THE MODERN POTATO PROBLEM 235 



fully demonstrated in the case of potato wart. Experi- 

 ments have shown that the susceptibility of the varie- 

 ties tested varies within wide limits. A number of 

 varieties were found to be absolutely immune. In- 

 cluded in this group are some of our leading commercial 

 varieties, as the Irish Cobbler and Green Mountain. 

 On the other hand it is unfortunate that some of our 

 other leading varieties are so susceptible that they 

 are practically destroyed when attacked by the wart 

 disease. The growing of immune varieties has been 

 found to be the only practical method of controlling 

 this disease in several European countries. 



There is a group of filamentous micro-organisms 

 Occupying an intermediate position between the moulds 

 and the bacteria. To this group of parasites belong the 

 Actinomyces. Certain species of this parasite cause 

 serious suppurative affections in animals. Another 

 species is the cause of one of the most common diseases 

 of the potato tuber, the common scab. It is recognized 

 by the rough pitting of the tubers, rendering them un- 

 sightly and greatly depreciating their market value. 

 This disease is no longer a serious problem in potato 

 culture aside from the expense and labour involved in 

 its control. By careful selection and disinfection of 

 the seed tubers, this source of the infection may be 

 controlled. The most effective, as well as the cheapest, 

 means of controlling the organisms in the soil are still 

 problems that are being studied. This organism thrives 

 best in soils of an alkaline reaction. The addition of 

 sulphur to certain types of soil has recently been recom- 

 mended as a means of controlling scab. The oxidation 



