Chapter X _163— Plant Diseases 



Addition of stable manure to soil has been considered to increase 

 the incidence of scab. This effect is believed to be due to the result- 

 ing alkaline reaction and accumulation of humus, both of which favor 

 the development of actinomycetes (270). Lime also increases scab 

 production because of the resulting alkalinity. Similar effects are ap- 

 parently obtained from excessive use of potassium. 



Frequently the application of organic materials such as green ma- 

 nures results in the reduction of scab. This may be due to one of the 

 following three factors: (a) an increase in soil acidity; (I?) an increase 

 in the buffering and moisture-holding capacities of the soil; (c) a stimu- 

 lating effect upon microbes which are inhibitory to the growth of the 

 scab organism. 



In a detailed study of the effect of soil moisture (287) and reaction 

 (285) upon the development of potato scab, Martin concluded that a 

 high soil moisture and a high acidity are the two important limiting 

 factors. Spore germination is limited by an acid reaction of about pH 

 5.3; it is favored by a higher pH value, 8.5 being the maximum. Se- 

 vere scab can be expected, however, in soils with pH values ranging 

 from strongly alkaline to about 5.2. The higher j;H values of the sap 

 in the tuber and the tendency of the pathogen to change the reaction 

 toward alkaline make growth possible at the lower pH values. 



DipPENAAR believed that the soil reaction is the most important 

 factor involved in a steady increase of the scab organism in soils which 

 are continuously used for potato culture. In soils having a pH of 5.2 

 or lower, the incidence of disease increases very slowly, if at all, with 

 continuous potato culture, provided care is exercised in selection and 

 treatment of the seed tubers. 



Soil moisture is also frequently considered as a highly important 

 factor in bringing about an increase or a decrease in abundance of the 

 scab organism in the soil and in the occurrence of scab on potatoes. An 

 increase in moisture increases the yield of the crop and decreases the 

 incidence of disease. This effect may be indirect and may be due 

 largely to the influence upon soil aeration. In general, the effect of 

 soil moisture upon scab development may give quite variable re- 

 sults (116). 



Soil temperature is least significant in influencing the occurrence 

 of potato scab. Jones (202) reported that the optimum soil temper- 

 ature for scab development is about 23 °C., whereas the optimum for the 

 percentage of scab to the total tuber surface is about 20.5°C. It was 

 concluded, therefore, that 22° C. is the optimum soil temperature for 

 scab development, under special Wisconsin conditions. Sandford (379) 

 also argued that since the scab organism grows over a wide range of 

 temperature (8° to 38 °C.) and since the potato grows and matures 

 between 13° and 32° C, temperature, as it affects host and pathogen, 

 cap. hardly be considered as a very important factor in the scab problem 

 under average field conditions in most potato growing areas. Most of 



