MacDonald's restaurants use 3.2 billion pounds of potatoes a year. Airborne sporangia are capable of travelling great distances in the wind. Mycelium survives in living tissues of tubers and thus over-winters in storage or as volunteer potato plants in the field.
Symptoms first appear on older leaves soon after flowering, following wet, humid weather conditions.
Dark green, water-soaked areas at the leaf margins spread inwards and later become dark brown and brittle. There is a fluffy white growth found on the underside of the leaves when humidity is high. Symptoms on tuber appear as irregular brown to purplish sunken lesions , often appearing around the eyes . The infected tissue is granular and reddish and can penetrate up to 2 cm into the tuber . Disease spread is rapid under ideal wet, humid, and windy conditions.