The Real Reason Your Tomato Plants' Leaves Are Brown This Summer (And How To Fix It)

Tomato plants don't hide their problems. Leggy plants are getting too much nitrogen, cracked fruit indicates overwatering, and leaves with holes are being chewed by insects. There are several reasons for brown spots on tomato plant leaves, but browning is most commonly caused by fusarium wilt, a disease that causes stunted plants with dying, brown leaves. There is no cure for fusarium wilt once the tomato plant shows symptoms. Infected plants must be dug up and discarded. While you may not be able to save individual plants infected by wilt, you can take measures to make your garden safe for the next growing season.

Fusarium wilt is in the soil and enters the tomato plant through the roots. It can't be transmitted aboveground, so you don't need to worry about plant-to-plant infection. Soil infected with wilt fungi can be introduced into the garden on contaminated garden tools, mulch, manure, or new plants.

Fusarium first appears as yellowing leaves that turn brown, with wilting plants that recover slightly at night when the fruits begin to ripen. This sometimes happens on one side of the plant. Verticillium wilt, a very similar pathogen, affects the entire plant uniformly, and could be the reason your tomato plants are wilting. Each of these invades the plant's vascular system and makes it hard for the tomato to get water. With verticillium wilt, affected plants may not die, and the tomatoes they produce are safe for consumption. Fusarium wilt, however, kills the plant entirely.

Fusarium only affects tomato plants; verticillium infects a variety of vegetables like peppers and squash in addition to tomatoes. Since infected tomato plants can't be treated, it's best to avoid introducing the fungi into your garden. It takes years for soil to be free of wilt. Only plant crops in affected areas of the garden that are not susceptible to the fungi. Choose one of the resistant tomato varieties for garden plots you are certain are free of disease. If you're uncertain about your soil, grow tomatoes in containers with commercial potting mix. Clean and sterilize your garden tools after each use, and keep your greenhouse and things inside it clean and sterile. Clean your shoes so you don't track wilt-infected dirt around the yard. 

Early, late, and Southern blight can also cause brown discoloration on leaves; tomato blight should be stopped in its tracks. Leaves on lower branches of tomato plants afflicted with early blight have brown spots encircled in yellow. This is frequently caused by water splashing on the lower leaves, and there are fungicides available to treat it. Late blight, the same water mold that caused the Irish potato famine, is uncommon. Dark spots grow until they've taken over entire leaves and the plant dies. There is no treatment once the spots have appeared. Southern blight first manifests itself as overall plant wilting followed by small, brown bumps. Plants infected with late or Southern blight should be discarded.