Answer: In my experience, rock or gravel road dust would not have such an adverse effect on so many different plants, however it would seem like something is happening. I would investigate the soil first of all to make sure that the pH is relatively neutral and that the nutrients are in balance. A healthy soil with plenty of organic matter in it also seems to help maintain plant vigor and help them ward off problems.
Then I would look to watering practices, with any watering being done at ground level rather than by overhead sprinklers so that foliage is not wetted as an invitation to fungal problems. Water to keep the soil evenly moist and mulch with several inches of organic mulch to help the soil stay more evenly moist. Water deeply to encourage deep rooting, avoid daily light sprinkling. Also, stay out of the garden whenever the foliage is wet or damp.
Next you might check to make sure that you are monitoring regularly and immediately remove any discolored foliage so that it is not reinfecting the area. The trimmings should be put in the trash rather than in the compost pile, again to prevent reinfection. At the end of the season, remove all the frost-killed plant debris and since things have been so bad this year, throw it in the trash. You might also want to replace the mulch or at the least add a clean layer of new mulch on top of the existing mulch. This will help keep some of the problems, such as black spot, from recurring.
In your plant selections, look for varieties known to be disease resistant. This step alone, particularly with plants such as roses or phlox or monarda, can make a huge difference.
Finally, there will be a certain amount of yellowing as the season progresses through fall and the plants begin to die back. Some of what you see in October can be just general end of the season effects. These should be minimal if you follow the steps above.
There are also some diseases that could cause foliar problems. You might want to take some samples to your county extension for consultation and their recommendations as to what to use to control them as this can depend on which specific plant is being affected.
I hope this helps you troubleshoot.
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