Geraniums (Pelargonium spp.), while resilient, are susceptible to several diseases that can compromise their health and aesthetics. Understanding these ailments from the plant's perspective is crucial for effective control and maintaining a vibrant garden.
This fungal disease, caused by Botrytis cinerea, thrives in cool, damp, and humid conditions. From our perspective as plants, it begins as water-soaked spots on leaves and petals, often starting on senescent or injured tissue. A characteristic gray, fuzzy mold develops, which are the spores of the fungus. These spores easily become airborne, spreading the infection to neighboring plants. To control it, you must improve our air circulation by providing adequate spacing. Avoid overhead watering that leaves our foliage wet for extended periods, and promptly remove any fallen flowers or dead leaves that accumulate around our base, as these provide a perfect habitat for the fungus to develop.
This disease is caused by Xanthomonas campestris pv. pelargonii. For us geraniums, it manifests as small, sunken, water-soaked spots on the undersides of leaves. These spots often become angular as they are constrained by leaf veins and may develop a brown, scabby surface. In severe cases, the bacteria cause systemic wilting and blackening of stems, effectively cutting off our water and nutrient supply. This pathogen spreads through water splash, contaminated tools, and infected cuttings. Control is preventative: always purchase certified disease-free plants, water us at the soil level to avoid splashing, and sterilize pruning tools between uses. There are no effective chemical controls, so infected plants should be removed and destroyed to protect healthy ones.
These soil-borne fungal pathogens, including Pythium and Rhizoctonia species, attack our root system, which is our lifeline. The first sign you may notice is wilting, despite adequate soil moisture. Upon inspection, our roots will appear brown, mushy, and rotten instead of a healthy white and firm. The stem base may also become soft and blackened. This condition is a direct result of oxygen deprivation in waterlogged, poorly drained soil. The fungus exploits this weakness and rapidly colonizes our compromised tissues. To prevent this, ensure we are planted in a very well-draining potting mix and that our containers have sufficient drainage holes. Avoid overwatering, allowing the top layer of soil to dry out between waterings.
This is a specific fungal disease that primarily affects zonal geraniums. It appears as distinctive pale yellow spots on the upper surface of our leaves. On the corresponding lower leaf surface, you will find brownish-yellow, powdery pustules filled with spores. This rust fungus weakens us by disrupting photosynthesis and causing premature leaf drop and deformity. The spores are easily spread by wind, water, and on hands or tools. Management involves promoting a dry environment by watering early in the day so our leaves dry quickly. Remove and destroy any infected leaves immediately upon discovery. Fungicides containing chlorothalonil or mancozeb can be used as a protectant spray according to label instructions.
