272 



THE ACTINOMYCETES, Vol. I 



Table 77 

 Effect of soil sterilization and moisture content on scab following inoculation with S. scabies (Goss) 



* Data based upon tuber weights. 



of competition between the soil saprophytes 

 and parasites. Further studies indicated that 

 several factors may be involved. These com- 

 prise an increase in soil acidity, an increase 

 in the buffering and moisture-holding ca- 

 pacities of the soil, and a possible stimulatory 

 effect upon those soil microbes which exert 

 an inhibitory effect upon the scab organisms. 



Soil sterilization and subsequent inocula- 

 tion lead to increased infection. This is 

 apparently due, according to Goss, to a result- 

 ant lack of competition of soil microorgan- 

 isms rather than to changes in the physical 

 or chemical structure of the soil. Addition 

 to such soils of a filtrate from unsterilized 

 soil tends to counteract this effect ; the effect 

 of sterilization is greatly reduced if the inocu- 

 lum is not added until after saprophytic or- 

 ganisms have become established in the soil. 



Various other factors affect the pathogen- 

 icity of the scab organism, such as passage 

 through the digestive tract of animals 

 (Morse), spread by potato residues (Lut- 

 man), the influence of the potato variety 

 (Longree), and the presence of antagonistic 

 organisms (Dailies). 



According to Fellows, if the scab disease 

 is to occur, the potato tubers must be in- 

 creasing in size. Stomata or young unsub- 

 erized lenticels must be present through 

 which the infection (!an take place. There 

 must also be dividing cells or cells which can 

 easily be incited to division b^^ the products 



of the organism, thus permitting the produc- 

 tion of the typical corky scab lesions. 



According to Goss and Werner, seed treat- 

 ments are effective in controlling seed-borne 

 scab ; however, even when healthy or treated 

 seed potatoes are used, the disease may be 

 very severe because of infection from the soil. 

 Crop rotation reduces the incidence of the 

 disease, but the fact that potato scab may 

 cause serious loss in soils never before 

 planted to potatoes suggests that other fac- 

 tors than the time interval between potato 

 crops affect the occurrence of the disease. 



Biological Control of Potato Scab 



No attempt will be made here to review 

 in detail the practical methods of control 

 of potato scab (Berkner and Schroder, Noll). 

 Suffice it to say that crop rotation (Werner 

 et at.) and acidification of soil (Martin, 

 Waksman, 1922, Duff and Welch, Cook and 

 Nugent, Blodgett and Cowan) were found 

 to be most effective. A complete review of 

 the literature, especially of the environ- 

 mental factors bearing on scab formation, 

 has been made by Hollrung. 



One of the most interesting aspects in the 

 production and control of scab on potatoes is 

 the possilile effect of other soil microorgan- 

 isms. Reference has already been made to 

 the work of Millard. Sanford (1926) has 

 shown that green rye plants, plowed into the 

 ground at the rate of 50 tons per acre, showed 

 no effect upon the reduction of scab in a well- 



