Certified GKH Gardening Expert
Water deficit (drought stress) can cause this, Chinese elms are not drought tolerant and need deep watering once or twice a month. Drip irrigation may not be adequate. Lawn irrigation is usually shallow and taken by the grass.
Mulching the soil surface under the foliar canopy helps to retain soil moisture.
Another condition that can cause summer leaf drop is a leaf fungal disease called elm anthracnose. It can be controlled with a diligent spray program starting in early spring at the time of leaf bud break, and repeated during the rainy season. If you contact a local tree service with a PHC (plant health care program), ask them about a fungicide with the active ingredient propiconazole. There is also stem injection treatment available.
There is no good do it yourself program available that I know of without spray equipment to reach the entire foliar canopy. But here's a diagnostic tip to differentiate between the two disorders I have mentioned. If you you are watering adequately, and the cause of the leaf drop is in fact anthracnose, the tree should re-foliate when the rains stop by April/May. If it is not leafing out again like that in late spring, beginning of summer, then it is more likely water deficit is the cause.
Along with water management, soil fertility is next most important. If you don't fertilize then the tree will be more susceptible to disease as well as drought stress.
