Geraniums, with their attractive flowers and wide range of sizes and colors, have been popular with home gardeners for over 300 years, according to the Colorado State University Extension’s website. These beautiful flowers come in all shades save blue or yellow, and range in form from trailing plants to upright, bushy flowers with scented foliage. All types of geraniums can suffer from problems that cause spotting on the leaves.
Xanthomonas campestris pv. Pelargonii and Pseudomonas erodii are bacteria that cause spots to develop on the leaves of geraniums. These little bacteria with the long names are often grouped together and called bacterial blight. Tiny spots develop on the leaves, which gradually get larger as the leaf turns yellow, then brown. Eventually, the leaves drop off and the stems of the plant rot. Pluck off infected leaves to stop the spread of the disease. If the entire plant is infected, it will die. In that case, remove it from other, nearby plants so the disease doesn’t spread. Prevent bacterial blight by watering at the soil level and letting air circulate around your geraniums.
Fungi can also cause leaf spots on geraniums. The spores spread on water and infect the leaves when water is allowed to sit on the foliage. Symptoms include orange or brown spots on the leaves. Yellow leaves are indicative of a serious infection and should be removed from the plant. One fungus that causes leaf spot is Puccinia pelargonii-zonalis. This spores of this fungus are bright orange, so it is also often called leaf rust. As with bacterial leaf spots, water at the soil level, avoiding wetting the foliage and give the plant plenty of room and sunlight. Spraying the plants with a fungicide can help prevent fungal disease as well.
Two viruses that cause yellowing and spotting of geranium foliage are tomato spotted wilt and impatiens necrotic spot. These viruses spread on insects — especially thrips — or tainted gardening tools. Some plants are even infected when they are brought home from the garden center. Only buy healthy-looking geraniums, and use insecticide to control bugs on your plants. Throw out infected plants and disinfect — dip in bleach — any tools used on infected plants before using them on other plants.
Freezing weather can damage the leaves, causing them to turn yellow and develop spots. These should be removed from the plant. Oedema also causes wet, greasy-looking spots to arise on geranium leaves; usually on the underside. This is caused by over-watering and can be remedied by waiting until the top layer of soil is dry to the touch before watering again. Oedema is not fatal to the plant.