When a tomato plant begins turning brown, it is exhibiting necrosis, which is the localized death of plant tissue. This visible change is a physical manifestation that the plant is under duress from its environment. Tomato plants are sensitive biological indicators, quickly displaying distress signals in response to unfavorable changes in their growing conditions. Diagnosing the specific cause requires careful observation of where the browning starts, how quickly it spreads, and the overall pattern on the leaves, stems, or fruit. Understanding these patterns allows growers to differentiate between environmental stress, nutritional deficiencies, and infectious pathogens.
Environmental and Cultural Stressors
Improper watering is a common cause of non-infectious browning in tomato plants. Overwatering restricts oxygen flow to the roots, leading to root suffocation and subsequent wilting, often followed by browning of the leaves. Conversely, severe underwatering forces the plant to draw moisture from the leaf edges first, resulting in crisp, dry, and brown margins. These issues generally affect the entire plant uniformly.
Sudden drops in temperature can cause cold damage, often appearing as a blackening or browning of the lowest leaves. Extreme heat and intense direct sunlight can lead to sunscald, where areas exposed to the sun develop bleached, papery brown spots, particularly on developing fruit or upper leaves. This type of damage is localized to the areas receiving the most intense exposure.
Accidental exposure to chemical agents, such as herbicide drift from nearby lawn treatments, can also induce necrosis. Herbicide symptoms often manifest as distorted growth coupled with browning or yellowing along the leaf edges. Physical damage, like stem breakage or rubbing against supports, creates localized points of injury that may turn brown as the plant attempts to heal the wound. These non-pathogen issues are often remedied by adjusting the immediate physical growing conditions.
Specific Nutrient Imbalances
Excessive application of fertilizers can lead to salt buildup in the soil, causing chemical dehydration of the roots. This condition, known as fertilizer burn, typically appears as browning and crisping that starts at the tips and outer edges of the leaves. Since the mineral salts are concentrated in the soil solution, the plant struggles to absorb water, resulting in marginal necrosis.
Browning caused by a lack of potassium often begins on the older, lower leaves of the plant. Potassium is a mobile nutrient, meaning the plant moves it from older tissues to support newer growth, leaving the older leaves deficient. The symptom starts as yellowing between the veins before progressing to a distinct browning along the outer leaf margins.
Calcium deficiency is a major cause of fruit browning known as Blossom End Rot (BER). This physiological disorder presents as a dark, sunken, leathery brown patch forming on the blossom end of the tomato fruit. BER is often caused by the plant’s inability to consistently take up calcium, which is an immobile nutrient, usually due to erratic watering. Consistent soil moisture is necessary to ensure the continuous flow of calcium into the developing fruit tissue, even when the soil contains adequate calcium.
Identifying Infectious Diseases
Early Blight, caused by the fungus Alternaria solani, is characterized by distinct dark brown or black spots that appear on the lower, older leaves first. A defining feature of Early Blight is the presence of concentric rings within the spots, creating a “target” pattern. As the disease progresses, these necrotic spots merge, causing the entire leaf to brown, curl, and drop prematurely.
Late Blight, caused by the water mold Phytophthora infestans, is a destructive pathogen that causes rapid, widespread necrosis on leaves and stems. Symptoms initially appear as dark, water-soaked lesions that quickly expand into large, greasy, and irregular brown or black patches. This disease thrives in cool, wet conditions and can cause an entire plant to collapse and turn brown within a few days. The undersides of the leaves may also display a fuzzy white mold growth.
Vascular wilts, such as those caused by Fusarium and Verticillium fungi, are soil-borne pathogens that invade the plant through the roots. These fungi block the xylem, which is the plant’s water transport system, leading to browning and wilting even when the soil moisture is adequate. A distinguishing symptom is that the wilting and subsequent browning often affect only one side of a stem or one branch of the plant.
Cutting a wilted stem lengthwise may reveal brown discoloration in the vascular tissue, which is a sign of a vascular wilt pathogen. Unlike cultural issues, infectious diseases often require specific management strategies, such as the application of targeted fungicides. In severe cases, the removal and destruction of the infected plant material is necessary to prevent the spread of the pathogen to nearby healthy plants.