136 



3. 



(2) Degeneracy is attributed by Stewart and East to be 

 due largely to diseases earned in the tuber. The organisms 

 have not been isolated, and some of these diseases may be 

 physiological, exhibiting themselves under unfavourable con- 

 ditions. Typical of these are those disorders knowns as Mozaic 

 Disease, Spindling Sprout and Curly Leaf. 



(3) It has been suggested that the continual asexual pro- 

 pagation may be the cause of this degeneracy, since the vigour 

 due to heterosis cannot manifest itself. East, however, main- 

 tains that this view is unproven. 



There is no doubt, however, that this so-called degeneracy 

 can be checked to some extent by hill selection. In localities 

 well suited to that plant, the productivity is often maintained 

 without recourse to any special form of selection. For instance, 

 in Ontario, where the climate is favourable, Zavitz has grown 

 five varieties in a variety test for 25 years without employing 

 any method of selection ; these varieties have given a slight 

 increase during the period of the trial. 



Emerson^ has shown that by mulching potatoes with 

 about 4 to 6 inches of straw, and thus modifying the moisture 

 and temperature of the soil, the productivity of varieties was 

 maintained. In this method the field from which seed 

 potatoes for the following year are to be obtained is mulched 

 with straw after planting and no subsequent cultivation given. 



Improvement by hybridisation is slow, laborious and un- 

 certain, and should be undertaken only by those having the 

 time and means at their disposal. The chief obstacles are 

 in getting crosses to take, and the time required to test the 

 yielding capacity of the hybrids. Improved varieties have, 

 however, been produced in this way. 



Mutations do occur, and if of some outstanding peculiarity 

 are naturally selected by breeders and growers, and often good 

 new varieties may be found. 



