REPORT OF THE SOCIETY 77 



Brunchorst, in Xorwaj", in 1886 studied the disease and named the causal organ- 

 ism Spongospora, placing it among the Myxomycetes. Gussow was the first 

 to find the disease on this continent, some potatoes with tj'pical scab spots being 

 sent to him from several localities in Canada in 1912. Morse and Melhus then 

 found evidence of the existence of the disease in Maine in the summer of 1913. 

 It is now known to be present in Great Britain. Ireland, Northern and Central 

 Europe, South America, Eastern Canada and in several northern localities 

 of the United States of which the most important was Maine. Dr. Morse, 

 Director of the Elaine Station, has informed the writer in correspondence that 

 Powdery Scab has not been found to anj' extent during the last five or six years 

 in Maine. That a similar situation occurs in New Brunswick is borne out by 

 correspondence between Dr. Morse, Mr. Cunningham and the writer. 



Symptoms 



The name of the causal organism indicates that only underground parts 

 are attacked, namelj' roots, stolons and tubers. Roots are affected earlier in 

 the season and on them white galls develop, looking remarkably like the nodules 

 on the roots of leguminous plants. Similar galls are produced on the stolons 

 and less commonh' on the part of the stem underground. Roots of all ages 

 may be affected. 



On the tuber the pustules begin to appear a little later in the season, usually 

 when the potatoes are already of a fair size. The stem end is frequently first 

 affected but as the tuber matures pustules may be found anywhere on the surface. 



The first sign is a tiny, brownish purple pimple on the surface surrounded 

 by a more or less translucent zone, the whole not exceeding one twelfth of an 

 inch in diameter. This spot increases in size and the skin is gradually separated 

 from the tissues below, which are stimulated to abnormal growth so that the 

 protuberance is formed. Usually the epidermis is ruptured by this growth just 

 about the time the organism is ready to go into the spore stage. The turned- 

 back epidermis is typical of the pustules of powdery scab in uninjured 

 specimens. As the parasite matures the host cells are invaded, broken down 

 and killed so that only remnants are found. At this time the parasite ceases its 

 vegetative life and forms spores which are aggregated into balls. It is the spore 

 balls together with remnants of cellular tissues which form the powder}^ mass 

 found in the pustules at maturity and which account for the common name 

 "Powdery Scab". If one rubs out the powdery mass the tissue below is seen 

 to resemble the epidermis of the tuber to a certain extent and in fact there is 

 present a layer of cells functioning as a cork layer, which fact accounts for 

 the other common name for the disease — "Corky Scab". 



In Europe, Great Britain and Ireland, a more advanced stage of the 

 disease is found where the tuber as a whole is malformed and tissues severely' 

 disorganized, this is known as the canker stage. That it does not occur in 

 this country is probably due to the shorter growing season. 



