Plant Diseases/coral bark maple
I have 2 -- 5yr old coral bark maples at the head of my gated entrance (2" trunk & 7' tall). Since July I noticed the leaves turning on one of them...it is now dead ( I think ). One nursery told me it was probably too much water from the daily lawn sprinkler system...another nursery said that it was probably not enough water during our longer hotter summer here in the Pac NW. The trunk has lost it's pink cast and is now grey. I assumed at first it was a virus or mildew problem........WHat do you think ???
Either nursery could be correct. I know you didn't want to hear that! Leaf yellowing/browning, especially starting around the margins of the leaf is usually a sign of a root issue. Too much water can lead to an anaerobic condition (roots suffocate). Oxygen levels go way down. The soil will often develop a sour or sewage like odor if they remain too wet for extended periods. It is very important that these and most trees need to be in a well drained soil. Clay soils can be a real problem since they hold lots of water due to the small pore spaces. Most places suggest an infrequent yet deep watering. Of course watering schedules need to be adjusted for hot/dry and windy conditions. Hard to say how much water a tree needs at a given watering. The soil should be moist, but not soggy around the root zone. It may be necessary to visually examine the soil around a portion of the roots. A good "rule of thumb" is that the soil should ideally just barely stick to your fingers when you squeeze a handful together. This is not real scientific as you can tell.
If you are not sure how much water your sprinkler is putting out during a cycle, try putting out a cheap rain gauge close to where your tree is and then measuring it when your system shuts off in that zone. "Generally", your lawn needs about 1"/wk. This is usually enough for an established tree the size of yours. Of course, as I mentioned, you may need to adjust up or down depending on the ambient rainfall.
Hope this helps.