Unlike late blight, this disease does not normally attack young, actively growing tissue. It attacks maturing portions of potato plants, potatoes under stress, and usually it shows up on the lower leaves. This is a disease of senescing plants. It thrives in plant parts with lower nitrate levels. As such early season early blight control can be very effectively managed by good plant nitrate levels.
Late season is when fungicides become really important.
Potato cultivars vary in their susceptibility to early blight. Often resistance is associated with how cultivars mature. Norkotahs are very determinate and vine maturity occurs at a very specific time, at which point they suddenly become highly susceptible. Nuggets, on the other hand, are more indeterminate and keep growing vines all season, and they are less susceptible to blight. Over fertilized Nuggets will have very little problem with early blight while lower fertilizer levels now used on Nuggets have lead to more problems with early blight late season in Nuggets.