REPORT OF THE SOCIETY 91 



shown by cutting away carefull}" the developing stolons around the base of the 

 stem when aerial tubers begin to develop. 



On the other hand, if root infection occurs the metabolism of the plant as a 

 whole is affected, the plant is definitelj^ dwarfed and the leaves cannot function 

 to a normal extent so that the potato tubers formed, if any, are small, giving the 

 condition known as "little potato". 



Life History of the Organism 



Usually the mj^celium does not give rise to the sporiferous stage but late in 

 the season forms the small compact blackish sclerotia on the surface of the tubers, 

 If the sporiferous stage is developed it occurs on the weft of mycelium around 

 the base of the stem just above ground. Here the spores are borne on basidia, 

 four spores, measuring 6-8 by 9-14 microns,, each on a short sterigma for each 

 basidium. They germinate and cause new infections in the field. The myce- 

 lium can grow through or over the soil and in one case north of Arras, France, 

 the writer found that the mycelium grew, during two months of a cool, moist 

 summer, ten yards along a row from the originally infected potato. Cultivation 

 between the rows prevented its lateral spread. 



Control 



Since Rhizodonia prefers heavy, moist soils, drainage and the avoidance of 

 heavy soils difficult to drain is important. If the soil is heavily infected it is 

 inadvisable to plant potatoes. With ordinary soils seed tuber disinfection 

 should be practised. In this connection Howitt at Guelph is doing some inter- 

 esting work on control by corrosive sublimate. His work for 1920 shows that 

 treating tubers with sclerotia on them for 2 hours in corrosive subUmate, 1 in 

 500 gave perfect control, while treatment for 2 hours in a solution 1 in 1,000 

 reduced disfigured tubers to 14 per cent. The experiments are still in progress. 



GROUP 7 



Diseases caused by Fungi Imperfecti (Adelomycetes or Deuteromycetes) 



(a) Early Blight 



This disease is common in years when Late Blight is not serious and while 

 it does not attack the tubers the yield is reduced because of leaf injury. The 

 amount of injury depends upon the season and it is greatest when hot spells 

 alternate with moist weather. In such cases losses in yield may reach as high 

 as 25 per cent, unless spraying is consistently practised. The disease is caused 

 by a fungus known as Alternaria solani (E. & M.) J. and G. 



