69 



able to boil or steam the produce from a diseased crop of "roots'" before 

 feeding it to stock. Rotation is very important, the disease {rradiially in- 

 creasing in severity when cruciferous crops succeed each other on the same 

 land at too short an interval until they cannot be grow^n at all. From 

 4 to 6 years should ela|:)se lietween such crops and since many weeds 

 can become infected and harbor the disease from year to year w^e have 

 here an additional reason for clean farming. Applications of lime to in- 

 fected soil at the rate of two to three tons per acre are of great value in 

 reducing the severity of the disease. The lime should be applied tw^elve to 

 eighteen months previous to the sow^ing or planting of a cruciferous crop. 



In some recent experiments conducted by the Division of Botany, Ot- 

 ta^va, on very badly infected land in P.K-I-, it was foimd that while un- 

 treated land sown with turnips yieldfed only 4800 lbs. of soimd roots per 

 a<rre, where lime was applied at the rate of 100 bushels per acre the yield 

 of sound roots was increased to 15,550 lbs. per acre. It was also found 

 that this quantity of lime gave a better yield than did the amount of 150 

 bushels and 75 bushels per acre respectively. 



Spongospora subterranea ( Wallr. ) Johns. This produces the 

 disease of potato tubers known as Pow^dery or Corky Scab. The dis- 

 ease is well-known in Europe but was first definitely identified in Can- 

 ada in the Autumn of 1912. It is now know^n to occur in the Provinces of 

 P.E.I. , N.S., N.B., Que., Ont. and Alta, being most abundant appar- 

 ently in Quebec. Whether the disease has really been introduced only in the 

 last few years or \vhether it has been established — in Quebec at any rate— 

 for many years is a point difficult to determine with any degree of cer- 

 tainty, owing to the close resemblance which it bears to the almost uni- 

 versal Common Scab ( due to Oospora scabies Thax ) . To the prac- 

 tised eye, however, there are certain differences usually quite sufficient to 

 distinguish the t\vo even \vithout the assistance of a lens, though on the 

 other hand cases may occur, as for instances with potatoes that have 

 been handled, w^here even the use of the microscope is not sufficient to en- 

 able one to make a satisfactory decision. A typical mature Powdery Scab 

 pustule differs from one of common Scab in usually being miore raised 

 and having a more even margin. The surface is formed by a smooth 

 membrane instead of being rough and irregular. When this membrane is 

 broken the pustule is seen to. be filled with a characteristic brown or 

 greenish po"wdler, which is composed of spore-balls. If this be rubbed 

 away it will be foimd that the pustule is marked off from the underlying 

 healthy tissue by a well-defined smooth corky layer. In diseased tubers 

 that have been much handled this is often the only remaining indication 

 of the disease. In the earlier stages of a mild attack the scab spots may 

 not be much larger than a pin head but as they increase in size they m.ay 

 coalesce until a considerable portion of the tubers is covered with large 

 diseased areas. In severe attacks the tubers are stunted and very much 

 deformed so that they are practically worthless. All the details of the 

 life history of the parasite are not known but it appears probable that 

 a zoospore obtains entrance into a cell through the thin skin of the 



