Tomato Diseases. Symptoms, Diagnosis, Prevention, And What To Do Next

Common tomato diseases include early blight, Septoria leaf spot, late blight, bacterial spot and speck, Fusarium wilt, Verticillium wilt, powdery mildew, gray mold, and several virus diseases. Tomatoes also develop disorders that look like disease, especially blossom-end rot, sunscald, physiological leaf roll, herbicide injury, and water or root stress. Start with where the symptom appeared, how quickly it is spreading, and whether leaves, stems, fruit, or the whole plant are involved.

Look at the oldest affected leaf and the newest affected leaf, then compare the shape and texture of the damage. Early blight usually makes larger brown lesions with yellowing and concentric rings on older leaves. Septoria makes many smaller gray or tan spots with dark margins, also starting low on the plant. Late blight produces irregular water-soaked lesions that can move quickly into stems and fruit. Tiny dark spots with halos can fit bacterial spot or speck. White powder on the surface points toward powdery mildew. Leaf symptoms overlap, so the direction of spread and the rest of the plant matter.

Lower leaves sit closest to splashing soil and often stay humid longer inside the canopy. Early blight and Septoria leaf spot commonly begin there, especially after rainy periods or repeated overhead watering. Lower leaves can also yellow first with Fusarium or Verticillium wilt, though those diseases usually add wilting and internal stem discoloration rather than a simple leaf-spot pattern. Remove leaves touching the soil, keep irrigation at the base of the plant, and watch whether new lesions continue moving upward.

Early blight usually starts on older lower leaves and produces brown lesions that develop concentric rings, often with yellow tissue around them. It tends to progress upward over time and can also affect stems and fruit. Late blight can appear on young foliage higher on the plant, makes irregular water-soaked lesions, and can move through leaves, stems, and fruit very quickly during cool wet weather. Rapid spread and stem or fruit involvement make late blight much more urgent.

Septoria usually produces many small round gray or tan spots with dark borders. Tiny black fruiting bodies may be visible in the centers of older lesions. Early blight lesions are usually larger and are more likely to show a bull's-eye pattern of concentric rings. Both commonly begin on lower leaves, both are favored by wet conditions, and they can occur on the same tomato plant at the same time.

Wilting can happen when roots cannot move enough water even though moisture is present. Fusarium and Verticillium wilt block the plant's vascular tissue and often begin with lower-leaf yellowing or one-sided wilt. Root damage, waterlogged soil, or severe root loss can create a similar canopy symptom. Check the root zone before adding more water. If one side of the plant yellows and wilts or the vascular tissue inside the lower stem is brown, a vascular wilt becomes more likely.

Blossom-end rot is a physiological disorder rather than an infectious disease. The dark sunken patch forms at the blossom end of developing fruit when calcium does not reach expanding fruit tissue consistently. Inconsistent watering, drought, root stress, low soil calcium, unsuitable soil pH, and excessive nitrogen can contribute. Management centers on steady soil moisture, mulch, appropriate soil fertility, and allowing the root system to function well.

Yes. Powdery mildew can infect tomatoes and appears as white or gray powdery growth on leaf surfaces. It looks very different from the water-soaked lesions of late blight or the small dark spots of Septoria and bacterial diseases. Cornell CALS greenhouse trials evaluated Lost Coast Plant Therapy against established powdery mildew on tomato, melon, and hemp, and the trial results report 96% +/- efficacy against powdery mildew across the evaluated crops.

Remove leaves that are heavily diseased, touching the soil, or contributing to a crowded wet canopy. For early blight and Septoria, removing infected lower foliage can reduce a source of spores and improve airflow. Avoid stripping a stressed plant of most of its foliage at once, because healthy leaves still feed and shade the fruit. Use clean tools, avoid handling wet plants, and remove diseased debris from the growing area rather than leaving it beneath the plant.

Lost Coast Plant Therapy can help when the tomato problem is powdery mildew or a supported soft-bodied pest such as spider mites, aphids, thrips, or whiteflies. Cornell CALS greenhouse trials reported 96% +/- efficacy against established powdery mildew across tomato, melon, and hemp. In a separate western flower thrips trial on lettuce, the strongest treatment reached 95% +/- efficacy against total thrips after five applications.