In my yard there are so many reasons. Some daylilies are inherently healthier. Thrips, spider mites and the plant tarnish bug will make foliage look horrible. Early and often is one of my first to bloom every year and the first to be attacked . Pore thing looks so hideous I was going to take ti out. Now I take the foliage off. It grows back really nice for the remainder of the year. Others like Just Plum Happy got leaf streak at the drop of a hat. That one I removed. All plants have their insects and diseases that come along to make their foliage look ratty. I still favor daylilies and roses because they are plants that survive almost everything. When it comes to thrips and spider mites they leaves the roses alone now as they seem to prefer daylilies. But, this still leaves JB's to trash them in summer. When purchasing more expensive daylilies,I ask about foliage health before getting them.