Powdery mildew of calibrachoa is caused by the fungus Podosphaera xanthii. Two other species of powdery mildew have been identified on calibrachoa in Germany but these have not been reported in North America.
Signs and symptoms of powdery mildew are most often observed on lower leaves of calibrachoa, where the dense canopy holds humidity. The powdery, white growth that is typical of the disease often appears early on and may be difficult to see. As the infection progresses, leaves may turn brown and drop. Upper leaves may also be affected, and flowers may be mottled.
The fungi that cause powdery mildews produce copious spores which move easily on air currents. Unlike the spores of most other fungi, powdery mildew spores do not require a period of leaf wetness in order to germinate and infect plant tissue; however, they do require high humidity.
Decrease relative humidity in the greenhouse by heating and cooling, increasing horizontal airflow, and spacing plants properly. Controlling humidity in and around the plant canopy is critical for management of the disease.
Avoid growing ornamentals and vegetable starts in the same greenhouse if possible. P. xanthii also affects cucurbits, verbena, and petunia. Research has shown that some strains of the fungus affecting calibrachoa may not affect petunias.
Very little information exists on the susceptibility of calibrachoa cultivars to powdery mildew, but there is anecdotal evidence that differences do exist. Keep good records and avoid cultivars that appear to be especially susceptible.
Apply fungicides at the first sign of disease. If you are uncertain whether or not powdery mildew is present, submit a sample to UMass Extension's Plant Diagnostic Lab for examination.
For a full listing of fungicides labeled for use on powdery mildew on ornamentals, please see the New England Greenhouse Floriculture Guide: http://ag.umass.edu/greenhouse-floriculture/publications-resources/new-england-greenhouse-floriculture-guide. Be sure to rotate products with different FRAC group numbers. Thiophanate methyl is currently not recommended for use on powdery mildew affecting cucurbits due to resistance issues. Strobilurins (Group 11) are also not recommended unless they are combined with a fungicide from another group, either as a tank mix or a combination product.
Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab