Tomato Diseases

Above: Early blight of tomato; photo by Southwest Purdue Ag Program

Those of us who enjoy vegetable gardening begin the year with high hopes. Then, when diseases and insects begin to affect our crops, we’re fighting these pests once again. Throw in unusual weather patterns, and it seems we’re always battling something.

Recently, heavy rains poured on our gardens, and in some gardens, the unwanted effect on tomatoes was that they began to crack and split. Perhaps this happened to your tomatoes?

Let’s take a deeper dive into some of the key tomato diseases and abiotic (non-living) factors that cause disease-like symptoms. Thanks to the staff at the Southwest Purdue Ag Program (SWPAP) in Vincennes, IN, for compiling much of the following information.

Anthracnose is a disease that often affects tomato fruit that are overripe and close to the ground. It appears as a sunken lesion on the fruit with orange-colored sporulation.

Bacterial canker is first noticed by the browning on leaf edges, and if the stem is cut low to the ground, it also has internal discoloration. The fruit may have a distinctive birds-eye lesion.

Bacterial speck is an early-season disease that causes small spots on leaves and on tomato fruit. Each speck has a chlorotic (yellow) halo around it.

Bacterial spot is a disease that causes lesions that appear “water-soaked,” and chlorosis occurs only when several lesions are in close proximity. Lesions on fruit may be large and scab-like.

Blossom-end rot is a non-infectious condition that almost every tomato gardener has experienced. Leathery lesions appear on the blossom end of the fruit (the bottom). This condition is thought to be caused by a calcium deficiency in the fruit brought on by wide fluctuations in moisture. Frequent watering and mulch can help mitigate the condition.

Buckeye rot causes a dark concentric pattern of necrosis (dead tissue) on the tomato fruit.

Catfacing is a non-infectious disorder resulting in malformed fruit with puckering, scarring, and cavities that may penetrate deep into the fruit. When the flower buds are exposed to cold, some blossoms stick to the young, developing fruit, resulting in the malformation.

Cercospora leaf mold is a disease causing large leaf lesions with diffuse chlorotic margins.

Early blight is one of the most common tomato diseases in the Midwest. Older leaves are more severely affected, having concentric ring lesions. The disease can also cause petiole (leaf stem) lesions.

Fusarium crown and root rot causes general decline and wilt of the whole plant. You’ll typically find a lesion on the base of the stem near the soil, which also causes discoloration inside the stem.

Fusarium wilt causes one-sided wilt on the tomato plant. Many of our modern hybrids have resistance to this disease.

Gray mold symptoms can be found on leaves, stems, and fruit. Leaf lesions are light brown, and stem lesions are darker brown. All parts of the plant may be covered with gray mold.

Intumescence is a condition, not a disease, in which plant parts may have bumpy growth. It is not an important or common problem.

Late blight is not common in Indiana, but when it does occur, quick action and specific fungicides are required to manage the disease. Necrotic leaf lesions may be ringed with white sporulation of the causal fungus. Fruit lesions appear to have a soft brown area.

Leaf mold causes chlorotic lesions with indistinct borders. On the underside of the leaf, you can see olive-green sporulation of the causal fungus.

Leaf roll is not an infectious disease, caused by plant stress. Some varieties are genetically more predisposed to the condition.

Pith necrosis is usually not an important problem. It causes irregular necrotic lesions on the stem.

Powdery mildew is one of the most easily recognizable diseases, with white talc-like lesions on leaves and stems. This is uncommon in field-grown tomatoes, and usually it is not a problem.

Septoria leaf spot is the disease I have personally observed the most. It is a common disease where symptoms first occur on older (lower) leaves and move upward on the plant. Leaf spots are medium brown with gray centers. Dark fungal structures can often be observed in the center of these lesions. Over time, lesions coalesce, and entire leaves turn brown. Fruit is unaffected.

Southern blight causes sudden wilt and decline of the plant. A stem lesion will be present at ground level. This is not common in Indiana.

Target spot causes ring-like lesions, and it is not common in Indiana.

Tomato spotted wilt virus may have several symptoms, including stunting, chlorosis, round lesions, and discolored fruit. The disease is transmitted by thrips (tiny insects).

Walnut allelopathy causes wilting of tomato plants grown near walnut trees.

White mold, also known as timber rot, causes wilting of tomato plants due to stem lesions. Inside the stem, dark, irregularly shaped structures are present. Stems can become dry and brittle, and fruit can be affected.

Lastly, zippering is a non-infectious disorder that produces a zipper-like scar from the stem-end toward the blossom end of the tomato fruit.

Access the SWPAP page on tomato diseases, complete with helpful pictures. Samples can be sent for analysis to Purdue’s Plant and Pest Diagnostic Lab.