Well, if both you and Hetty think it's environmental rather than a disease or insect, I'm thinking it's the well water, then. Plumies do like it acidic, right?
I have 3 kinds of water. My general irrigation is untreated, straight from the aquifer well water and has a pH of 8.2. Other sensitive plants show chlorosis after about 2 weeks of no rain. Tap water (city) the pH is 7.4 and the rainwater from the cisterns just tested out at 7.0, perfectly neutral. (surprisingly) But the cistern may have been low during July and kicking on the well water fill would put high pH water into the cistern.
Thinking that more rain the last week or so has helped already, hence I'm seeing healthier new leaves. I'm going to augment the water when it doesn't rain with extra rainwater for a while, and also give a Mg douse to see if that helps the plant. Can't hurt, right?
From now on maybe I'll turn off the sprinkler head to that bush when the weather's dry and hand water it with rain water from the barrels. (yes, I have three 75gal. rain barrels, too
) They run dry pretty fast when it stops raining though - the fatal flaw with collecting rain water. When you need it most, you run out.
