Years ago zonal geranium (Pelargonium x hortorum) was among the few crops I rarely saw in the Entomology Diagnostic Lab, aside from occasional fungus gnat damage to roots or stems. Foxglove aphid, despite its long history as a greenhouse pest, then started taking a liking to the crop where it usually causes a kind of odd leaf distortion.
by Dan Gilrein
We also began seeing twospotted spider mites in vegetatively propagated (never in seed-grown) zonals, leaving symptoms of yellowing, brown spotting and edema on the undersides of leaves. Often older leaves show the greatest symptoms, which can be easily mistaken for a disease or cultural problem.
The issue has not been restricted to zonal geranium as we occasionally see it on ivies and some of the other hybrids. I encounter foxglove aphids rarely in geranium now though twospotted spider mite still bears watching.
If either pest is suspected turn over leaves to look for the aphids or mites; you may see the aphids’ cast ‘skins’ or honeydew fallen on upper leaf surfaces from above, or in the case of mites there may be some fine webbing beneath where the leaf veins join the petiole or their small, spherical translucent eggs visible there with a hand lens. Check younger leaves too, as the mites tend to move onto younger leaves from the declining foliage below.
If confirmed, there are highly effective controls for both, but note geraniums can be sensitive to Kontos and Savate as well as insecticidal soap.
Source: eGRO Blog