Some of my corn leaves are splitting. It looks as though someone grabbed them by the tip and shredded them into three strips. It has happened only on a few leaves, but I am afraid it might start spreading. I don't see any sign of insects, so I am at a loss. The corn is putting up tassels and a few ears have started to appear. I am growing it in a raised bed with black plastic mulch. thanks for any help you can give.