Brown spots on plant leaves indicate that tissue has died, a process known as necrosis. These symptoms are not a single disease but a general reaction to damage, making diagnosis challenging because many underlying causes look similar. Accurately determining the cause requires close inspection of the spot’s size, shape, color, location, and the overall growing environment. Understanding the difference between infectious and non-infectious causes is the first step toward successful recovery.
Spotting Caused by Fungi and Bacteria
Brown spots caused by living pathogens, such as fungi and bacteria, are infectious and can spread, often requiring high humidity for dispersal. Fungal leaf spots typically appear dry, rounded, and may exhibit distinct patterns like concentric rings, resembling a target. A fungal spot often has a dark margin surrounding a lighter tan or gray center. In high humidity, tiny black dots—the fungal fruiting bodies—may become visible within the lesion.
Bacterial leaf spots, in contrast, frequently present as water-soaked lesions that look mushy or translucent, sometimes with a greasy appearance. These lesions are often angular, meaning their expansion is stopped by the leaf’s vascular tissues, giving them straight sides that follow the leaf veins. A bacterial spot may also be surrounded by a bright yellow halo where bacterial toxins have inhibited the plant’s chlorophyll production. Bacteria typically enter the plant through natural openings like stomata or through physical wounds.
Abiotic Factors: Water, Light, and Temperature Stress
Non-living environmental factors are responsible for a significant number of brown spot issues. Improper watering is a frequent culprit, with both extremes causing tissue death. Underwatering deprives leaf margins and tips of moisture, leading to desiccation that appears as dry, crispy brown edges. Conversely, chronic overwatering leads to root rot, impairing the roots’ ability to absorb water and nutrients, which mimics drought stress on the leaves. Excessive water uptake can also cause edema, where cells burst from internal pressure, leaving behind small, corky brown bumps or scars.
Light and temperature extremes also cause localized tissue damage. Sun scorch, or sunburn, occurs when a plant is suddenly moved into intense light, causing exposed cells to overheat and die, resulting in bleached, papery, or large brown necrotic patches. Cold stress, such as frost, causes water within leaf cells to freeze and rupture, manifesting as dark, water-soaked, and slimy brown spots. High temperatures can also cause general wilting followed by widespread browning due to excessive water loss.
Chemical and Nutritional Imbalances
The chemical environment of the plant, including soil composition, is a source of brown spots. Specific nutritional deficiencies can lead to distinct patterns of browning, particularly with mobile nutrients that the plant moves from older to newer growth. For example, a severe potassium deficiency often presents as a scorched appearance along the tips and edges of older leaves. Magnesium deficiency causes yellowing between the veins of older leaves (interveinal chlorosis), which can progress to brown, necrotic spots. These symptoms are symmetrical across the leaf and affect specific ages of foliage.
External chemical exposure can result in phytotoxicity, commonly called chemical burn. This damage appears as irregular brown or black spots, streaks, or scorched margins where a concentrated substance made contact with the leaf surface. Common causes include over-fertilization, where high salt concentrations draw water out of the plant cells, or the incorrect application of pesticides or herbicides. Mineral buildup from hard tap water can also accumulate on leaf surfaces, leading to localized brown spots or tip burn.