The appearance of brown spots on tomato leaves is one of the most common and frustrating issues for anyone growing the plant. This symptom signals a breakdown in leaf tissue, known as necrosis, which can be triggered by various factors, making accurate identification challenging. Spotting is a general visual cue, not a specific disease, meaning careful observation of the spot’s size, shape, location, and pattern is necessary for proper diagnosis. Understanding whether the cause is a living pathogen or a non-infectious environmental issue is the first step toward determining the plant’s problem.
Fungal Pathogens
Fungal diseases cause many leaf spot issues and thrive when leaves remain wet for extended periods. The resulting brown spots are distinct and often provide a clear visual signature of the underlying pathogen.
The fungus Alternaria solani causes Early Blight, which typically begins on the oldest, lowest leaves first. The characteristic lesion is a dark brown spot up to a half-inch in diameter, featuring dark, concentric rings that create a classic “bullseye” pattern. The surrounding tissue may turn yellow, and as multiple lesions expand and merge, the entire leaf turns brown and drops prematurely.
Another common fungal culprit is Septoria lycopersici, which causes Septoria Leaf Spot, often appearing shortly after the plant begins to set fruit. These spots are noticeably smaller than those of Early Blight, usually measuring no more than one-eighth of an inch across. Septoria lesions are circular with dark, defined borders and a lighter center that is gray or tan.
A distinguishing feature of Septoria Leaf Spot is the presence of tiny black specks, which are the fungal fruiting bodies (pycnidia), visible inside the light-colored center. Both pathogens favor warm, humid conditions; Septoria develops rapidly between 68 and 77 degrees Fahrenheit, while Early Blight prefers 75 to 84 degrees Fahrenheit.
Bacterial Infections
Brown spots caused by bacteria often present differently than fungal lesions, lacking the defined patterns and fruiting structures. The pathogens are frequently seed-borne or survive on plant debris, spreading via wind-driven rain and water splashing. Bacteria enter the leaf tissue through natural openings or small wounds.
Bacterial Spot, caused by Xanthomonas species, results in lesions that are initially small, dark, and water-soaked. These spots are often angular because their expansion is restricted by leaf veins, giving them a jagged edge. Under humid conditions, the spots can coalesce, giving the foliage a blighted appearance before the tissue dries out and turns necrotic brown.
Bacterial Speck, caused by Pseudomonas syringae pv. tomato, produces very small, dark brown to black spots that are typically more rounded. These specks are often surrounded by a faint yellow halo, which helps differentiate them. This disease is favored by cooler, wetter conditions compared to the warmer temperatures that accelerate bacterial spot. Both pathogens can infect the foliage, stems, and fruit, causing small, slightly raised, scab-like lesions.
Environmental and Nutritional Stress
Not all brown spots result from infectious disease; many are physiological disorders caused by environmental imbalances or nutritional issues. These abiotic factors result in necrosis distinct from pathogen-induced spotting, helping to narrow the diagnosis.
A common non-infectious cause is Edema, a physiological disorder where the plant absorbs water faster than it can transpire it. This leads to pressure buildup within the leaf cells, often triggered by high humidity, low light, and cool temperatures. The excess pressure causes cells, typically on the underside of the leaves, to swell, blister, and burst, resulting in tiny, warty bumps that turn brown and corky.
Nutritional Deficiencies
Certain nutrient deficiencies manifest as necrotic brown spotting, generally starting on the plant’s older leaves. Potassium deficiency first causes the edges of the oldest leaves to turn yellow, progressing into a distinct browning or “scorch” along the margins before dark spots appear. Magnesium deficiency displays as interveinal chlorosis (yellowing between the veins), and this yellowed area eventually develops necrotic brown spots or blotches.
Physical Damage
Physical damage can also create brown spots, such as Chemical Burn (phytotoxicity), which occurs when fertilizers or pesticides are misapplied. This damage often appears as spots, blotches, or scorched browning along the leaf edges where the chemical concentrated. Sunscald occurs when foliage is suddenly exposed to intense direct sunlight, often after a dense leaf canopy is lost. This exposure results in patches of white or brown necrotic tissue on the most exposed areas of the leaf.