Q. I have been treating my thundercloud plums for an aphid infestation. We have been using neem oil and insecticidal soap once a week for about two to three weeks. Now, the trees are losing a significant number of leaves. Is this because the aphids drained the leaves of moisture and they have dried and fallen off? Is it possible that we burned the leaves with the neem oil? Is the tree simply in shock? Do we need to try a different pest-control treatment? Do we need to prune at all? Will our trees recover from this? Please help Feather Asmussen – Everett, WA
A. The condition where leaves are lost or damaged due to the spraying of a chemical is called phytotoxicity. This can often happen with summer oil sprays especially when the plants are under stress from lack dry soils and/or when temperatures get above 80 degrees. When you say you are spraying with neem oil and insecticidal soap it sounds like you are applying them simultaneously. This is not necessary as both products on their own will control aphids. More is not necessarily better. It is likely that the combination of the two ingredients that magnifies their phytotoxicity. I would back off to just using the insecticidal soap until you get the aphids under control. Of course the amount of concentrate added to a specific quantity of water should be as indicated on the product label. Increasing the amount does not improve the efficacy and can increase the chance of phytotoxicity. If the trees have not been defoliated before they will rebound and send out new leaves and therefore will likely recover.