132 



PLASMODIOPHORALES 



nutritional conditions are more favorable in that re- 

 region of the cell. 



The walls of the infected host cells are also mark- 

 edly changed by the parasite. As the pseudopods of 

 the Plasmodium push down between the cells, the 

 walls become swollen, gelatinous and wavy. These 

 walls have a greater affinity for Orange G than those 

 of healthy cells, according to Kunkel's and Wild's 

 observations, which indicates that they have under- 

 gone a change in composition. The middle lamella is 

 also usually dissolved by the action of the Plasmo- 

 dium. 



Control 



The shipment and importation of infected tubers 

 appear to be the primary means of dispersal of pow- 

 dery scab from one region or country to another. 

 Most countries have accordingly enacted legislation 

 against the importation of diseased potatoes and es- 

 tablislied inspection and certification bureaus within 

 their boundaries to insure planting of healthy tubers. 

 Locally, the disease may be transferred from one 

 field to another by fertilizing with contaminated ma- 

 nure, by farm implements, contaminated sacks, and 

 soil on the shoes of laborers. Sanitary practices must 

 accordingly guard against dispersal by such means. 

 Since fungus spores will survive passage through 

 the digestive tract, infected tubers and parings 

 should be boiled or sterilized before feeding to hogs 

 and other animals to avoid contaminated manure. 

 Other sanitary measures involve selection of disease- 

 free tubers for planting and the avoidance of con- 

 taminated land. 



Inasmuch as the spores of S. suhterranea may re- 

 main viable in the soil for 3 to 5 years or longer 

 (Melhus, et al.). crop rotation, fallowing, or pastur- 

 ing the land are essential in regions where the dis- 

 ease is abundant and destructive. In such regions the 

 potato crop may be largely destroyed if rotation is 

 neglected (Petlivbridge, '26). Dorojkin ('36) thus 

 advocated compulsory crop rotation of no less than 

 3 years in Russia, but it is apparent that a longer 

 period may be necessary to starve out the parasite. 

 The rotation period obviously depends to some ex- 

 tent on climatic conditions, and the character of the 

 soil. In Scotland, for instance, a rotation of 6 to 10 

 years or longer lias been recommended for loamy soil 

 in regions where high rainfall and low tempartures 

 normally occur during the potato growing season. 



Eradication of wild hosts is of doubtful value at 

 present because S. suhierranea has a comparatively 

 limited host range and very little is known about its 

 occurrence outside of the potato. Since the fungus 

 develops on tubers only after they are partially ma- 

 ture early harvesting may sometimes be effectual in 

 avoiding the disease, provided infection does not 

 occur early in the season. However, it is not prac- 

 ticable because no marked above-ground symptoms 

 of infection occur, which would indicate whether or 

 not the tubers are infected. 



Liming the soil as practiced in the control of club 

 root of crucifers stimulates instead of inhibiting 

 the development of powdery scab. Massee ('10, '15) 

 recommended dressing the land with quicklime in 

 the spring when the spores germinate in the soil, but 

 Pethybridge, Home ('12), and others found that 

 lime increases the amount of diseased tubers. Mel- 

 hus, et al., likewise noted that lime at the rate of 

 3,000 lbs. per acre increased infection 13.2 per cent 

 in one case but reduced it in another. These varia- 

 tions, however, were probably due to differences in 

 soil types in the two test blocks. Phillips ('32) ad- 

 vised the application of lime to loosen the soil and 

 make it more porous and thereby increase drainage. 

 Since damp, water-logged soil favors the develop- 

 ment of the parasite proper drainage is essential. 



Other control measures involve seed tuber disin- 

 fection before planting, soil disinfection with chemi- 

 cals and fungicides, and the use of resistant potato 

 varieties. As to tuber sterilization various disinfec- 

 tants have been advocated and used. Johnson ('08) 

 found that soaking infected tubers for 18 to 2i hours 

 in 2 per cent Bordeaux mixture, 1 Vo hours in corro- 

 sive sublimate, or 2 hours in a weak formalin solu- 

 tion is effective in killing the spores of S. suhierra- 

 nea. Subsequent workers have confirmed these re- 

 sults to some degree. Pethybridge ('13) in particu- 

 lar observed that seed tubers treated with weak solu- 

 tions of formalin, copper sulphate. Burgundy mix- 

 ture, or rolled in flowers of sulphur checked the dis- 

 ease to a marked degree. Tubers soaked in 1 per cent 

 copper sulphate for 3 hours yielded no diseased off- 

 spring, while those rolled in sulphur gave only 1.03 

 per cent infection. Melhus, et al., however, found 

 that tuber treatment with 2 pts. of formalin per 30 

 gals, water at -16 to 50° C. for 5 minutes, or mer- 

 curic chloride, 1 ozs. to 15 gals, water at -i-l to -IS C. 

 for 5 minutes, gave better results than the usual 

 long cold treatments with either of these substances. 

 Rolling wet tubers in sulphur or soaking them in 5 

 per cent atomic sulphur for fl/o hours was less effec- 

 tive than treatment with formaldehyde and mercuric 

 chloride. While these disinfectants reduce infection 

 considerably, the results obtained are not to be re- 

 garded as absolute, according to Melhus, et al., since 

 other factors such as variations in soil moisture and 

 texture, drainage, and temperature have a marked 

 effect on the results. Later workers, including Ab- 

 bott ('28), Dorojkin ('31, '36) and Rovdo ('36), 

 iiave reported similar beneficial results from the use 

 of corrosive sublimate, formalin, and mercuric chlo- 

 ride on infected seed tubers. Dorojkin found that 

 soaking tubers 20 to 30 minutes in a .2 per cent solu- 

 tion of meranin, a liquid organic mercury prepara- 

 tion containing less mercury than mercuric chloride, 

 gave excellent control in Russia. 



Disinfection of contaminated fields by the appli- 

 cation of sulphur at the rate of 300 to 900 lbs. per 

 acre has been reported by Melhus, et al.. Cotton 

 ('22), Abbott, and Boning and Wallner ('38) to re- 

 duce the incidence of infection considerably. Melhus 

 and his collaborators reported that better results 



