16. 



2. What is tne rate of decrease of the popu- 

 lation number wnen no potatoes are grown? 



If hign degrees or infestation are present, 

 a third question is mportant: 



3. At wnat level of inl'estation of the soil 

 does damage occur? 



The dan,'^er level . The third question has boen 

 answered first. Although the amount of damage to 

 the potato also depends on soil fertility, all 

 authors agree fairly well about the maxlinum level 

 of infestation which the potato can stand without 

 being damaged. CARROLL (1933) found a relation 

 between degree of infestation of the soil and degree 

 of potato sickness when the first was not over 3-U 

 cysts per gram of soil. At higner degrees of in- 

 festation the crop was irregular but always very 

 poor. 



The National Agricultural Advisory Service in 

 England found that damage occurs where more than 

 5 viable cysts per 10 gram of soil are found (equals 

 roughly 15 eggs/g.). This figure also depends on 

 Whether soil and weather conditions are favorable 

 to the potato. 



Rate of increase . In the early experiments in 

 Europe very variable rates of increase were found. 

 This was due to the fact that the initial degrees 

 of infestation were always high. The first experi- 

 ments which gave a better understanding of the fac- 

 tors governing the rate of increase were not done 

 in Europe but on Long Island, W.Y., by CiiITWuOiJ and 

 mj^MESSER (19U8). These show clearly that the rate 

 of increase depends on initial degree of infestation. 

 At very low levels there may be one ratio betx^een 

 initial degree of infestation and number of eggs 

 foCTied on a potato plant. At higher levels the 

 increase becomes smaller the higher the degree of 

 infestation, whereas at very high levels there may 

 be a reduction in number of the parasite. An in- 

 creasing portion of the larvae hatched from the eggs 

 in the cysts cannot complete their life cycle as the 

 potato roots die too soon because of the heavy attack. 

 The decrease in number of eggs per unit of soil when 

 potatoes are grown in heavily infested soil explains 



