The most important are early blight and late blight--both fungous diseases. Early blight affects only the foliage. Late blight kills the foliage and often rots the tubers. Two serious troubles often mistaken for blight are: (1) Tip burn, the browning of the tips and margins of the leaves due to dry weather; and (2) flea-beetle injury, in which the leaves show numerous small holes and then dry up. The loss from blight and flea-beetles is enormous--often, one-fourth to one-half the crop. For blight-rot and flea-beetles spray with bordeaux, 5-5-50. Begin when the plants are 6 to 8 in. high and repeat every 10 to 14 days during the season, making 5 to 7 applications in all. Use 40 to 100 gal. per acre at each application. Under conditions exceptionally favorable to blight it will pay to spray as often as once a week.
Scab is caused by a fungus that attacks the surface of the tubers. It is carried over on diseased tubers and in the soil. In general, when land becomes badly infested with scab, it is best to plant it with other crops for several years. (See page 190.)