My client had cottony aphids in her post oaks and had tree sprayed. Shortly afterwards about 10% of the leaves turned brown and finally dropped. At first sight of brown I inspected and noticed the insects had rolled the leaf at the base of the blade. I thought maybe that this rolling action cut off the leafs vascular system and caused it to die, but there are green leaves with the same little roll which are still alive. I considered spray pressure could of broke the petiole or small stems but this was not the case.The tanks were rinsed well prior to another solution being used in that tank.What else should I be looking for or considering? And what is the right spray for this application? Malathion?

First, ask these questions:
What pest is it, specifically?
Is it really harming the tree, and necessary to control?
If necessary, what product should I use, and when?

Get help if you aren’t sure of any of these. Contact your Cooperative Extension for identification and control measures. Every state has its own rules and regulations, so I can’t post a recommendation on what product to use. Be sure you target it carefully.

The leaf drop may be due to several things. The earlier sprays may have caused a phytotoxic reaction, either from the product itself, or from environmental conditions such as drought or heat. It could also have been caused by the insects affecting the leaves enough to initiate leaf drop of the most damaged leaves, even after spraying. This is why I recommend first identifying whether it is still an active pest. Perhaps the first treatment was just a bit late to stop all damage.

If the tee is doing OK now and the pest is not continuing to affect it severely, maybe the best thing to do now is.... nothing. Then watch it for a recurrance.

Russ, Thanks for the directional focus. Being fairly new at this I freak out when these unexpected things( sudden brown leaves) jump out.My Coop-Ext. agent ID'd the cottony aphids last year as I also did and let local spray for them again this year. This is a high profile, beautiful, old Q.stella which belongs to our local tree board prez. and I figure that even if the browning has now stopped; should I maybe take some samples and send to our entomology dept. and ask for toxicity report??? What should I ask lab to look for? Me wannum know what cause-um brown leaves....
P.S. I really enjoy and appreciate this site for the information which is of great help to the trees in my area. Thank you everyone!!!! Darbo