Geraniums that have leaves that are turning yellow and light brown may have contracted a fungal disease. Both Alternaria and Cercospora leaf spots cause leaf discolouration and health problems for the geranium plant. It is important for gardeners to diagnose their plant's disease to offer the best management practices and protect their surrounding geranium plants.
Alternaria Leaf Spot Symptoms
Alternaria leaf spot causes older leaves to produce blister-like looking patches. Gardeners may not notice the disease unless they look underneath the leaves. These blisters become sunken and turn brown with yellow haloes, according to the University of Georgia. Eventually, these brown and yellow blisters will spread to the top of the leaves. If the geranium is experiencing a severe infection, the plant may lose its leaves. Lack of foliage inhibits the geranium's ability to photosynthesise.
Cercospora Leaf Spot Symptoms
Cercospora leaf spot causes small sunken lesions on leaves. Gardeners who closely examine these lesions may see fungal spores growing near these lesions, according to the University of Georgia. Signs of yellowing or cholorosis may appear next to the lesions, which indicates the geranium is having a difficult time photosynthesising. Depending on the severity of the infection, the geranium may lose it leaves. It is important to monitor the plant for additional health problems that may occur because the plant is under stress from the Cercospora leaf spot.
Apply a fungicide every seven to 14 days. The frequency ofr fungicide treatment will depend on the severity of the infection and rainfall. The more often it rains, the more often you will need to treat the geranium with fungicide. Remove diseased leaves with a pair of bypass shears. Sterilise the bypass shears with 70 per cent denatured alcohol and 30 per cent water to prevent spreading either one of the fungal diseases to the rest of the geranium.
Avoiding wetting the leaves of the geranium. When watering the plant, pour the water a few inches from the base. Water the plant in the morning, as suggested by the University of Georgia. By watering in the morning, you allow for the leaves to dry out before the cool nighttime temperatures. Cool nighttime temperatures and wet leaves encourages both Alternaria and Cercospora leaf spots. Also clean up all diseased debris around the plant. Fungal spores can continually infect a geranium.