It is most severe on maturing or under-fertilized potato vines. The disease often first appears in late July. Spores may be carried to leaf surfaces by splashing water or wind. A narrow light green to yellow halo frequently surrounds the spot. The spots may enlarge and coalesce, causing the leaflets to die.
Secondary infections occur when spores from primary lesions are spread to healthy leaves.
Lesions may develop on stems, but it is rare for significant injury to occur from this infection. Tubers may become infected when they are lifted through soil infested with spores that are concentrated at ground level. Lesions first appear on the older, lower leaves, spreading to the younger leaves under favourable weather conditions. Advanced lesions often have angular margins as they move up to leaf veins. Although rare, tuber infection may occur resulting in irregular, brown-black, sunken spots, surrounded by a raised violet border.