For the past 30 years, I've allowed large beds of violets (wild I guess) to grow in my yard, mostly under trees. I pull some but not all weeds among them, and periodically - maybe every 5 years or so - I mow most of the area. This year, for the first time in recent memory, many violets are dying. First the leaves become fringed with yellow, then eventually the plant reduced to a mushy pile of yellow. Can you tell me what is happening, whether I should be worried, and what I could/should don? Thank you!!