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).