This past week in the Diagnostic Clinic, we received some photos of a Japanese Maple whose younger leaves were exhibiting curling at the edges. No insects were found and so the search began for the cause of the damage. As more information was forthcoming, it was also noted that the new buds on some of the blue spruces were turning brown as well. We also learned that a nearby plum tree had recently been sprayed with an insecticidal soap. Since the symptoms on the Japanese Maple and the Spruce were associated with new growth, not the older growth and no insects were found, it appeared that there was most likely an abiotic issue. So we did some research and discovered that insecticidal soaps can cause phytotoxicity damage to certain plants if sprayed at the wrong time, and that newer growth on Japanese Maple is extremely susceptible to damage from insecticidal soaps. An excellent fact sheet from Cornell University provided the information that we based our diagnosis on, which also indicated that if the insecticidal soap was applied when temperatures were above 90F, it could increase the incidence of phytotoxicity damage. The link for the fact sheet can be found at http://ccesuffolk.org/assets/Horticulture-Leaflets/Using-Insecticidal-Soap-In-Your-Home-Garden-Or-Landscape.pdf.
The take-home message from all this is that reading the label is important to make sure your plants won't be unnecessarily harmed by the use of any product.