Tomatoes are one of the most popular plants in the home vegetable garden; a variety of problems, however, can stand in the way of a good harvest.
An awareness of the more common problems can allow you to take steps to prevent them or deal with them quickly, if they appear in your garden.
Problems due to unfavorable growing conditions
Blossom drop occurs when a plant blooms but fails to set fruit; the blossoms just wither and drop. Blossom drop can result from a variety of unfavorable growing conditions but is commonly associated with night-time temperatures below 55 degrees or extended daytime temperatures above 85 degrees, excessive nitrogen fertilizer and insufficient light.
Blossom end rot is characterized by the development of water-soaked spots on the “blossom end” of the fruit (the end opposite the stem) that enlarge and turn black and leathery. Blossom end rot is caused by insufficient calcium uptake by the fruit which may result from fluctuations in soil moisture, excessive nitrogen fertilizer, high salt concentrations in the soil, or a soil pH that is outside the recommended range for tomatoes (6.0-6.5 according to Washington State University). Insufficient soil calcium is rarely the cause of blossom end rot.
Oddly-shaped tomatoes with crevices and leathery scar tissue are the hallmarks of cat-facing. Cat-facing is caused by damage to the blossoms, cold temperatures during fruit set, and exposure to growth-hormone-type herbicides. If herbicide damage is the cause of the deformity, the leaves and vines are also affected.
Physiological leaf roll is a poorly understood condition that presents with upward cupping and rolling of the leaves. Physiologic leaf roll has been associated with drought, high soil moisture, high temperatures and heavy pruning. It does not affect general plant health but is of concern because it resembles damage due to viral infections and herbicides.
Fruit cracking occurs when rapid changes in soil moisture cause the fruit to expand quicker than the skin can grow. Cracks can form concentric circles around the stem or vertical splits radiating out from the stem area. Vertical splits tend to be larger and lead to secondary bacterial and fungal infections.
Diseases caused by fungi, bacteria, viruses
Early blight is caused by Alternaria tomatophila, a fungus that overwinters in plant debris. The infection causes dark spots on leaves that enlarge forming target-like concentric rings surrounded by a yellow halo. Early blight is encouraged by wet weather and typically stops in dry, hot weather.
Late blight is caused by a fungus-like microorganism called Phytophthora infestans. It starts as gray-green water-soaked spots on leaves and stems that expand into large brown blotches that affect all above ground parts of the plant. In humid conditions, a thin powdery white fungal growth can appear on infected leaves, fruit and stems. Late blight spreads most rapidly in cool, rainy weather and can wipe out an entire planting in a matter of days.
Anthracnose is caused by Colletotrichum coccodes, a fungus that over-winters in plant debris and is encouraged by wet conditions, moderate temperatures and poorly drained soils. The infection leads to small, dark spots with yellow halos on leaves and sunken, water-soaked spots with dark centers on the fruit
Curly top is a viral disease that causes puckering and twisting of the leaves and stems. It resembles physiologic leaf roll but affects the entire plant resulting in poor growth and fruiting; leaf veins may also appear purplish. Curly top affects a number of other crops including beets, beans, peppers and squash and is transmitted by the beet leafhopper which thrives in low humidity and high light intensity.
Verticillium and Fusarium wilt are caused by soilborne funguses that invade the vascular system of tomato plants, preventing the upward movement of water and nutrients. Leaves wilt and turn yellow and then brown; one side of the plant is often affected initially. If the stem of a wilted plant is sliced open a brown discoloration of the water-conducting tissue can be seen. These fungi invade the plant through the roots and remain viable in the soil for years.
Aphids are soft-bodied, pear-shaped insects (about 1/8 inch long) that come in a variety of colors. Some have wings, some do not. Aphids feed on plant juices and cause foliage to curl and pucker. Feeding aphids secrete honeydew, a sweet, sticky material which may attract ants. High levels of nitrogen encourage aphid reproduction.
Spider mites are tiny, eight-legged creatures that resemble moving dots when found on a plant. Spider mites feed on plant juices causing leaves to develop whitish-yellow speckling near the mid-rib and lose color. Delicate webbing on the undersides of the leaves is a common finding. Spider mites thrive in hot, dry conditions.
Tips for Prevention
Here are a few tips that can help prevent problems from cropping up in your tomato patch:
• Select tomato varieties that are suited to our climate. Look for varieties resistant to common diseases. Plant labels may be coded; for instance, ‘F’ indicates resistance to Fusarium wilt and ‘V’ indicates resistance to Verticillium wilt.
• Test your soil before planting and amend it as indicated by the results. Do not over fertilize. Excessive nitrogen encourages vegetative growth (which prevents good air circulation). It also encourages reproduction in aphids as well as a buildup of salts in the soil.
• Wait for the right planting time. Plant tomatoes when soil temperatures are about 60 degrees which is late May or early June for most local gardeners.
• Space tomato plants about 3 feet apart. Provide vertical support (for example cages or staking) to improve air circulation, keep fruit off the ground and protect plants from wind damage.
• Water plants evenly throughout the growing season. Use drip irrigation or soaker hoses to prevent wetting the leaves. Mulch to moderate soil moisture fluctuations.
• As plants grow, prune for good air circulation and light penetration. Be careful not to remove too much foliage as excessive sunlight can sunburn the delicate fruits.
• At the end of the growing season, remove all pieces of your tomato plants — including the leaves and roots — from the garden. Do not compost this vegetation if affected by diseases or pests.
• Finally, rotate your tomato plants to a new garden spot next year to limit the buildup of diseases and pests in the soil.
Check your plants regularly for diseases and pests. If you find a problem, consult local Master Gardeners for help. Call the Master Gardener Plant Clinic Desk at 360-417-2514 or send an email describing the problem (with a photo, if possible) to firstname.lastname@example.org.
Jeanette Stehr-Green is a WSU-certified Clallam County Master Gardener.