 Next on our list is spray coverage. Fungicides move in the xylem of the plant, which only moves up. If you don't spray the lower leaves or even the lower halves of leaves, those parts of the crop will be unprotected. Also, when new leaves emerge, those are unprotected as well. We usually figure fungicide residual lasts two to three weeks. Between that and the fact that new leaves since your last treatment won't have fungicide on them, if you have late season diseases you're trying to stop, spraying an additional time or two may be best.