First off, I’m Sunset Zone 16 or USDA Hardiness Zone 10A in the SF Bay Area. Mediterranean type climate, though the Summers don’t have the heat of other parts of California, especially those inland. Wet winter & early spring, also fog influence in late afternoon/evening in summer.
I planted a number of bareroot trees last winter, and they’ve done pretty well in terms of growth and vigor, except the Pluot.
But, both the Flavor King pluot and the Weeping Santa Rosa plum have similar looking “shot hole” type of leaves, pictures are at bottom. It may be worth noting also that the branches and leaves shown didn’t exist until mid to late summer, so these holes were made late summer/fall, even though we had very VERY little rain until the past few days (but did have fog at times). I was looking at what to do this winter to have them fare better and found the following from University of California: UC IPM: Fungicide Efficacy and Treatment Timing for Plum
There are a number of statements in this page that make me confused, where I had previously thought I should use Bordeaux mixture to treat the shot hole:
Note 4 under the first table: “Shot hole disease rarely occurs on plums in California. The small holes often observed on leaves in spring are caused by either a genetic disorder or by other agents including environmental factors.”
Note 2 under the second table: “No treatment is recommended for shot hole because the shot holes found on plum leaves only rarely are caused by the shot hole fungus.”
Basically, my questions boil down to:
Is this really shot hole considering it’s apparently very rare based on the UC documentation?
I always think a debilitating disease is worth the money to get a pathologist to put under a microscope, but I think you have bacterial leaf spot. I treat that with copper- Kocide if you can control it very early in the growing season or/and with one of the copper soaps if you need to continue treatment after leaves become vulnerable to the burn of Kocide.
I never had a problem with this disease in CA on J. plums, but I never grew any kind of Santa Rosa and pluots were just a twinkle in Zaiger’s eye back then. I left CA in the late '70’s.
Is your pluot on the same rootstock as other plums?
Here, some shothole type leaf damage is common but not debilitating to J. plums at some wetter sites, so I’ve never had it properly diagnosed- so take my advice with a grain of salt.
Where it has been a larger problem here is when it attacks the fruit, which I’m confident is BLS. It happens when late spring and/or early summer is particularly wet for susceptible varieties. Especially when a tree is shaded to the east.
Pluot and plum are on different rootstocks. Pluot on Citation, WSR on Myrobalan 29C. The plum, especially at the top where the picture is, is not shaded from any direction but it is along a fence so lower portions of the tree are shaded east-southeast. Pluot shaded by the house along the north side (so not much shade ).
I may be willing to send leaf samples depending on cost to analyze, although all but ~5 of the leaves on the Pluot have now fallen and I put them in muni compost stream as didn’t want them around on the ground propagating whatever bug this is.
No fruit as this was the first season in the ground, but do want to understand how to protect them in the years to come.
Myro produces a lot more vigor than Citation. One thing I hate about pluots coming from DW is that they generally put them on citation, and some pluots aren’t all that vigorous to begin with, such as Flavor Grenade.