Geraniums, whether the common annual Pelargonium species or the hardier perennial Geranium varieties, are popular flowering plants that signal distress through visible changes in their foliage. When geranium leaves begin to turn yellow (chlorosis) or red/purple (anthocyanin pigments), it indicates underlying stress, not a disease itself. This discoloration is the plant’s reaction to a suboptimal environment, a nutritional imbalance, or a biological threat. Understanding the specific patterns of this discoloration is the first step toward restoring the plant’s robust health.
Environmental Stressors
The most frequent causes of leaf yellowing relate directly to improper water management, which affects root function. Overwatering, characterized by soggy soil, suffocates roots, leading to root rot and uniform yellowing. Conversely, extreme underwatering can also cause leaves to yellow, typically starting at the tips and edges.
Temperature shifts can induce dramatic color changes, particularly reddish-purple pigments. Exposure to sudden cold snaps or cool nighttime temperatures causes the plant to produce anthocyanin, resulting in a distinct red or purple hue, common on older leaves in spring and fall. Too little light results in pale green or uniformly yellow leaves due to insufficient photosynthesis.
Nutrient Deficiencies Causing Discoloration
Yellowing and reddening are specific indicators of nutrient deficiencies, depending on where the discoloration appears. Nitrogen (N) deficiency causes generalized yellowing (chlorosis) that first manifests in older, lower leaves. Since nitrogen is a mobile nutrient, the plant relocates it to support new growth, leaving older foliage pale yellow. The distinct red or purplish tint is a classic sign of phosphorus (P) deficiency, causing older leaves to take on a deep reddish-purple coloration. Magnesium (Mg) deficiency results in interveinal chlorosis on older leaves, where the tissue between the veins turns yellow while the veins remain green.
In contrast to mobile nutrients, deficiencies of immobile nutrients like iron or manganese present their symptoms on the newest growth first. A lack of iron, for example, results in pronounced interveinal chlorosis on the young, terminal leaves. This occurs because the plant cannot move the element from older tissue to the growing tips. The precise location and pattern of the discoloration are essential clues for diagnosing the specific nutrient imbalance.
Pests and Pathogens
Biological threats, including fungal pathogens and insect pests, cause discoloration by interfering with the plant’s cellular structure. Geranium rust, caused by the fungus Puccinia pelargonii-zonalis, creates pale green or yellow spots on the upper leaf surface. Diagnosis is confirmed by observing the corresponding reddish-brown, raised pustules on the underside of the leaf. Bacterial leaf blight (Xanthomonas campestris pv. pelargonii) first appears as small, water-soaked spots that develop into V-shaped necrotic areas with a yellow border. Once this bacterial infection takes hold, the plant is generally incurable and must be removed to prevent spread.
Certain pests, such as spider mites and whiteflies, contribute to yellowing by extracting cell contents, resulting in tiny yellow or white stippling on the leaves. Viral infections, such as those that cause leaf curl or mottling, can lead to general yellowing and leaf distortion that is often permanent.
Diagnosing the Issue and Restoring Plant Health
Effective recovery begins with a systematic diagnostic checklist, starting with an assessment of the plant’s current environment and watering habits. Closely examine the leaves, paying attention to whether the discoloration is uniform or interveinal, and whether it affects old leaves (mobile nutrient or water stress) or new leaves (immobile nutrient).
Diagnosis Checklist
The initial assessment should include:
- Checking the soil moisture and drainage.
- Inspecting the plant’s exposure to light.
- Reviewing recent temperature fluctuations.
- Determining if discoloration is uniform or interveinal.
Treatment and Prevention
If overwatering is suspected, allow the top inch of soil to dry out completely before watering, ensuring the container has sufficient drainage holes. For cold stress, move the plant to a warmer location or cover it during cold nights. When a nutrient deficiency is confirmed, use a slow-release fertilizer or liquid feed tailored to the specific need.
If a pathogen like geranium rust is identified, immediately remove the affected leaves to limit the spread of spores. For bacterial blight, the entire infected plant should be discarded, as no chemical cure exists. Good cultural practices are the most effective preventative measures, including ensuring adequate air circulation, avoiding overhead watering, and using sterile potting mix.