We’ve seen quite a few cases of calibrachoa with powdery mildew this season. An e-Gro Alert was written on this topic in 2015 (http://www.e-gro.org/pdf/2015_436.pdf), but, since we’ve been seeing a number of samples this year, let’s review.
by Nora Catlin - nora.catlin@cornell.edu
Carefully inspect incoming liners and plants on your bench in order to catch this disease early. Early symptoms and signs of powdery mildew on calibrachoa are typically subtle, the obvious powdery and dusty fungal growth on the leaves may not be visible at first. Keep an eye out for leaves that are chlorotic, yellow, or with purple-gray spotting; be sure to look at the lower and interior leaves. If you notice discolored leaves, inspect carefully for signs of subtle white or light-colored fungal mycelial threads or spores.Using high light and magnification can help – I find that holding and rotating leaves in a bright ray of sun helps me to better see what is on the surface of the leaf. Identifying powdery mildew in its very early stages sometimes requires the help of a microscope, and assistance from a diagnostic lab might be necessary. Yellow mottling might also be an indication of a virus infection (e.g. Tobacco mosaic virus and/or Calibrachoa mottle virus), so keep this possibility in mind as well.
As powdery mildew progresses it can also cause lower leaves to turn brown and dry and sometimes there is also leaf drop. Be aware that that dead and/or shriveled lower leaves, or a sparse-looking plant from dropped leaves, can be symptoms of powdery mildew. In later stages you will also see the typical white, powder-like fungal growth that is indicative of the disease. On some cultivars, powdery mildew affects blooms, causing faded color with fungal growth on petals.