Powdery Mildew on Calibrachoa (Million Bells)

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A calibrachoa (million bells) with dried, brown lower leaves was submitted this week to the NCSU Plant Disease and Insect Clinic by a commercial grower (Fig. 1). Slightly younger leaves were turning yellow and had fine necrotic flecks (Fig. 2). There was no evidence of disease in the roots or stems. In this situation there are three likely causes: nutritional problems, spider mites, and powdery mildew. It was the latter. The powdery mildew fungus was present on both sides of the leaves, but the colonies were so thin that they were not visible to the unaided eye or even with a 10x hand lens. It took a microscope to confirm the diagnosis (Fig. 3).

An e-Grow Alert on this problem was published in May 2015. You may also be interested in the new Calibrachoa sheet in the e-Gro Nutritional Monitoring Series.

Note that a similar situation was encountered on greenhouse-grown dahlia plants on the NC State University campus last month (Fig. 4).

drying lower leaves

Fig. 1. Powdery mildew symptoms on greenhouse-grown calibrachoa

Leaf with moderate symptoms, not yet dried out

Fig. 2. Incipient symptoms of yellowing and blotchiness on a mid-level leaf.

chain of five barrel-shaped spores (conidia) on a small conidiophore

Fig. 3. Chain of microscopic spores (conidia) of the powdery mildew fungus Podosphaera

Lower leaves of potted dahlias starting to turn yellow and dry out.

Fig. 4. Greenhouse-grown dahlias with powdery mildew.