August, 1922 



SCIENTIFIC A ^RICULTURE 



417 



Diseases oF the Potato 



by B. T. DICKSON, 

 Professor of Botany, Macdonald College. 



(Continued) 



GROUP 8. "PHYSIOLOGICAL" 

 DISEASES. 



(a) Black Heart 



As the name indicates the symptoms of 

 black heart are confined usually to the me- 

 dullary region. The blackened tissues are 

 irregular in outline and almost always elong- 

 ated from stem to bud and with branches 

 extending towards the buds. Occasionally 

 there may be large black spots scattered ir- 

 regularly through the flesh of the tuber and 

 sometimes the vascular tissues are blackened. 

 Surface discoloration of the tuber occurs in 

 cases of insufficient aeration but usually no 

 surface symptoms are evident. 



There are two conditions which bring about 

 black heart. The first is overheating and this 

 is a shipping trouble. Among potatoes ship- 

 ped during the winter in stove-heated cars 

 those in the vicinity of the stove are fre- 

 quently found to be badly affected with black 

 heart. The experiments of Bartholomew in 

 1913 showed that heating tubers to 100-113° 

 F. for 14-18 hours brought about changes in 

 the respiration rate of the tubers so that in 

 the overheated environment more oxygen is 

 demanded than is available. The innermost 

 tissues are the first to be affected by the in- 

 sufficient oxygenation and they become ne- 

 crotic and brown to black in colour. Gra- 

 dually the dead cells dry out becoming 

 shrunken, tough and black. 



The second condition was demonstrated by 

 Stewart and Mix at the N. Y. Geneva Sta- 

 tion. They showed that black heart occurs 

 at low temperatures if the oxygen supply is 

 markedly deficient. Tubers confined with a 

 volume of air equal to the volume of potatoes 

 required up to 40 days for black heart to 

 develop at 40°F. 



Whether affected tubers sprout or not de- 

 pends upon the extent of the collapsed tissues 

 and upon whether the injury is due to in- 

 sufficient oxygenation at high or low tem- 

 peratures. Usually the latter require a longer 

 time, for sprouting but if no surface injury 

 is present and the buds sprout well there is 

 no harm in using such tubers for seed. It 

 is obviously unwise to use tubers with sur- 

 face lesions for seed since the lesions will 



become breeding grounds for fungi and bac- 

 teria. 



Control 



It must be remembered that potatoes stored 

 at higli temperatures and for a long period 

 require more ventilation than those stored 

 at lower temperatures or for short periods. 



1. Potatoes should not be stored more 

 than six feet in depth at temperatures below 

 45 °F. for a lengthy period of time. At tem- 

 peratures of 50-' F. to 70 °F. they should not 

 be piled more than 3 ft. deep if they are to be 

 kept longer than a month. 



2. Small potato pits do not need extra 

 ventilation but the tubers should be pro- 

 tected from frost. 



(b) Frost Necrosis 



In Canada, as in the northern tier of states 

 of our neighbour, the main crop is in constant 

 danger of exposure to freezing temperatures 

 from the time of harvest to planting or to re- 

 tail sale. There are two types of frost in- 

 jury, viz: — that in which complete freezing 

 occurs and that in which no superficial symp- 

 toms are noticeable. In the first case freez- 

 ing is due to exposure to a very low tem- 

 perature or to prolonged freezing at a some- 

 what higher temperature. The tubers are 

 frozen solid and the tissues collapse on thaw- 

 ing. The soft, wet condition resulting is 

 easily recognized. 



The second type is due to exposure to tem- 

 peratures just below the freezing point or to 

 a very low temperature for a short time. In 

 this case, the tuber is never frozen completely 

 but certain tissues only are affected. Upon 

 exposure to ordinary .storage temperatures the 

 affected tissues become necrotic and are black- 

 ish in colour after being exposed to oxida- 

 tion in the air. The necrosis is of three 

 general types, viz. net, ring, and blotch. 



In "net" necrosis there is a general black- 

 ening of the finer vascular elements extend- 

 ing from the vascular ring into the medulla 

 and cortex. 



A more pronounced blackening in the vas- 

 cular ring and adjacent tissues gives the 

 "ring" type of frost necrosis. This is often 

 most apparent at the stem end. 



When the necrotic areas are larger arid less 



