Brownish water-soaked spots appear on leaves. They enlarge rapidly, turn black, and kill the leaves, then the leafstalks and main stems. In moist weather, gray mildew grows on the lower surfaces of leaves. Both in the ground or in storage, brownish purple spots that become a wet or dry rot infect the tuber skins.
If late blight is an annual problem in your area, apply a fungicide labeled for this disease, following label directions. Use drip or furrow irrigation. Wait at least a week after plants die naturally before digging the tubers. This allows time for the spores to die. Handle the tubers gently to avoid wounding them. Clean up and destroy plant debris after harvest. Plant certified disease-free seed pieces.