The most common threats to tomato plants and how to stop them

Tomato plants face three main categories of damage: insect pests that eat leaves and fruit, fungal and bacterial diseases that spread through wet foliage, and weather stress from heat, cold, or heavy rain. You can prevent most of these by removing dead leaves, spacing plants so air flows between them, watering at soil level rather than overhead, and checking plants twice a week for early signs of damage. Catching problems early—when a few leaves show spots or you see the first hornworm—stops them from spreading to the whole plant.

The specific threats vary by region and season, but the protective steps are the same everywhere: keep the plant dry above ground, remove anything that looks diseased, and remove pests by hand or with water spray before they multiply. You do not need chemicals to grow healthy tomatoes; most home gardeners stop problems with these basic practices.

Key Takeaways

  • Water tomato plants at the soil level in the morning, never on the leaves, to prevent fungal disease from taking hold.
  • Remove the lowest leaves on the plant once it is established, and prune away any leaf that touches the ground or shows spots or yellowing.
  • Space plants 24 to 36 inches apart and stake or cage them so air can flow freely around all sides of the foliage.
  • Check plants two or three times per week for hornworms, aphids, and spider mites, and remove them by hand or spray them off with water before they spread.
  • Mulch the soil around the base with 2 to 3 inches of straw or wood chips to keep soil from splashing onto leaves during rain or watering.

Watering and moisture control

The single most important protection is keeping water off the leaves. Fungal spores live in soil and water; when leaves stay wet, spores land on them and germinate. Water your tomatoes at the base of the plant in the early morning, directing the stream to the soil only. A soaker hose or drip line is ideal because it delivers water exactly where roots need it and keeps foliage completely dry.

If you water by hand, hold the hose low and move slowly around the plant, letting water soak into the soil. Avoid watering in the evening, because leaves will stay wet overnight and give fungal disease the conditions it needs to spread. Morning watering allows any accidental splash to dry quickly as the sun comes up.

Mulch the soil around each plant with 2 to 3 inches of straw, wood chips, or shredded leaves. Mulch keeps rain from splashing soil—which carries fungal spores—onto the lower leaves. It also keeps the soil temperature steady and reduces how often you need to water.

Removing diseased leaves and improving air flow

Once your tomato plant is 12 to 18 inches tall and has several sets of true leaves, remove all leaves from the bottom 6 to 8 inches of the stem. These lower leaves are the first to touch soil, collect splash, and develop disease. Removing them stops spores from establishing a foothold. Continue removing any leaf that yellows, spots, or touches the ground as the plant grows.

Space plants 24 to 36 inches apart—not closer—so air can move freely between them. Crowded plants stay damp longer after rain or watering, and disease spreads faster in still air. If you are growing in containers, do not cluster pots tightly together; leave space on all sides.

Stake or cage your tomatoes so the plant stands upright and leaves do not rest against the soil or each other. Leaves that touch the ground or press against other foliage stay wet and become disease entry points. Staking also makes it easier to spot pests and remove damaged leaves.

Identifying and removing common pests

Tomato hornworms are large green caterpillars, up to 4 inches long, that eat holes in leaves and fruit. They are straightforward to spot if you look for them. Check the plant two or three times per week, especially the undersides of leaves and the top of the plant where new growth appears. When you find a hornworm, pinch it off and drop it into soapy water, or let it be if you see a small white cocoon on its back—parasitic wasps have already laid eggs in it and will kill it naturally.

Aphids are tiny soft-bodied insects, often green or black, that cluster on new growth and the undersides of leaves. They suck sap and weaken the plant. Spray them off with a strong stream of water from the hose; most will not return. If they come back within a few days, spray again. Spider mites are even smaller and cause leaves to look speckled or bronzed; they also respond to water spray.

Whiteflies are tiny white insects that fly up when you brush the plant. They live on the undersides of leaves and spread disease. Spray them off with water, or use a yellow sticky trap hung near the plant to catch them. Check traps twice a week and replace when covered.

Protecting against common diseases

Early blight starts as brown spots with concentric rings on lower leaves, usually appearing in mid-summer when nights are warm and humid. Remove the spotted leaf when ready and dispose of it in the trash, not the compost pile. Do not compost any diseased plant material. Continue removing leaves as spots appear, working upward from the bottom of the plant.

Late blight causes water-soaked spots on leaves and stems and spreads rapidly in cool, wet weather. It is less common in most regions but devastating when it appears. Remove affected leaves and improve air flow by pruning lower foliage and spacing plants farther apart. If late blight appears on many leaves, remove the entire plant to stop it from spreading to neighbors.

Septoria leaf spot appears as small circular spots with dark borders and gray centers, usually on lower leaves. Remove affected leaves and improve air circulation. These diseases thrive when foliage stays wet, so all three are prevented by the same practices: overhead watering, crowding, and poor air flow.

Protecting tomatoes from weather stress

Tomatoes grow best when daytime temperatures are between 70 and 85 degrees Fahrenheit and nights stay above 55 degrees. In very hot weather, afternoon shade cloth can prevent sunscald—a pale, papery patch on fruit exposed to intense sun. Drape shade cloth over the plant during the hottest part of the day, or plant tomatoes where they get morning sun and afternoon shade from a taller plant or structure.

Heavy rain can split ripe fruit or cause blossom-end rot, a dark sunken spot on the bottom of the fruit caused by inconsistent watering. Mulch helps regulate soil moisture and reduce splitting. If you know heavy rain is coming, water deeply the day before so the soil is already saturated and cannot absorb more water when rain falls.

Cold snaps below 50 degrees Fahrenheit slow growth and can damage flowers. If frost threatens, cover plants with a sheet or frost cloth in the evening and remove it the next morning. Early-season plants benefit from being started in containers so you can move them to a sheltered spot if cold weather arrives.

Staking, caging, and pruning for health

Determinate tomatoes (bush types) usually need only a cage or short stake. Indeterminate tomatoes (vining types) grow 6 feet or taller and need sturdy stakes or a tall cage. Staking keeps fruit off the soil, improves air flow, and makes it easier to spot and remove pests and diseased leaves.

Prune suckers—shoots that grow in the angle between the main stem and a branch—on indeterminate varieties to direct energy into fruit production and improve air circulation. On determinate varieties, light pruning of lower foliage is enough. Never remove more than one-third of the plant's foliage at once, because leaves power fruit ripening.

Once the plant has set fruit and you are in mid-summer, you can remove some upper leaves to let sunlight reach developing tomatoes. This speeds ripening but should be done gradually over several weeks, not all at once.

Frequently Asked Questions

Can I use fungicide spray to prevent disease?

Fungicide works best as prevention, applied before disease appears, rather than as a cure. Sulfur and copper fungicides are approved for organic gardening and work on early blight and other fungal diseases. Follow the label directions exactly, and do not spray when temperatures are above 85 degrees or within two weeks of an oil spray. Most home gardeners prevent disease successfully with watering practices and leaf removal alone.

What should I do if my tomato plant has blossom-end rot?

Blossom-end rot is caused by calcium deficiency linked to inconsistent watering, not by a disease or pest. Mulch the soil, water deeply and regularly so the soil stays evenly moist, and remove affected fruit. New fruit forming after you fix the watering pattern will be normal. The problem usually stops within two to three weeks.

How do I know if my tomato plant is overwatered?

Overwatered plants have yellow leaves, soft stems, and a musty smell from the soil. Water only when the top inch of soil is dry to the touch. In hot weather that may be daily; in cool weather it may be every three days. Soil should be moist, not soggy. If your soil drains poorly, add compost or mulch to improve it, or grow tomatoes in containers with drainage holes and potting mix.

Should I remove flowers to help the plant grow stronger?

No. Removing flowers reduces your harvest and does not strengthen the plant. Tomatoes naturally drop some flowers if they are stressed by heat, cold, or inconsistent watering, but removing healthy flowers is unnecessary. Focus on consistent watering, good air flow, and removing diseased leaves instead.

Can I save seeds from my tomato plants?

Yes, but only from determinate (bush) varieties and heirloom indeterminate varieties. Hybrid tomatoes do not produce seeds that grow into the same plant. Let a ripe tomato fully mature on the plant, cut it open, scoop seeds and gel into a jar, add a little water, and let it ferment for two to three days. Rinse the seeds, dry them completely on a paper towel, and store them in a cool, dry place. They stay viable for three to four years.