I have 2 Japanese plum trees, one is a Santa Rosa and the other is unknown, but probably either Satsuma or Santa Rosa. Basically all the new growth has started to turn yellow and wilt. It hasn't started to affect older growth yet though. I've had a fairly wet spring and I haven't needed to do any supplemental watering. I also have clay soil, although it drains pretty well. My only guesses are either brown rot (but I see no brown spores on the declining foliage) or phytophthora root rot. I can probably post some pictures tomorrow, but just based on that description can anyone say what this problem likely is?

Disease can be strengthened by a lack of balance of what it needs. Yellow foliage is often a sign of lack of nitrogen, and Asian plums have about the highest need for nitrogen of any of the fruit trees. My advice would be to pee on it at the dripline. It;s what I usually do with foliage that is too yellowish green, and it almost always helps. Many fertilizer lists mention to add "urea" and my thought is that I have a convenient and free source of that nutrient.
John S
PDX OR

Haha, that is probably one of the strangest, yet best suggestions I've ever heard. I don't doubt it one bit though. I have plenty of fertilizer (10-10-10 I think) and more than a healthy supply of urine. Thanks, John.