Ive been growing sunflowers for my entire life. For some reason this year they are dying when they never have before, ever. I have 4 kinds & so far 3 kinds have died, about 8 plants. 2 have taken 1 whole day to die, the others went from 100% looking great to 100% dead in 2-3 hours. Disease, fungus? Here is a pic of the problem. Down at the base, the dark color, going from thick green stalk, to small constricted brown death. I see no signs of any bugs, no snails, nothing inside when cut open.