<i.rii HOOT or ( mciFKHS 



I'ii) 



Walker. J. C. aiul H. H. I.arscn. 1!);U. Wisconsin 



Aftr. Kxp. Sta. Hull. I-28. 

 . K. 1*. Link, and .^. Morrll. l!i:»7. Wise. .Xgr. 



Exp. .>^ta. Hull. t.-C). 

 Willman. 1'. I,. liL'tO. f. .S. Dipt. .Vijr. T.cli. Hull. 



7S1. 

 Wilson. J. D. 193t. Ohio .Xsir. Exp. .Sta. Hiniontlily 



Hull. 19: 58. 



Davies. 1). W., M. (irittitli. and G. Evans. 192S. 



Welsh .lour. .\gr. !•: 293. 

 Preston. N. C. 1928. //>»/. i: 280. 



. 1931. Jour. Min. Agr. 38: 272. 



Whitehead, T. 1922. Ihid. 29: 362. 



. 192.5. Welsh .lour. .Vgr. 1: 17(i. 193."). IbitL 



11:228. 1936. //'»/. 12: 183. 19 K), //)/(/. 16:99. 



POWDERY SCAB OF POTATOES 



Powdery scab of potatoes is now almost world 

 wide in distribution. Lagerlieim's discovery of its 

 presence in Ecuador suggests that it may be endemic 

 to South .\merica. since that continent is generally 

 regarded as the native home of the potato. In that 

 event powdery scab may be of greater antiquity 

 than is generally supposed. It has doubtless attained 

 its extensive distribution by the shipment and im- 

 portation of infected tubers. From South America 

 it may have been imjiorted to Europe and then back 

 to North .\merica and other parts of the world. Pow- 

 dery scab was first re))orted from Germany by Wall- 

 roth and Martius in 18 12. but it had doubtless been 

 known by potato growers for many years before that 

 time. Shortly afterwards, it was described from Eng- 

 land by Berkeley {' i6) and later from Wales, Scot- 

 land. Norway and Ireland. The first record of its 

 occurrence in North America was made in 1913 on 

 potatoes in Quebec, and in the same year it was also 

 found in Maine and other states. This disease is 

 known by a variety of names throughout the world. 

 In (iermany it is described as Knollenbrand, Kartof- 

 felgrind. Kartoffelgnatz. .Schorfkrankheit. and Kar- 

 toffclschorf. In England, .Scotland, Wales, Ireland 

 and the U. S. A., it goes by the names of potato 

 canker, corky end, corky scab, spongy scab. Spoiif/o- 

 gpora scab, and jiotato tumor, although powdery 

 scab is the term most commonly used. The names, 

 potato canker and tumor are employed when the 

 lesions and tumors are unusually deep and conspicu- 

 ous. 



As a destructive disease of potatoes, powdery 

 scab is of secondary importance compared with late 

 blight, virus, Fuxarium wilt and rot, and common 

 scab. In relatively dry and warm regions the dam- 

 age caused may be so slight as to go unnoticed, while 

 in other places with high )irecipitation and low tem- 

 peratures the losses may lie quite serious. ))articu- 

 larlv if the disease is of the cankerous type and is 

 followed by powdery scab dry rot in storage. In 

 England, VVales (Pethybridge, '24), New Zealand 



(Anony., '27), Peru (Abbott, '31), and Russia 

 (I)orojkin, '36), destruction of 30 to 50 per cent 

 .•nul more of the cro)) h.is been rejiorted in years of 

 he.ivy rainfall .-iikI low teui|)eraturc. Likewise, Mel- 

 luis. (•/ al., found that 30 to 73 ))er cent of infected 

 tubers may be destroyed by dry rot in storage or 

 rendered useless for table or ))lanting. Such losses, 

 however, appear to be exceptional, but the disease is 

 nevertheless of sufficient importance to warrant the 

 establishment of strict (pi.-irantine .md tuber inspec- 

 tion and certification laws by most tountries througll- 

 out the world. 



Predisposing Factors 



The occurrence of powdery scab and incidence 

 of infection are dei)endent on climatic conditions. 

 Heavy rainfall, fairly low tenii)eratures, damp, 

 poorly-drained and water-logged soils favor infec- 

 tion and development of the disease. Melhus, et al., 

 observed that periods of rainfall, followed by cool, 

 damp, cloudy weather during the growing season are 

 highly essential to the development of the disease, 

 and these observations were subsequently confirmed 

 by Ramsey ('18) from greenhouse experiments. He 

 found 83 per cent infection in pots of potatoes grown 

 at 57°-60° F. under moist conditions, while no in- 

 fection occurred in pots at 76°-80° E. and in rela- 

 tively dry soil. \\\\A ('29), on the other hand, found 

 no clear correlation between incidence of scab and 

 the prevalence of any particular climatic conditions 

 in Switzerland. Koltermann's ('31), Phillips' ('32), 

 and Naumov's ('36) observations on the disease in 

 Germany and Russia confirm those of Melhus. Ram- 

 sey, and others in America. Naumov found powdery 

 scab to be more prevalent in soils with 60-90 per 

 cent moisture content and with pH values from (■.7 

 to 5.9 than in soils with 40 jier cent moisture and 

 high pH values. 



That unfavorable climate is an etieetive barrier 

 to the spread of the disease is evident from experi- 

 ments conducted by Melhus, et al., which involved 

 ])l.uiting of heavily infected tubers in fifteen differ- 

 ent regions along the Atlantic Coast from Massa- 

 chusetts to Florida. All of these plantings yielded 

 clean crops. These results are sup])orted by the ob- 

 servations of She])herd ('35), Nattras ('38), and 

 Littlejohn ('39) that heavily infected imported 

 tubers planted in Mauritius and Cyprus give 

 healthy crojis and that iS'. suhicrranea does not re- 

 main viable under prevailing climatic conditions on 

 those islands. 



Hydrogen ion concentration ap])arently does not 

 influence the incidence of infection, since .S'. nuhtcr- 

 ranea appears to tolerate both .alkaline and acid re- 

 actions. Wild. Phillii)s, and Naumov found that in- 

 fection mav readily occur in soils with pH values 

 ranging from 1.7 to 7.6. Furthermore, the incidence 

 of infection is not affected by the carbonate or 

 hexosan content of the soil, according to \\'ild. 



The ])hysieal character of the soil, however, is an 

 imi)ortant factor. A close correlation between cer- 



