Answer: Bishop's Weed can develop fungal leaf spots in wet weather and a disease called leaf blight in hot, humid weather. While a fungicide might control the problem, you might also consider mowing the plants down (or cutting them back to just a few inches). This will promote the development of healthy new foliage. There's not much else you can do other than try to keep water off the foliage. You can't control the rain, but you can avoid overhead watering.
Best wishes with your garden!
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