why do my sunflowers begin to look horrid in mid/late august every year-sometimes sooner? the leaves get mottled and begin to fall and the plant looks as if it's about to die, although it's still producing flowers and it's much too early for that to be a result of the season-i think. It doesn't happen to all - some stay fresh so much longer than others, even within the same variety in the same area. I am mystified. any ideas? thanks so much.