Answer: Unfortunately, based on your description I am not able to tell you specifically what is happening to your plum tree. These trees are potentially subject to a wide variety of pests and diseases. I would suggest you work with your local county extension to obtain a specific diagnosis and then based on knowing that, determine how to proceed. They may appreciate photos of the overall plant and closeups of the foliage showing the discoloration pattern, and possibly also a sample that is freshly cut, enclosed in a plastic bag and kept cool so it stays fresh. If it is something that requires a chemical control, they will have the most up to date information on what to use and how/when is best to apply it for maximum results. In the meantime, clean up any fallen foliage and dispose of it in the trash to limit sources of reinfection; do a thorough clean up this fall for the same reason.
Also, I should mention that daily watering is not the best way to water new trees and shrubs. Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not saturated/sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is stil damp, do not water yet. When you do water, apply the water to the soil (avoid wetting the foliage) and water very slowly and thoroughly so it soaks down to where the deepest roots are. After watering, wait a few hours and then dig down to see how far it went; sometimes it can be surprising. Watering deeply but less often encourages deep roots and helps the plants withstand drought better in the long run.
Using an organic mulch several inches thick in a flat layer over the root zone also helps keep the soil moist; it also helps feed the soil slowly over time as it breaks down.
With new plants, correct watering is far more important than fertilizer. Fertilizer is not a good idea for a plant that is stressed. In general, an annual application of good quality compost along with a spring application of a general purpose complete fertilizer such as 10-10-10 per the label directions should be adequate. For a more precise fertilization recommendation you would need to run some basic soil tests. (Your county extension can also help with the soil testing and interpreting the results.) You could do a topdressing with compost now, but I would not fertilize until next spring.
I'm sorry I can't be more specific for you but I hope this helps.
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