Tomato leaves turning brown signals that the plant is under stress. This browning, or necrosis, is the plant’s final response to various issues, ranging from environmental oversights to infectious diseases. Identifying the precise cause is the first step toward recovery, as the location and pattern of the browning offer important clues. Symptoms generally fall into categories such as damage from pathogens, environmental imbalances, or acute external injury.
Fungal and Bacterial Pathogens
Infectious diseases frequently cause leaf browning, often characterized by specific patterns. The fungal disease Early Blight (Alternaria solani) typically begins with dark, circular spots on the oldest, lowest leaves. These lesions develop concentric rings, creating a “bullseye” pattern, and are often surrounded by a yellow halo. Spores survive in the soil and are splashed onto the lower leaves by rain or watering, allowing the infection to spread upward.
Another common fungal culprit is Septoria Leaf Spot, which also starts on the lower leaves but presents differently. Its spots are smaller, usually 1/16 to 1/4 inch in diameter, appearing as numerous circular lesions with dark margins and a tan or grayish center. A unique feature is the presence of tiny, pinhead-sized black dots, visible within the center of the spots. Both diseases thrive in high humidity and moderate temperatures, eventually causing the infected leaf to yellow, wither, and turn brown before dropping off.
Bacterial diseases, such as Bacterial Canker, cause browning that is less spot-focused and more systemic. Initial symptoms often appear as marginal browning or necrosis, sometimes called “firing,” which spreads inward from the edges of older foliage. This bacterium (Clavibacter michiganensis subsp. michiganensis) can also cause one-sided wilting as it invades and clogs the vascular system. Systemic bacterial infections are more difficult to manage than localized leaf spots because they affect the plant’s entire internal structure.
Water and Nutrient Imbalances
Browning not caused by a distinct spot often points to a physiological disorder related to moisture or nutrient management. Under-watering, or drought stress, causes leaves to turn uniformly yellow before becoming dry and crispy, particularly at the edges. This occurs as the plant sacrifices older foliage to conserve water, leading to the death of tissue on the edges first when water supply is insufficient.
Conversely, overwatering can cause browning by leading to root suffocation and root rot. When roots sit in saturated soil, they cannot take up oxygen, impairing their ability to absorb water and nutrients, resulting in symptoms similar to drought stress. The key difference is that with overwatering, the soil will be clearly saturated, and the wilting will not recover even after the sun goes down.
Severe nutrient deficiencies can also lead to leaf browning, most notably a lack of potassium. Since potassium is a mobile nutrient, the plant moves it from older leaves to newer growth, causing symptoms to appear first on the lowest foliage. This deficiency manifests as yellowing along the margins of older leaves, progressing to a scorched brown appearance, known as marginal necrosis. A soil test is the most reliable way to confirm if a nutrient imbalance is the underlying cause.
Environmental and Chemical Damage
Acute external factors, rather than slow-developing diseases, can cause immediate and localized browning. Sunscald, the plant equivalent of a sunburn, affects foliage, especially in newly transplanted seedlings or plants suddenly exposed after heavy pruning. The exposed leaves develop bleached, gray-white, or brown blotches on the side directly facing the sun.
Windburn is another physical stressor that leads to browning, typically appearing as dry, scorched edges or tips, similar to drought stress. Strong, constant wind increases the rate of water evaporation from the leaves, overwhelming the plant’s ability to supply moisture and causing the tissue to dry out rapidly. This damage is most apparent on plants positioned in exposed areas.
Chemical damage, such as from fertilizer burn or herbicide drift, causes immediate, distinct symptoms. Excess fertilizer or salt buildup in the soil can cause the edges of the leaves to look scorched and brown. Herbicide drift, even from a distant application, is indicated by distorted, curled, or twisted new growth, followed by blotchy browning or yellowing. The pattern of chemical damage tends to be irregular or localized to one side of the plant, depending on the drift’s direction.
Immediate Steps for Plant Recovery
Regardless of the diagnosis, the first step for recovery is the immediate removal of all affected foliage. Prune off any leaves displaying spots, yellowing, or browning, cutting them back to the main stem using sterile tools. This sanitation measure is important for managing infectious diseases, as it reduces the amount of spores available to spread to healthy upper leaves. All removed plant material should be bagged and discarded, not composted, to prevent pathogens from overwintering.
Next, the plant’s immediate environment requires attention, starting with a check of soil moisture. If the soil is dry, water deeply and consistently at the base of the plant to alleviate drought stress, ensuring water penetrates the root zone. If the soil is saturated, stop watering immediately and allow the top few inches to dry out completely before resuming a less frequent watering schedule. Applying organic mulch around the plant base helps prevent soil-borne spores from splashing onto the lower leaves during watering or rain.
Improving air circulation is a quick, effective preventative and recovery action, especially in humid conditions that favor fungal growth. Strategic pruning of non-diseased leaves in the lower canopy can open up the plant, allowing faster drying of the remaining foliage. For plants diagnosed with a nutrient deficiency, a soil test provides the data needed for a targeted soil amendment, avoiding reliance on general-purpose fertilizers.