Brown spots can range from a cosmetic nuisance to a sign of serious infectious disease. The shape, texture, and location of spots carry diagnostic information — learning to read them is the fastest path to effective treatment.
Dr. Sarah Green
PhD Horticulture · Updated February 2026
4
Distinct cause categories
#1
Most infectious symptom type
65%
Cases are fungal or bacterial
Match your spot description to the most likely cause before reading further.
Fungal spores thrive in humid, poorly ventilated conditions — producing circular spots with defined halos.
Fungal pathogens are the leading cause of brown spots on both indoor and outdoor plants. They thrive in warm, humid conditions with poor air circulation. Common culprits include Alternaria leaf spot, Botrytis (grey mold), and Cercospora, each producing characteristic spot shapes and colors.
Fungal spots typically have a defined edge — often with a darker brown or yellow halo around a lighter center, sometimes with concentric rings. Spots start small and expand over time, and adjacent spots may merge into larger necrotic patches. White, grey, or rust-colored powder on spots indicates a surface fungal infection like powdery mildew.
Remove all affected leaves and dispose away from other plants — do not compost. Improve air circulation by spacing plants further apart. Apply a copper-based fungicide or neem oil spray (1 tsp per liter with a drop of dish soap). Repeat every 7–10 days. Avoid wetting foliage when watering.
Angular, water-soaked lesions that dry brown — spread rapidly through water splash and contaminated tools.
Bacterial infections produce water-soaked spots that begin as small, dark, greasy-looking lesions. Over time these dry out and turn brown, often surrounded by a yellow halo as the plant's immune response attempts to contain the infection. Bacterial diseases spread through water splash, contaminated tools, and physical contact.
Unlike fungal spots, bacterial spots are often angular and geometric, bounded by leaf veins that act as barriers to spread. The lesions may appear translucent and wet when held up to light. Bacterial diseases accelerate rapidly in warm, wet conditions and can devastate a plant if not caught early.
There is no cure once established, but spread can be controlled. Remove affected leaves, sterilize pruning tools with 70% isopropyl alcohol between cuts, and avoid overhead watering. Copper-based bactericides offer some protection. Avoid excess nitrogen fertilization, which produces soft tissue more susceptible to bacterial invasion.
Bleached, papery patches on the sun-facing side — appear suddenly after a change in light exposure.
Sunscorch produces bleached, tan, or crispy brown patches on leaf areas that receive the most direct sun. The damage is caused by UV radiation and heat intensity exceeding what the plant's tissue can tolerate, killing cells and causing rapid dehydration of affected areas.
Scorch marks appear suddenly following a change in sun exposure — moving a plant outdoors in spring, rotating it toward a window, or a rare sunny day in an otherwise overcast location. The spots are dry and papery rather than wet or soft. Scorch is always non-infectious and will not spread.
Move the plant out of direct midday sun. If transitioning to higher light, acclimate gradually over 2–3 weeks. Scorched leaves will not recover but new growth will emerge undamaged once the plant is repositioned. Increase watering frequency slightly during hot sunny periods.
Mushy, irregular brown patches — the plant cannot transport water due to rotted roots below.
When root rot progresses, the plant's inability to transport water results in soft, brown patches forming on leaves — often starting at edges or tips and working inward. These spots differ from disease-caused spots in that they are mushy rather than dry, and may be accompanied by overall yellowing and a foul odor from the soil.
Root rot-related spotting tends to be irregular in shape compared to fungal or bacterial lesions. The entire plant looks stressed — not just isolated leaves. Checking soil and roots is essential to confirm this diagnosis.
Remove the plant from its pot, trim rotted roots, treat with a hydrogen peroxide solution (3%), and repot in fresh dry mix. Reduce watering frequency and ensure the pot has adequate drainage. Do not water again until the top 2 inches of soil are dry.
Inspect spot texture
Dry and papery = sunscorch. Wet and soft = bacterial or root rot. Dry with defined circular margins = fungal disease.
Check spot location on the leaf
Top surfaces only suggest scorch. Underside spots suggest fungal spores or pest damage. Both surfaces indicate systemic infection.
Examine spot shape and pattern
Angular spots bounded by veins = bacterial. Circular "target" spots = fungal. Irregular spreading patches = overwatering damage.
Check neighboring plants
If multiple plants are developing similar spots, an infectious pathogen is spreading. Quarantine affected plants immediately.
Test soil moisture and inspect roots
Soggy soil with mushy leaf spots confirms overwatering. Remove from pot to check for brown, soft root tissue.
Space plants so air flows freely around all foliage. Crowded shelves create the humid microclimates where fungal spores germinate.
Bottom watering eliminates wet foliage — the primary vector for fungal and bacterial infections. If top watering, do it in the morning.
Isolate new plants for 2–3 weeks before placing near your collection. Even nursery plants can carry dormant spores.