Chapter II — 45 — Important Types 



causative agents of scab, have been described. Because of lack of ex- 

 perimental demonstration, it is difficult to state how many of these actu- 

 ally cause scab. The ease with which numerous saprophytic actino- 

 mycetes are isolated from the surface of material that has been in contact 

 with the soil justifies these doubts. 



In a study of the effect of environmental conditions upon the growth 

 of S. scabies, the following conclusions were reached: 



S. scabies grows within a wide range of temperature (8° to 38° C.)- 

 Good growth and maturity occur between 13° and 32° C, and the opti- 

 mum temperature is about 27° C. Therefore, under average field con- 

 ditions in most potato growing areas, it appears that temperature, as it 

 affects host and pathogen only, cannot be a very important factor in 

 the scab problem. The spores survive temperatures up to 90 °C. (moist 

 heat) for ten minutes. 



S. scabies is a strong aerobe. The spores will germinate with an 

 extremely small supply of oxygen, but a large amount is required for 

 subsequent development. Maturit)', as indicated by dark aerial hyphae, 

 will not take place in the absence of oxygen. Amount of oxygen, not 

 partial pressure, is the limiting factor for germination and growth. 



It was found that the germination of spores of S. scabies on nutrient 

 agar was greatly retarded by a lagging film of excess water. The inocu- 

 lum of S. scabies appeared to increase most rapidly at a soil moisture 

 content about optimum for plant growth. 



The limiting acid reaction for germination of the spores of the strain 

 of S. scabies used was found to be about pH 5.3. Germination oc-' 

 curred most quickly at about pH 8.5, and an optimum development took 

 place at this point. Because of the higher pH of the tuber and a strong 

 tendency of the pathogen to make its habitat (scab pustule) alkaline, 

 severe scab may be expected in soils ranging from a strongly alkaline 

 reaction to at least pH 5.4. 



