseases:
Early Blight
Early blight is caused by a fungus that spreads during the spring when spores are dispersed via wind and splashing rain to wet surfaces on your plant, generally the low leaves of tomato plants.
Symptoms: When the fungus is first observed it presents as small brown lesions mostly on older foliage. As it progresses, the spots enlarge and concentric rings in a bull’s-eye pattern form in the center of the diseased area. As the disease spreads the tissue surrounding the lesions may turn yellow. Most of the time, early blight occurs on the lower third of the tomato plant, affecting the leaves, stems and even fruit.
Treatment: Try to prevent early blight by planting disease-resistant cultivars, pulling weeds and volunteer tomato plants, rotating crops, spacing plants so that they do not touch when mature, mulching and fertilizing properly and avoiding watering foliage with irrigation water.
If you notice signs of early blight, prune off all leaves and infected lower branches within 12 inches of the ground. Dispose of them by burning, deep burial or double bagging them for disposal in a landfill. Do not put them in your compost pile. Try not to remove more than 20% of the plant’s total mass. Disinfect your pruners before storing them.
Septoria Leaf Spot
Septoria leaf spot is another fungal disease, which affects tomato foliage and stems. However, the fruit is not affected.
Symptoms: The first signs of infection usually occur on the lower portion of the plant, manifesting in the oldest leaves first, after the plant begins to grow fruit. You’ll likely notice dark spots that will eventually develop a whitish to tan center. Severely spotted leaves will turn yellow, die and fall off the plant, resulting in defoliation, fruit sunscald and poor fruit flavor and color.
Treatment: It’s important to take preventative measures to combat septoria leaf spot by rotating crops, pulling weeds and volunteers, properly spacing plants, watering plants without getting the foliage wet and cleaning up your garden at the end of the season. Septoria leaf spot is not soil-borne but can overwinter on residue from previous crops, decaying vegetation or on some wild hosts.
If you notice signs of septoria leaf spot, remove and dispose of diseased leaves and stems to prevent further spread.
Bacterial Spot
Bacterial spot is becoming an increasingly devastating tomato disease, potentially made more prevalent by the frequency of severe weather events that favor disease development.
Symptoms: In its early stages, bacterial spot lesions look very similar to early blight. These lesions spread rapidly under warm, wet conditions, causing a severe foliar infection that leads to defoliation reducing both the quality and quantity of the fruit. The pathogen can also directly damage the fruit.
Treatment: Use preventative measures — rotating crops, pulling weeds and volunteers, properly spacing plants, watering plants without getting the foliage wet and cleaning up your garden at the end of the season — to avoid bacterial spot in your garden. Once it’s been identified in your garden, remove affected plants and avoid getting the foliage wet when watering plants to stop further spread.
Late Blight
Late blight is a potentially serious disease of potato and tomato plants that occurs most frequently during cool, wet weather, affecting all parts of the plant.
Symptoms: In its early stages, small, dark, water-soaked spots appear on the leaves of tomato plants. As the spots enlarge, white mold appears along the margins of the affected area on the lower surface of the leaves. Within 14 days of the first symptoms appearing, the plant’s leaves and stems will turn brown and shrivel. Infected fruits develop shiny, dark or olive-colored lesions.
Treatment: Make sure you check your tomato seedlings thoroughly for suspicious leaf spots before purchase. Thereafter, ensure your plants are getting the right amount of nutrients, moisture and light for growth, and limit environmental stressors.
Blossom End Rot
Blossom end rot is a very common problem that affects the fruit of tomato plants.
Symptoms: Blossom end rot appears as a sunken, dark brown spot, ranging 1/2 to 1 inch in diameter, on the blossom end of tomatoes. It is caused by low levels of calcium in the garden soil and wide fluctuations in available moisture.
Treatment: Prevent blossom end rot by reducing plant stress and maintaining steady growth. Further support fruit production by providing an ample supply of moisture that ensures the calcium from the soil is reaching the fruit. Mulching can also help keep the soil around your plants moist. If your plants experience blossom end rot, remove affected fruit, so fruit that matures later will develop normally.
Note: Applying nitrogen fertilizer in excess can make blossom end rot more serious.
Fruit Cracking
Fruit cracking occurs when fruit develops too rapidly due to water fluctuations. Tomatoes experience two types of cracking — radial and concentric.
Symptoms: Radial growth cracks extend from the stem area, and concentric cracks encircle the fruit, towards the top.
Treatment: Make sure a steady supply of water is provided to your tomato plants throughout the growth process, especially during dry periods. Cracking occurs when fruit that reached the ripening stage during a dry period is exposed to heavy rains and high temperatures right after.
