Cause of leaf discoloration on new BoBo Hydrangea plants

Question: Hello and thank you for taking my question.  About 4 weeks ago now I planted 5 very healthy appearing BoBo Panicle Hydrangea plants from a very reputable nursery.  For planting they recommended using Organic Mushroom Compost Blend to blend with the existing soil in the bed.  And fertilized with Myke Naturally Powerful Mycorrhizae for transplanting.  The plants have been receiving an irrigation system watering 3 days a week for 15 minutes. The plants/leaves have never been surface watered.  After about two weeks I noticed many of the leaves on all of them were starting to discolor with multiple shades of rusty brown. I immediately went back to the nursery, with photos, for their advise. Three of them assessed the issue to be a leaf spot fungus.  They sent me back home with a fungicide called "Copper Fungicide".  I proceeded to remove all affected leaves as they would not recover and thoroughly sprayed the rest of the plant with Copper Fungicide.  Now today I notice the same thing starting to show up on all five plants again, but this time there are many leaves which have turned yellow. The yellow leaves seem to be more within the lower part of the plant.  Back to a different reputable nursery and the person I spoke with semi agreed that it was a fungus problem and to keep using the spray.  Personally, after looking at photos online of this leaf spot fungus, I don't think my plant's leaves really look that similar to those on line.   Sooooooo.... I am wondering if there could be some sort of nutritional problems where the soil just isn't the right PH or not enough iron or whatever.  Or could they be getting too much sun?  They are in total shade until about 11:30 when they finish out the day in fun sun. Hopefully these photos will show well enough for you to get a good idea of what could be possibly be going on here. Totally appreciate you assistance here !!!

responder: Alison O'Connor, PhD  Replied June 27, 2024, 4:07 PM EDT
response: Hi Jeannie,  Yes, I think you identified the issue - the afternoon sun has just been too hot for these plants. More than likely they were grown in a protected location at the nursery (or were shipped from a place with lower elevation and less sun intensity) and when exposed to our intense sunlight, the leaves became burned.   While this plant can tolerate full sun, our summer afternoon sun might just be too intense.   I don't believe this is nutritional - or a fungus. Could you plant in a location where they have morning sun and afternoon shade?

responder: The Question Asker Replied June 29, 2024, 6:14 AM EDT
response: Thanks so much Alison for confirming one of my suspicions.  Still working on a solution to give these lil hydrangeas a break from our intense Colorado sun since I really don't have a spot in the yard where they would be happier.  It's looking like I may just have to use garden netting for 'em during these intensely hot and sunny days.  Fingers crossed it works!!!  Someone needs to come up with a spray on sunscreen for plants... There's your first million, Alison    Have a great rest of your summer and thanks again for sharing your expertise!!! Jeannie     On Thu, Jun 27, 2024 at 2:07 PM Ask Extension <<personal data hidden>> wrote: