Diseases of the Potato. 311 



the tubers are small. When older tubers become affected, the 

 injury generally assumes the form of a corky crust upon the sur- 

 face. 



The conditions which favor the growth of the potato scab fun- 

 gus are dampness and an alkaline condition of the soil. It is a 

 common experience that scab is more prevalent in soils rich in 

 organic matter, and abundance of air and moisture allow a more 

 vigorous growth of the parasite to take place. 



The fact that an alkaline soil causes more severe attacks of 

 potato scab has an important practical bearing. A soil may be 

 made alkaline in a number of ways, especially by the addition of 

 certain fertilizers. Lime has a very strong tendency in this di- 

 rection, and stable manure exerts a similar influence. This fact 

 has undoubtedly given rise to the popular belief that stable 

 manure will increase the amount of scab upon potatoes. The 

 appearance of the fungus may be favored merely by the alkaline 

 condition of the soil, or it may actually be applied to soil which 

 is free from the disease, and thus an entire field be infected by the 

 fertilizer. If both the soil and the manure are free from disease, 

 no scab will appear, unless it exists upon the tubers used for seed. 

 All fertilizers which have a tendency to produce an acid condi- 

 tion of the soil may check the growth of the fungus, and by the 

 proper selection of fertilizing material sometimes a very marked 

 effect may be produced upon the crop.* 



During 1894, a third cause for the injury generally known as 

 scab was advanced by A. D. Hopkins, entomologist of the West 

 Virginia Experiment Station. The first account of his investi- 

 gation of this disease was read before the West Virginia State 

 Horticultural Society April 6.f In this paper it was said that 

 the larvse of one or two species of insects (Epidapus scabies, Hop- 

 kins, and Sciara sp.^ disfigure potatoes in a manner very similar 

 to that of the scab fungus. In the more northern states, however, 

 these insects do not appear to be so troublesome, and the methods 



*An excellent account of the effect of various chemicals and manures upon the 

 amount of scab upon potato tubers bas been published by Wheeler and Tucker 

 in Bull. 23, of the R. I. Agric. Exp. Sta. Oct. 1895. 



See Special Bull. 2. W. Va. Agric. Exp. Sta. pp. 97-111. Also Proceedings 

 of the Washington Entomological Society, May 3, 1894; Insect Life, vii. p. 147. 



