Cutting Rot (fungi and bacteria): Geraniums are grown from seed but often propagation is done with cuttings. Cuttings are susceptible to invasion by numerous soil-borne organisms, and, thus, treatment of cuttings with a fungicide is often necessary. Allowing the cuttings to “heal” before planting will help reduce stem rot. Wound healing takes place if cuttings are laid on damp sand in heavy shade for approximately three hours. Bacteria (Xanthomonas sp.) and fungi (Pythium sp., Rhizoctonia sp., Fusarium sp., and Botrytis sp.) cause stem rot. They may infect stems singularly or in combination.
Black Leg (fungus – Pythium spp.): Generally a disease of cuttings and young plants. Stems and petioles blacken and a soft rot develops. Rotting starts at the base of the stem and may extend well above the soil line. Plants wilt and die. Symptoms progress rapidly. Control may require sterilizing potting mix and tools. Treat cuttings with a fungicide. A soil drench may retard spread of the disease in propagating benches.
Botrytis Blight (fungus – Botrytis cinerea): Affects blossoms, leaves and stems. On flowers, petals darken at edges and wilt prematurely. Affects central florets first. If humidity is high, spore masses may be found on flowers and leaves. Spots on leaves are irregular, brown and have a water-soaked appearance. Botrytis will cause a soft rot of cuttings. Proper sanitation in greenhouses by removing and destroying infected plant material will prevent spread. Use of a foliar fungicide when conditions are favorable for infection will help to prevent the disease. Improving ventilation and air circulation among the plants will also reduce infection.
Rust (fungus – Puccinia pelargonii): Rust occasionally is found on cultivated geraniums. Distinct, reddish pustules form on the underleaf surfaces in a circular pattern. The upper surface is yellow in areas where pustules form. Control rust by removing infected leaves and spraying with a fungicide.
Bacterial Leaf Spot and Stem Rot (bacterium – Xanthomonas campestris pv. pelargonii): Leaf spots begin as small, water-soaked spots on the underside of leaves. Spots become well defined and slightly sunken in a few days. Necrosis and wilting of the entire leaf follows, but the spots do not coalesce. Another symptom is rapid wilting of the leaf margin resulting in an angular pattern bound by the veins. Stem rot begins with one or two branches showing wilt. Eventually, the entire stem turns black with only a few leaves remaining at the terminals. Within the stem, the vascular fibers remain intact but the support tissue around the fibers is destroyed. Controls consist of using disease-free plants for cuttings, controlling the humidity in greenhouses, avoid wetting leaves when watering.
Viruses: Several viruses are known to affect geraniums. They cause mosaic patterns, mottling, crinkled or cupped leaves. Rogue out diseased plants as soon as they are noticed and use disease-free plants for propagation.
Oedema (physiological): At first, oedema appears as water-soaked spots on bottoms of leaves which later become corky and brown. The leaves may turn yellow and fall off. Oedema is caused by moist, warm soil and moist, cool air or cloudy conditions that result in more water being absorbed by roots than is being transpired by the leaves. As a result, cells swell and burst. Avoid over-watering and increase ventilation.