Late blight is the most destructive disease of potato; it is a fungal disease which is practically universal, and it causes destruction both of the foliage and the tuber. A devastating epidemic of late blight fungus on potatoes beginning in Europe in 1845 brought about the famine, particularly in Ireland, that caused starvation, death and mass migration. Of the other fungal diseases the most important is early blight, which may be as destructive as late blight under hot, dry conditions. Viruses cause the most economically important diseases. Over two centuries ago, English farmers realized that when locally grown seed was used year after year, yields of potato deteriorated. This was due to the spread of virus diseases, of which 20 have been recorded as occurring in potatoes. Of these, it is generally accepted that leaf roll is responsible for the largest reductions in yields. It is found practically everywhere potatoes are grown commercially. Other virus diseases include various mosaics and spindle tuber. A few of the bacterial diseases of potato are black-leg and various rots and wilts.