MAINE AGRICULTURAL EXPERIMENT STATION. I914. 2/ 



to cooperate, as it has done at various times in the past, by 

 sending in suspected specimens in order that the presence and 

 distribution of this and other dangerous plant diseases might be 

 known as soon as possible. 



Powdery scab differs from the ordinary scab, which is well- 

 known practically everywhere that potatoes are grown, in quite a 

 number of ways. The common scab is caused by a minute 

 fungus ; the powdery scab by an organism of larger size and 

 belonging to a dift'erent order of plants called slime moulds. 

 Common scab produces relatively large, more or less irregular 

 brownish spots, usually with a decidedly uneven surface. Pow- 

 dery scab forms only small spots which are at first in the form 

 of pustules containing a brownish or olive colored powder. 

 Later the tops of these pustules become rubbed oft*, leaving small, 

 scab-like spots. These may run together into larger patches but 

 even then the original limits of the spots or pustules can usually 

 be made out. 



A very important difference between common and powdery 

 scab is that the former produces only one type of injury to the 

 potato tuber. In addition to the ordinary type, powdery scab 

 in severe cases may produce true cankered areas where the tis- 

 sues are eaten into and hollowed out. Also in severe cases of 

 the ordinary, pustular or scabby stage of the disease, potatoes 

 attacked by powder}- scab exhibit a strong tendency to wither 

 and dry out and show an apparent dry rot. 



Methods of Control. 



When it became evident that powdery scab was prevalent in 

 at least limited sections of the state, the Station pathologists 

 were at once delegated to assist the state and national authorities 

 in making a preliminary survey to determine the limits of the 

 infected areas. One of the pathologists also spent some time in 

 assisting in training the inspectors when the state inaugurated 

 its inspection service. A bulletin has been published which 

 summarized the information regarding the disease to date, and 

 studies are now being carried on with reference to control meas- 

 ures and life-history studies. The following are some of the 

 control measures adv^ocated in order to prevent the spread and 

 distribution not only of powdery scab, but of several other im- 

 portant tuber-borne diseases as well : 



