36 DEPARTMENT OF AGRICULTURE. 



the poison to make it dangerous to use them for food. The solution becomes weaker 

 with use, and should not be used more than three or four times. The same precau- 

 tions must be observed to prevent subsequent infection as in the last case; i.e., the 

 floor, etc., should be disinfected. Planters, containers, and other implements which 

 are to come in contact with the potatoes should be similarly disinfected. After treat- 

 ment the tubers are cut and handled in the usual way. > 



Many treatments have been tried for soil that is scab-infested, but beyond what 

 has already been mentioned, little of practical value has so far been found. 



POWDERY SCAB (Sponyospora subterranea) . 



This disease was first noticed in Canada about three years ago. and was found, 

 subsequently, in various areas of the Maritime Provinces and Quebec. As a con- 

 sequence a complete embargo against Canadian potatoes was put in force by the 

 United States at the beginning of 1914. In the early summer of 1915. owing to a 

 shortage of potatoes in the Pacific States, the embargo was temporarily raised to 

 allow of the shipment of potatoes from British Columbia. The discovery, however, 

 of powdery scab in a consignment sent to Seattle caused the embargo to be replaced. 

 Since January 1st, 1916, the embargo has been again raised. During the past season 

 a careful survey has been made with a view to ascertaining the extent to which the 

 disease may be present in the Province. So far, it has been discovered only in the 



Island District*and the immediately adjacent Delta country. None of the disease 

 has been found in the Interior, although a careful inspection of the Ashcroft area 

 was made. 



Superficially, this disease much resembles common scab, but there are certain 

 differences which enable a person familiar with them to distinguish most cases at 

 sight. A certain number of cases, however, are met with in which microscopic 

 examination is necessary, and suspected cases should be sent to the Plant Pathologist 

 for examination. 



Compared with common scab, the spots or pustules of powdery scab are. in 

 general, smaller and with a more even outline. Even where they are numerous they 

 retain their individuality better, and have not the same tendency to run together into 

 large affected areas in which the individual spots cannot be recognized. The surface 

 is smooth when the spot is young, consisting, in fact, of the ordinary skin of the 

 tuber instead of the rough, irregularly thickened, corky tissue of common scab. On 

 breaking this skin the pustule is found to be filled with a brownish or greenish 

 powder, mixed with a certain amount of dried-up tissue. If this is rubbed away. 

 another distinct skin is generally found beneath, marking off the spot from the 

 sound flesh of the tuber. In the case of common scab there is nothing comparable 

 to this, the rough corky tissue lying directly on the sound flesh. The powdery mass 

 which has given the name to the disease is composed of spore-balls, each one of 

 which is a mass of minute spores adhering firmly together. Many of these are set 

 free in the soil before or at digging-time, while they may be very readily distributed 

 on implements, bags, or the skin of apparently healthy and desirable tubers. In 

 this way the disease is spread. Once introduced into the soil it is probable that 

 potatoes planted there within a period of several years will be liable to the disease. 

 As in the case of common scab, the chief injury lies in the unsightly appearance of 

 affected tubers. There is. however, a much greater tendency for affected tubers to 

 shrivel in storage. In bad cases, especially when the potatoes have been grown year 

 after year on the same wet. heavy soil, the injury may be very great, although it 

 must be admitted that under such circumstances good yields can hardly be expected 

 in any case. At the present time the disease in British Columbia is, on the whole, 

 of a very mild type, a careful inspection of a 5-acre field at digging-time often 

 resulting only in the finding of two or three slightly affected tubers. With proper 

 precautions, therefore, on the part of the growers, there is every reason to hope that 

 the spread of the disease may be checked. 



