100 PROTECTION OF PLANTS, 1921-22 



Figure 12 illustrates the development of sclerotia in a Petri dish culture on 

 dextrose agar. 



Other Hosts 



Ducomet states that this organism is parasitic on Tomato and Physalis 



'peruviana. I have artificially infected tomato but have not 3-et tried Physalis 



sp. 



Control 



This disease is serious only in that it weakens plants and therefore reduces 

 the crop. Exactly to what extent it is ot economic imortance it is as yet impos- 

 sible to state. 



McAlpine advocates burning the haulms of diseased plants and selecting 

 clean tubers for planting. 



GROUP 8 



* ' Physiological ' ' diseases 



(a) Black heart 



As the name indicates, the symptoms of black heart are confined usually 

 to the medullary region. The blackened tissues are irregular in outline 

 and almost always elongated from stem to bud and with branches extending 

 towards the buds. Occasionally there may be large black spots scattered 

 irregularly 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 usualty 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 shipped during 

 the winter in stove-heated cars those in the vicinity of the stove are frequentty 

 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 insufficient oxygenation 

 and they become necrotic and brown to black in colour. Gradual^ the dead 

 cells dry out becoming shrunken, tough and black. 



The second condition was demonstrated by Stewart and Mix at the N. Y. 

 Geneva Station. 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. 



