352 AGRICULTURE OF MAINE. 



The next picture illustrates a bad case of scab resulting from 

 inoculation with a pure culture of the scab organism. 



While it is well known that potato tubers carry the disease, 

 it is a matter of considerable practical interest as to whether or 

 not the manure of stock fed with scabby potatoes is also a 

 source of infection. Some experiments carried on at the Maine 

 Agricultural Experiment Station a few years ago bear upon this 

 point. For two years in succession a horse and a cow were fed 

 scabby potato tubers in amounts equal to or in excess of those 

 likely to be used on the farm. After this had gone on for 

 several days the manure from these animals was collected in 

 sterilized receptacles and then mixed with pots of sterihzed 

 soil, in which potato tubers free from scab spots and disinfected 

 with formaldehyde were planted. At the same time similar 

 potatoes w^ere planted in pots of the same sterilized soil, but 

 without the addition of manure. In the first case about one- 

 fourth of the potatoes produced in those pots where the horse 

 manure was used were scabby. None appeared where the cow 

 manure was used or in the check. The second year, nearly four- 

 fifths of the tubers where the horse manure was used were 

 scabby, and one-sixth W'here the cow manure was used. The 

 check again was clean. The first picture shows the check, the 

 second, the average condition of the scabby potatoes w^here the 

 cow manure w^as used and the third, the average appearance of 

 the diseased tubers w^here the horse manure was used. 



The final conclusion was that limited amounts of uncooked, 

 scabby potato tubers could be fed to cows with a fair degree of 

 safety, but that the germs of this disease readily pass through 

 the digestive tract of a horse in a living condition. 



The next picture is intended to show the difiference between 

 common and pow^dery scab. As will be seen, common scab pro- 

 duces relatively large, more or less irregular brown spots, usually 

 with a decidedly uneven suface. Powdery scab forms only small 

 spots W'hich are at first in the form of pustules containing a 

 brownish or olive-colored powder. Later, the tops of the pus- 

 tules become rubbed off, leaving small scab-like spots as shown 

 in the lower and middle part of the picture. 



One important difference between common and pow^dery scab 

 is that the first produces only one form of injury upon the 

 potato. Severe cases of powdery scab may lead to quite different 



