Hybrid tomatoes are commonly grown in many home gardens because many varieties offer greater disease resistance and consistent fruit. While hybrids are usually modified to be resistant to many diseases and pests, they are still subject to cultural issues that can lead to leaf problems. And while a hybrid tomato may be resistant to some pests, it isn't resistant to all of them; proper diagnosis and treatment is needed if the leaves begin to brown.
Improper watering is the first cause to look for if the tomato leaves begin browning. The culprit is often too much water, not too little. Improperly irrigated tomato plants often show browning around the edges of the leaves. There may also be a yellow tinge. Tomatoes require about 2 inches of water a week. Irrigate them once a week deeply, keeping the soil moist but not soggy. Apply a layer of mulch around the plants to help retain moisture without the need for overwatering.
Leaves begin to brown at any stage of development if they are infected with bacterial canker. This disease may only affect leaves on one side of the tomato plant, with the other side appearing healthy and unharmed. Eventually, affected plants will wither and die. Planting resistant, certified disease-free hybrids is the best course of action to prevent this disease. Any plants affected with the disease must be dug up and destroyed, or it can spread to nearby healthy plants.
Gray mold affects damaged leaves first. The damaged area turns brown, then a gray, fuzzy mold begins growing on the affected spot. Gray mold attacks plants through the stem, so inspect it for small lesions or damage near the soil level if gray mold is suspected. Unfortunately, even hybrid tomatoes fall victim to gray mold (though some varieties, such as the Duke hybrid, are less prone to the fungus). Prevention is the only method of control. Spray the plants with a fungicide after planting if you live in an area prone to the fungus or have had problems in the past.
Tomato Spotted Wilt
Spotted wilt causes brown spots or streaks on the leaves, and streaks appear on stems as well. Spotted wilt is spread by insects, primarily thrips. While some hybrids are less susceptible, in areas where spotted wilt infections are common, proper precautions must be taken. Applying insecticide throughout the growing season prevents both thrips from feeding on the plants and the spread of spotted wilt.