sometimes too much rain, not enough watering, soil conditions. I just trim them off, I get them at the bottom of the plants. In the fall when you take those plants out of the ground use a plastic trash bag and trash them. Amend you soil with horse or cow manure and lime and turn it under for next year. After you plant next year put 2 sheets of newspaper overlapping 6 to 8 inches and put grass clipping over it. I asked a person who did lawn cutting to give me clean cut grass with no trash in it. Worked like a charm and the plants grew very well. The ground stayed moist during the drought, I only watered after dark.