What's Wrong with My Tomato Plant?

The dark, leathery patch at the bottom of the fruit is a calcium-delivery failure, not a disease. Tomatoes pull calcium through water movement, and when the soil swings from dry to saturated the plant can't keep up with demand at the far end of each developing fruit even when calcium is present in the soil.

Early blight is a fungal disease that starts on the oldest, lowest leaves as dark brown spots with concentric rings and a yellow halo. Tomato foliage stays damp longer than most vegetables because the dense canopy traps humidity, and the warm-season growing window lines up perfectly with the conditions this fungus loves.

Septoria causes small, round spots with a dark border and a pale tan center, usually appearing first on lower leaves after the first fruits set. Like early blight it spreads by rain or irrigation splash, but the spots are smaller and more numerous. Both fungi overwinter in soil and debris and return season after season in the same beds.

When a tomato plant gets a sudden surge of water after a dry spell, the inside of the fruit expands faster than the skin can stretch. Indeterminate varieties with larger fruit are more prone because the skin is under more tension as the fruit grows. Cracks can run radially from the stem end or circle the fruit near the top.

Tomato hornworms are among the largest caterpillars in the garden and can eat an entire branch in a single night. They are bright green with white diagonal stripes and blend in almost perfectly with tomato stems and foliage. Their frass (dark pellets on the leaves below) usually gives them away before you spot the caterpillar itself.

Tomatoes are heavy feeders and exhaust nitrogen from the root zone quickly, especially in container soil or after heavy rain. The yellowing starts on the oldest lower leaves and moves up, since the plant pulls nitrogen out of aging tissue to feed new growth and developing fruit.

Early blight announces itself as yellowing around the brown spots on lower leaves before the spots are obvious. The whole lower canopy can turn yellow-green as the fungus spreads upward, which is sometimes mistaken for a nutrient issue. If you see any brown spots or concentric rings in the yellow area, blight is the culprit.

Tomato pollen goes sterile when daytime temperatures stay above 90°F or nights dip below 55°F. The flowers open, look normal, and then drop without setting fruit. This is a physiological response built into the plant as a warm-season Solanaceae crop, and fruit set resumes on its own once temperatures return to the 65–85°F range.

A tomato pushed hard on nitrogen produces lush, dark green foliage but delays flowering and drops flowers before they set. Indeterminate varieties are more susceptible because they keep vegetative growth running all season alongside fruiting.