Leaf bumps are raised areas or physical deformities on plant foliage, indicating a reaction to an external irritant or an internal physiological issue. These bumps are symptoms caused by various factors, including pests, environmental imbalances, or infectious pathogens. Often, these growths are the plant’s attempt to wall off an invader or seal damaged tissue, resulting in abnormal cell growth. Identifying the specific nature of the bump is the first step toward effective management and ensuring the plant’s continued health.
Identifying Insect and Mite Activity
Many noticeable leaf bumps are galls, which result from arthropod manipulation of plant tissue. A gall is an abnormal growth of plant cells stimulated by the feeding or egg-laying of an insect or mite. The gall-forming organism, such as a gall wasp or midge, injects chemicals that hijack the plant’s growth hormones. This manipulation forces the plant to create a protective, nutrient-rich structure where the pest’s larvae can develop safely.
Galls can appear as hard spheres, blister-like swellings, or velvety, fuzzy patches called erineum galls. Although cosmetically unsightly, galls rarely threaten the overall health of a mature plant. The damage occurs early when leaves are developing, and once the gall is formed, the organism is sealed inside and protected from topical treatments.
Scale insects are another common source of bumps, though the bump is the insect itself covered by a protective shell. Scale insects are mostly immobile, anchoring themselves to leaves and stems to feed on plant sap. Armored scale insects create a hard, waxy covering separate from their body and do not excrete a sticky substance.
Soft scale insects are covered in a softer, waxy layer and produce a sugary liquid called honeydew. This honeydew often leads to the secondary development of sooty mold, a black fungus that grows on the residue. Unlike galls, scale insects can be physically scraped off the leaf surface, which aids in diagnosis.
Bumps Caused by Environmental Stress
Some leaf bumps are physiological reactions to unsuitable growing conditions rather than living organisms. Edema, or dropsy, is a non-contagious disorder occurring when roots absorb water faster than leaves can release it through transpiration. This imbalance is common when soil is warm and moist, but the air is cool and saturated with humidity.
Excess water pressure causes leaf cells to swell until they rupture, usually on the underside of the foliage. These burst cells dry out, forming tiny, permanent, blister-like growths or corky patches. Edema is often seen on houseplants and signals a need to adjust cultural practices, especially watering and air circulation.
Small, hard bumps can also be external residue rather than a plant defect. When hard water, containing high concentrations of dissolved minerals, is used, evaporation leaves behind a subtle mineral deposit. Chemical residues from foliar sprays or fertilizers can also dry on the leaf surface, creating a rough, gritty texture.
Fungal Infections and Pathogen Lesions
Infectious agents, particularly fungi and some bacteria, cause distinct, raised spots on leaves during disease progression. Rust diseases, caused by fungi in the order Pucciniales, are characterized by the production of highly visible, raised pustules. These bumps are typically orange, yellow, or rusty brown and contain masses of powdery spores spread by wind or water.
The pustules are often clustered on the underside of the leaf, sometimes with corresponding yellow spots on the upper surface. Rust thrives when leaf surfaces remain wet for extended periods. While rust rarely kills the host plant, heavy infections cause significant leaf yellowing and premature drop.
While many leaf spots are flat, sunken areas of dead tissue, some pathogens cause the plant to form raised, textured lesions. Fungal diseases, such as scab, create small, corky, or rough spots as the plant forms a protective boundary around the infection site. These raised lesions often have a darker, defined border and may be accompanied by chlorosis, indicating an active infection.
Managing and Treating Leaf Bumps
The approach to managing leaf bumps depends entirely on accurately identifying the cause, whether it is a pest, a pathogen, or an environmental factor. For bumps caused by insects, specifically galls, intervention is rarely possible once the abnormal structure has formed, as the pest is protected inside. Instead, management focuses on prevention by applying a targeted insecticide or horticultural oil early in the spring when the causal insect is active and before it lays eggs on the newly emerging foliage.
Scale insects can be treated by mechanical removal, such as scraping them off or dabbing them with rubbing alcohol. For heavier infestations, treatment involves smothering the pests with horticultural or neem oil, which suffocates the insect. Systemic insecticides are sometimes used for soft scale, as the pesticide moves through the plant’s vascular system to kill feeding insects.
Edema requires immediate cultural adjustments to correct the water imbalance. This involves reducing watering frequency, especially during cool or cloudy periods, and ensuring excellent air circulation to promote healthy transpiration. The damaged leaves will not recover their smooth appearance, but the condition will stop spreading once environmental factors are corrected.
For infectious bumps like rust and fungal leaf spots, sanitation is the most immediate step. This involves the careful removal and destruction of all infected leaves and plant debris. Avoiding overhead watering is important to keep the foliage dry, preventing the germination and spread of fungal spores. Fungicides containing sulfur or copper can be applied preventatively at the first sign of disease.