The appearance of yellow or brown leaves on a geranium signals plant stress, but the discoloration is a symptom, not the root problem. Identifying the precise cause requires examining the pattern and location of the damage on the foliage. Geraniums, specifically the common Pelargonium species, display distinct visual cues that differentiate environmental distress from nutritional imbalances or biological threats. Correctly diagnosing the issue is the necessary first step toward restoring the plant’s health and preventing further damage.
The Most Common Culprits: Watering and Light Issues
Improper watering is the most frequent cause of foliar discoloration because geraniums are drought-tolerant and susceptible to “wet feet.” Overwatering or poor soil drainage starves the roots of oxygen, creating an environment ripe for fungal root rot. This manifests as general yellowing of the oldest, lower leaves, which become limp and may turn brown and mushy as the root system collapses.
In contrast, signs of underwatering appear as dry, crispy brown edges that progress inward from the leaf margin. The entire leaf may become brittle, and the plant will wilt, but the soil will be noticeably dry. A geranium that is consistently too dry prioritizes survival, leading to a reduction in leaf size and overall plant vigor.
Light exposure plays a significant role in leaf color; too little sunlight causes chlorosis. When a geranium is kept in a heavily shaded spot, the leaves turn a pale yellow-green as the plant struggles to produce chlorophyll. Growth becomes weak and spindly as the plant stretches in search of brighter light.
Conversely, intense, direct afternoon sun can cause sunburn, especially after cloudy weather. This appears as bleached, patchy white or brown spots that develop quickly on the most exposed leaves. The sudden heat and intensity cause the leaf tissue to scorch and turn necrotic.
Nutritional Deficiencies Mimicking Damage
Once watering and light are ruled out, yellowing may stem from a lack of essential nutrients. Deficiencies are distinguished because certain nutrients are mobile, meaning the plant can relocate them from older leaves to new growth. Nitrogen deficiency, for example, causes uniform yellowing (chlorosis) that starts on the oldest, lowest leaves while new growth remains green.
Magnesium deficiency, also a mobile nutrient issue, causes interveinal chlorosis on older leaves. The tissue between the veins turns yellow while the veins remain green, creating a distinct marbled pattern. This indicates the plant cannot properly manufacture chlorophyll, which relies on magnesium.
Potassium deficiency is visible as yellowing or scorching along the margins of the older leaves. This marginal necrosis occurs because potassium regulates water and nutrient transport, and a lack of it causes the leaf edges to dry out. Addressing these imbalances usually requires a complete fertilizer application or an amendment like Epsom salts for a magnesium boost.
Identifying Pests and Pathogens
When discoloration is accompanied by specific spots or textures, the problem is biological. Geranium rust, a common fungal disease, is identified by distinct orange or reddish-brown pustules on the underside of the leaves. These raised spots correspond to small, yellow-green specks on the top surface, and the infection often spreads in concentric rings.
Bacterial blight (Xanthomonas campestris) is serious and often begins with V-shaped yellow or brown lesions starting at the leaf margin. As the infection progresses, the leaves wilt, and the vascular tissue inside the stem can turn black, indicating the entire plant is compromised. Infected plants should be removed and destroyed immediately to prevent spreading through water splash or contaminated tools.
Small insects can lead to discoloration through feeding damage. Spider mites, which are nearly invisible, suck contents from leaf cells, causing a fine, yellowish stippling or a dull, dusty look. Whiteflies are visible as tiny white insects that fly up when disturbed; their feeding results in plant weakness and a sticky residue called honeydew.