Chapter X _169— Plant Diseases 



control of potato scab. These are based largely upon seed selection, 

 seed treatment, soil fertilization, crop rotation, and soil treatment. 



The methods of seed treatment (60, 149, 288) are based upon 

 surface sterilization of the seed by means of various chemical disin- 

 fectants. Here belongs the use of hot formaldehyde, of mercuric chlo- 

 ride, and numerous others. Organic mercur)^ compounds have not 

 been found satisfactory (216). In view of the fact that the scab-pro- 

 ducing organisms persist largely in the soil, seed treatment has not 

 been universally accepted (35). 



The use of acid-forming fertilizers and of sulfur, which result in 

 making the reaction of the soil sufficiently acid to prevent the develop- 

 ment of the scab organism, has be.en found to give the most satisfactory 

 results in the control of potato scab. In using sulfur, it has often been 

 considered advisable to inoculate it with actiye cultures of the sulfur- 

 oxidizing»organism Thiobncilhis thiooxidans (286). The plowing un- 

 der of green cover crops combines the favorable effect of reducing scab 

 with that of improving soil. When used in connection with acid-pro- 

 ducing fertilizers, the results obtained from such practices are highly 

 satisfactory. The use of stable manures, however, increases the amount 

 of scab. 



Rotation of crops is essential when scab infestation is particularly 

 heavy. Several years of alfalfa, for example, preceding the growth of 

 potatoes greatly reduces the amount of scab. Where short rotations 

 are practiced, sweet clover is recommended preceding the potatoes (148). 



Since an excess moisture has been found to be injurious to the 

 growth of actinomycetes in soil, one would expect that excessive rainfall 

 would not favor scab development. This is fully justified by the popu- 

 lar expression: "A dry season is a scab year, a wet season is a scab-free 

 year." Heavy watering of crop actually reduces the amount of scab 

 (325). This general impression has frequently been questioned, how- 

 ever, since not only has more severe scab been frequently observed in 

 irrigated than in dry land, but the lower portions of irrigated fields often 

 gave the more severe scab."^ The conclusion may, therefore, be reached 

 that the whole complex of soil texture, moisture and aeration is involved 

 rather than merely moisture. 



R. W. Goss. Private communication. 



