According to Colorado State University Extension, about 300 known species of hardy geraniums exist. Also known as cranesbill, hardy geraniums need little care to thrive. They can withstand temperatures as cold as -20 degrees Fahrenheit and resist various pests and diseases. However, a few diseases can still affect these plants.
Bacterial Leaf Spot
The bacteria known as Xanthomonas and Pseudonomas cause bacterial leaf spot or bacterial blight. Xanthomonas cause small brown spots with angular or circular shapes and yellow halos about 1/16-inch in diameter to develop on leaves. Pseudonomas cause reddish-brown spots to form on leaves and distort the shape of the leaves. These bacteria could cause hardy geraniums to eventually wilt. No fungicide or bactericide exists to control bacterial leaf spot, so send infested plants to a lab for diagnosis. The University of Massachusetts Extension recommends that you wait for clearance from the United States Department of Agriculture before destroying infested plants.
According to Purdue University, downy mildew often resembles bacterial leaf spot in appearance. Peronospora or Plasmopara bacteria cause downy mildew, causing grayish-brown, angular lesions on upper leaf surfaces, often within the boundaries of leaf veins. The lesions eventually come together, damaging large amounts of tissue and causing the leaves to drop. Sanitation could help control the disease, but severely infected hardy geraniums might not heal. Apply fungicide every seven to 10 days to the hardy geraniums to prevent downy mildew from infecting the plant.
Southern Bacterial Wilt
The bacteria known as Ralstonia (Pseudomonas) solanacearum cause Southern bacterial wilt in the southern regions of the United States. The symptoms include yellowing leaves, the death of plant tissues and wilting. The bacteria spreads within diseased cuttings, through water movement and through vegetative propagation. Isolate hardy geraniums that look suspicious in garbage bags in a cool area until you can get a lab to test them. Don’t discard them without USDA clearance to prevent the disease from spreading.