Answer: It probably isn't the same fungus attacking the tree's leaves as is attacking the marigolds. A common disease on apples is apple scab, which causes fuzzy brownish gray spots on the leaves. This fungus overwinters on the dead leaves, then, next spring, the fungus reinfects the tree. So one way to help control scab is to clean up all the fallen leaves under the tree at the end of the growing season. However, it's also true that the disease spores can spread on the wind for neighboring trees.
Are you sure the marigolds are diseased? Do you see evidence like brown or gray spots on the leaves, or water-soaked areas on the stems or flowers? Because the plants are also susceptible to insect attack, especially aphids and Japanese beetles, and treatment for these would be very different than treatment for disease.
I suggest you choose some different flowers for that bed. You might even try a mix of several different types, to see which ones do the best. If you've been planting marigolds there every year, it's possible that a disease has "taken hold" and will plague you each year. Take a break from marigolds for a few years, then perhaps you can start including a few plants here and there in future years and see how they fare. Here are some annual flowers to consider:
Cosmos 'Cosmic Orange'
Nasturtiums (these flower best in relatively poor soils, with little added fertilizer)
mini sunflowers ('Elf' - 16" tall; 'Sundance', 24", Mini-Sun, 16")
Tithonia (Mexican sunflower), 'Fiesta del Sol'
Zinnias, especially the Splendor Hybrid series, the Border Beauty series, and 'Profusion'
Good luck! Your neighbors are lucky to have your garden to enjoy!
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