94 PROTECTION OF PLANTS, 1921-22 



iug a tendency to "spindling sprout" should be discarded. If the tubers have 

 not been sprouted the stem-end of suspicious tubers can be clipped and those 

 with net -necrosis discarded, or the stem-end half only discarded if the bud end 

 is healthy. 



3. Plant disease-escaping varieties, i,e. early types. 



(c) Common Scab 



In Europe, Africa, Australasia, and North America common scab occurs 

 probably everywhere the potato is grown. The disease is caused by a fungus 

 closely related to the higher bacteria and known as Actinomyces scabies (Thax.) 

 Gussow. It is often present to such an extent as to prohibit potato culture but 

 usually the chief loss comes from depreciation in the sale value of the. tubers. 

 Estimates show that there is a variation from 5 p. c. to 75 pc. of the crop which 

 is unfit for sale. 



Symptoms 



This is a tuber disease and the symptoms vary from shallow, rough pitt- 

 ings if the attack occurred when the tubers were nearly mature, to deep rough 

 pittings with furrows and cracks if the potatoes were infected when young. At 

 first the scab appears as a minute red-brown spot. It gradually extends out- 

 ward, becomes irregularly corky and deeper brown in color. Scab mites, white 

 grub and wire worms enlarge and deepen the injury, making the tubers still 

 less valuable. 



Life History of the Organism 



The fungus lives overwinter in the scab spots on the tubers or in the soil. In 

 the soil it may persist for many years and in that case long rotation is not suffi- 

 cient but soil treatment must be considered. AlkaHne soils favour the fungus 

 and acid soils check it. Thus the addition of lime, stable manure, potash or 

 ashes to soil is dangerous when it is known that the scab organism is present. 



Other Hosts 



Turnips, beets and mangels are susceptible to attack by this organism and 

 therefore these crops cannot be used in a rotation. 



Varietal Susceptibility 



Irish Cobbler, Sir Walter Raleigh and Carmen among others are fairly 

 resistant to scab. 



Control 



Two essentials are clean seed tubers and clean soil. If the soil is clean 

 great care should be taken regarding the seed tubers and when there is an}' doubt, 

 disinfection should be thoroughly performed. Even in the case of infected soil, 



