Fungus on Geraniums

Botrytis Blight

• Botrytis blight is a common fungal disease that affects geraniums, typically appearing during periods of excessive moisture or high humidity. It may infect the plant during any stage of growth and throughout any section of the geranium producing a gray fuzzy growth caused by the pathogen Botrytis cinerea. Lesions develop on both the stems and leaves of the plant, often rotting away the entire stem. Gardeners should avoid overhead watering to minimize the possibility of the geranium contracting this disease.

Leaf Rust

• Leaf rust is brought on by the fungal pathogen Puccinia pelargonii-zonalis and results in the development of tiny yellow spots on the upper surface of geranium leaves accompanied by a rust-colored fungus on the underside of the foliage. The rusty fungi serve as the reproductive center of the disease by releasing their spores into the air. Geranium leaves typically turn yellow and fall prematurely from the plant. Avoiding overhead irrigation also reduces the likelihood of this fungus developing.

Root & Stem Rot

• Root and stem rots caused by the fungi Pythium or Rhizoctonia produce a yellowing, wilting and stunting of the growth of a geranium. A black or brown lesion normally displays at the soil line where the fungus initially attacks the plant girdling -- strangling -- it and depriving the geranium of necessary water and nutrition distribution. A gray or white growth may also be seen on the plant as the roots gradually decay. Improving the drainage of the soil and avoiding standing water is vital to managing root rots.

Leaf Spots

• Alternaria leaf spots make their presence known on the under-surface of older geranium leaves causing a sunken, brown spot with a yellow halo. The disease eventually becomes apparent on the upper surface of leaves, with the foliage gradually turning completely yellow and falling away.

  Cercospora leaf spots are pale, depressed lesions that morph to a gray color as the fungus releases spores into the environment. Leaf tissue near the lesions will turn yellow with the infected foliage prematurely falling away. Infected leaves and plant litter should be removed and destroyed.