Geraniums (Pelargonium sp.) are flowering plants that produce clusters of colourful and showy blossoms in a range of colours, including white, pink, orange or variegated selections. They usually grow between 12 and 20 inches tall depending on the cultivar. Geranium enthusiasts cultivate them as container plants, in hanging baskets or in flower beds. Geraniums are susceptible to several plant diseases, including powdery mildew.
The sphaerotheca and erysiphe fungi cause powdery mildew infections in geraniums. The disease normally infects outdoor plants during warm weather, but it can infect indoor plants at any time of the year. The dormant pathogen spends the winter on spore-contaminated leaf litter or on diseased leaves or stems. It emerges during the spring and sends up ascospores, which are reproductive spores that travel on the air and cause new infections when they land on susceptible hosts. The fungus spreads the disease by producing new spores throughout the growing season.
The powdery mildew fungus infects geraniums when temperatures are between 15.6 and 26.7 degrees Celsius with high humidity. Geraniums that grow in shady locations or in overcrowded plantings are more prone to powdery mildew infections than plants that grow in sunny locations with good air circulation. Powdery mildew fungi do not need water to germinate, unlike many other types of fungi, and can be particularly problematic in hot or dry climates.
Small, round, powdery white spots appear on geranium foliage. The patches grow until they cover entire leaves, stems and blossoms in a layer of fungal growth. Infected leaves curl, distort and turn yellow. They often fall from the plant. Geranium blossoms may appear disfigured, and infected buds may not open. Small black spots appear in the fungal matter. The powdery mildew fungus that infects geraniums is plant-specific and usually does not infect unrelated species. Geraniums rarely die from powdery mildew infections, but the disease makes them unattractive and reduces their aesthetic value.
Plant geraniums in sunny areas and space them far enough apart that they receive adequate air circulation. Rake up and destroy all leaf litter to prevent plants from becoming infected the following spring. Reduce the surrounding humidity level by watering plants from below rather than above, and avoid working with geraniums when their leaves are wet. Plant disease-resistant cultivars whenever possible. Spray susceptible geraniums with an appropriate fungicide every seven to 10 days during the growing season.
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- Iowa State University Plant and Insect Diagnostic Clinic: Geranium
- Cornell University: Powdery Mildew Factsheet
- The Connecticut Agricultural Experiment Station: Cranesbill (Geranium)
- Ohio State University Extension; Powdery Mildews on Ornamental Plants; Stephen Nameth, et al.
- Colorado State University Extension; Powdery Mildews; B. Edmunds, et al.; May 2009