REPORT OF THE SOCIETY 95 



it is essential that clean seed tubers be used since otherwise the infestation is 

 being increased. Long continued investigations are demonstrating the value of 

 sulphur as a medium for soil treatment. Martin in 1921 found that only 8.9 p.c. 

 of clean tubers developed in infested, untreated soil but when 600 lbs. per acre of 

 finely ground sulphur were applied there were 33.5 p.c. clean tubers and when 

 600 lbs of inoculated sulphur were used per acre the clean tubers reached 50.9 

 per cent. This, it is to be remembered, is in soil severely infested with Acti- 

 momyces scabies. In some soils 600 lbs. of sulphur per acre might be disadvan- 

 tageous while 300 lbs. would be satisfactory. By inoculated sulphur is meant 

 sulphur having mixed with it soil containing sulphur oxidising bacteria. 



(d) Skin spot {Oospora pustulans) 



Reference has already been made to the fact that skin-spot has been shown 

 T3y Shapovalov to be a stage of Powdery scab. 



(e) Silver scurf 



Harz in 1871 first described this disease when it occurred in Austria and 

 since then it has been found in Germany, Great Britain, Ireland and North 

 America. It is caused by a fungus named Spondylodadium atrovirens Harz. 



Symptoms 



In its early stages silver scurf, under moist conditions, causes olive black 

 patches to appear on the surface of the potato. These dingy patches are the 

 conidiophores and spores of the fungus. The spots may be small, or may cover 

 an area half an inch or more in diameter. Later in the season the surface layers 

 are slightly raised by the abundant growth of mycelium in the cells below and a^ 

 this time small, black, superficial sclerotia are formed. When washed the 

 raised areas appear whitish or silvery, giving rise to the name "Silver scurf". 

 Still later in the season these raised areas gradually become depressed owing to 

 the death and collapse of infected cells. If the season is moist enough for rapid 

 fungus growth the areas increase in extent until in severe infections the whole 

 surface of the tuber is involved and the fungus penetrates more deeply into the 

 tissues. As a result it is not unusual to find tubers which are entirely discolored 

 and shrunken. Surface layers with sclerotia slough off and thus infect the soil. 



Life History 



The fungus overwinters as sclerotia which are very minute. These may 

 be on the surface of the potato or in the cells of the outer layers or in debris 

 sloughed off into the soil. How long such sclerotia may live in a dormant c; ndi- 

 tion is doul)tful, but given moist, warm environment they germinate readily 

 by conidiophores. Vegetative development is rapid and from spore to spore 

 occupies a period of only four or five days under optimum conditions. The 

 conidiophores are erect, dark-brown, and tall (120 microns) and the spores are 

 borne in whorls on the upper parts of the conidiophores. The spores are dark 



