Brown spots on plants are a common symptom of underlying stress, representing necrotic tissue (dead plant material). This tissue death can be caused by factors ranging from incorrect watering to microscopic pathogens. Since the spot is only a symptom, accurately identifying the precise cause is necessary for effective treatment. Treating an insect problem with a fungicide, for example, wastes valuable time and resources.
Environmental Stress (Water, Light, and Temperature)
Improper care and unfavorable growing conditions are the most frequent culprits, and the appearance of the necrotic tissue often provides clues about the nature of the stress.
Overwatering is a common cause, leading to root rot when the soil remains saturated and oxygen is depleted. This root damage prevents the plant from transporting water and nutrients, resulting in soft, dark brown, or mushy spots, often starting at the leaf base or margin. These spots may sometimes be surrounded by a distinct yellow ring, or halo, indicating the plant’s attempt to isolate the damaged area.
Conversely, underwatering causes the plant to lose turgor pressure, leading to dehydration, especially in areas furthest from the stem. This manifests as crispy, dry, and brittle brown edges or tips on the leaves. Low atmospheric humidity, particularly for tropical houseplants, creates a similar effect by causing moisture loss faster than the roots can absorb it, resulting in brown, desiccated leaf tips.
Intense, direct sunlight can result in sun scald, causing the leaf tissue to rapidly overheat and die, appearing as bleached or scorched brown patches. Extreme heat can also cause cell walls to rupture, a condition known as edema, which forms small, pitted brown scars on the leaf surface. Temperature extremes, such as sudden drops or exposure to frost, cause the water inside the plant’s cells to freeze and expand. This cellular rupture leads to the sudden appearance of brown, wet, or slimy spots on the foliage.
Identifying Fungal and Bacterial Infections
Infectious diseases caused by fungi and bacteria also produce necrotic brown spots, but their appearance differs significantly from environmental damage. Fungal leaf spots, such as those caused by Septoria or Anthracnose, often have a dry, papery texture and a well-defined circular shape. Many fungal infections display a “target spot” pattern, featuring concentric rings of different shades of brown or tan, showing the organism’s outward growth from the initial point of infection.
Bacterial leaf spots, typically caused by genera like Pseudomonas or Xanthomonas, present a different visual signature. These spots often appear water-soaked, slightly sunken, and possess a wet or oily look. Unlike the defined circles of many fungi, bacterial spots are frequently angular, limited by the leaf veins. Both pathogens are easily spread by water splashing onto the foliage, but high humidity and poor air circulation often enable fungal spores to germinate and infect the tissue.
Damage Caused by Plant Pests
Several common plant pests feed in ways that directly cause localized tissue death. Sucking insects, including scale, mealybugs, and thrips, use piercing mouthparts to extract sap from the plant cells. The small wounds and the removal of cellular contents result in minute, localized necrosis.
Thrips often leave behind silvery streaks on the leaf surface where they have scraped the tissue, with the damaged areas eventually turning into large brown patches. Spider mites cause a fine stippling pattern—tiny brown or bronze speckles—which can merge into larger discolored areas over time. When diagnosing, look for physical signs of the pests themselves, such as the immobile, shell-like brown bumps of scale insects or the fine, silken webbing left behind by spider mites.
Chemical and Nutrient Issues
Brown spots can also signal problems related to the chemical composition of the soil or water, or an imbalance of essential minerals. Fertilizer burn occurs when an excess of soluble salts in the fertilizer accumulates in the soil, which draws water out of the plant’s roots, causing localized dehydration. This toxicity typically manifests as crispy, brown scorching along the leaf edges or tips.
Similarly, using hard tap water can lead to a buildup of minerals like fluoride, which is toxic to many plant species. This chemical toxicity often results in marginal necrosis, causing the leaf tips to turn brown. Specific nutrient deficiencies can also lead to necrotic spots; for example, a lack of potassium can cause the leaf margins to die. Chemical overspray from pesticides or herbicides can also directly burn the foliage, resulting in irregularly shaped brown patches where the chemical has made contact.