
Almost always, when the cause is foliar or drought — and the treatment is mostly not a spray. Water deeply and infrequently through drought, mulch out toward the dripline, rake and remove fallen infected leaves to cut next year's spore load, and prune for airflow. Fungicide is worth considering only for a high-value tree, only for a diagnosed disease, and only applied preventively at bud break; spraying a tree that already dropped its leaves in August accomplishes nothing and is one of the more common ways homeowners get sold a program they do not need. What changes the answer: a tree defoliating for the third or fourth straight year, leaf drop combined with upper-crown dieback, or a suspected vascular wilt. Those need real diagnosis, and oak wilt in particular changes what you should do and when you are willing to prune. Removal is a last step and follows a diagnosis, never the leaf drop itself.
One season of early leaf drop is not dangerous and does not put anyone on a rope. The tree either refoliates or coasts to next spring; one year is affordable. Repeat years are what matter — each defoliation draws down stored reserves, and by the third consecutive year you generally see twig dieback, a thinner crown, and secondary borers or cankers arriving. That is where a survivable annoyance turns into a declining tree. The exception that moves fast is vascular wilt: a red oak with oak wilt can go from first symptoms to dead within a single growing season and spread to neighboring oaks through root grafts, so wilting and dropping in a red oak in midsummer deserves a same-week look rather than a wait.