The potato, the fourth most important food plant of humanity, is subject to a recorded 70 pathogens and 198 pests. Fortunately only a few are of widespread economic importance, the seriousness of which varies in different regions and in different years according to the prevailing weather. The potato is especially subject to disease because it is propagated vegetatively. This permits transmission of pathogens (especially viruses) from generation to generation, and produces genetically homogenous populations which favour epidemic diseases.
In the temperate zones of North America and Europe the Colorado potato beetle is the principal insect pest. In warmer climates the potato tuber moth causes major losses in the field and stores. Similarly, cyst nematodes, which originated in the Andes of southern Peru, are important pests throughout the warm regions of the world. Late blight is the most serious disease of potatoes during cool, wet weather in most of the 129 countries where the potato is grown. Soft rot and wilt diseases caused by bacteria, and wilt diseases of fungal origin may also cause significant losses. Early blight may be destructive under warm, dry conditions. The international trade in certified seed tubers owes its importance to the need to replace virus degenerated seed stocks. The potato spindle tuber viroid is of special quarantine significance since there is no known resistant varieties. In some developed countries rigid control practices minimize the effects of a particular pest or pathogen.
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.