80 PROTECTION OF PLANTS, 1921-22 



Control 



The following points are important in control: 



1. Seed tuber selection. 



2. Seed tuber treatment (formaldehyde, hot or cold). 



3. Destruction of spore balls in diseased tubers by boiling before using as 

 feed. 



4. Sulphur 900 lbs. per acre applied broadcast reduces the amount of infec- 

 tion. 



5. Practise long rotation if the attack is severe. 



Skin spot 



"Skin spot" caused by Oospora pustulans is placed among the diseases 

 caused by Fungi imperfecti. Potatoes with a spot apparently exactly like 

 that caused by 0. pustulans, as described in England, were found in shipments 

 to the Spokane market from British Columbia. In fact Heald found that 95 

 per cent of the tubers showed lesions. The spots are circular, brown and small 

 when yoang, darkening with age. At first the spots appear slightly elevated 

 but later somewhat depressed. Under moist conditions sporulation and growth 

 occur in the lesions so that further necrosis occurs. Shapovalov, in a paper 

 given at the Toronto meeting of the American Phytopathological Society, states 

 that the pustules are identical with those of the immature or closed-sorus stage 

 of powdery scab. This author investigated the fungi which, as secondary organ- 

 isms, invade the "skin-spot" pustules and found that they varied with the 

 locality. 



GROUP 3. 



DISEASES CAUSED BY BACTERIA 



Black-leg or Black stem-rot 



(J5. airosepticus v. Hall). 



The most important disease of potatoes caused by bacteria is that known 

 commonly as "Black-leg". Other names are: — "Basal stem rot", "Bacterial 

 black rot", "Black shank disease", "Black stem-rot". It is obvious that "Black 

 leg" is an unfortunate name and that "Black stem-rot" or "Bacterial black rot" 

 would be preferable. Throughout the course of this discussion, the name used 

 will be "Black stem-rot". 



