Marion berry with leaf disease
It sounds like Septoria leaf spot, a common marion berry problem. I attached a photograph of it in an article so you can compare and decide if that is it. We have had hot humid conditions, and if you water the plant from the top, that sets up perfect conditions for it. You need to maintain good hygiene of the plants, watering from the bottom, cleaning up any fallen leaves and double-bagging and disposing of them, and applying a fungicide to kill the spores. Make sure you read the directions carefully and wear protective clothing.
Thank you, but the photos I can find of Septoria look like there is white surrounded by dark areas, which my leaves do not show, and I see no mention or photos of bright orange yellow spots under the white spots near the edges of the leaves, as in my posted photos. The brown in my photos are just dry spots, are rare, and seem unrelated to the problem. I just found this description of Cane and Leaf Rust, which sounds a lot like what I am seeing - can this be ruled out as a possible cause of the problem with my berries? "The first symptom of cane and leaf rust (Kuehneola uredinis) is lemon yellow pustules (uredinia) that split the bark of the fruiting canes of susceptible blackberries. Spores from these pustules (urediniospores) infect leaves and produce small yellow pustules (uredinia) on the underside of leaves during early summer." Can I cut out the canes that have produced fruit and dispose of them at this time without damaging the plants to help control whatever the disease is? Also, my canes are trellised with not many ending up on the ground during the winter and I have only watered a few times with a hose during this very hot year, with not much water ending up on the leaves. There are not a lot of weeds, but I was considering putting a mulch of bark dust down - would that be good or bad for a fungus problem? Thanks!
It could be rust, it could be septoria. Here's an article on rust. Fungicide is a good protectant for rust, but once the disease is established. Wet leaves and leaving old plant material (canes and leaves) again are a cause. If it is rust, apply lime sulphur now, and again in early fall. I recommend you bring sample leaves in with you to an extension office for positive identification.