POWDERY SCAB OF POTATOES. 



13 



Table II. — Yield of diseased potatoes when soil was untreated and following various 



treatments for powdery scab . 



No. of 

 plat. 



Treatment of land, if any. 



Total 

 yield 

 per 

 square 

 perch. 



Yield 

 of dis- 

 eased 

 tubers. 



Extra superphosphate added, 4 hundredweight to statute acre. . . 



Each tuber planted in a handful of wet sawdust 



Extra sulphate of potash added, 1 hundredweight to statute acre 



No treatment 



do 



Extra muriate of potash added, 1 hundredweight to statute acre. 

 Flowers of sulphur applied, 65- hundredweight to statute acre 



Pounds. 



99 



74 



102.5 

 100 



97 



94 



ioe 



Per cent. 

 30.3 

 34 

 38 

 51 



52.5 

 52.1 

 23.6 



Pethybridge writes as follows regarding these experiments: 



With the exception of the muriate of potash, it will be seen that considerable diminu- 

 tion in the weight of diseased tubers produced has been effected by the methods of 

 treatment used, although the use of sawdust has reduced the total yield. The yields 

 given in the above table are pounds per square perch. 



The best results were obtained with sulphur, where not only was the amount of 

 disease reduced to less than one-half of that in the untreated plats, but the total yield 

 was higher than in any other case. This result confirms previous experiments carried 

 out at Clifden, which have always shown that sulphur added to the soil increases the 

 yield of potatoes and diminishes the attack of scab. * * * 



Substantial as are the reductions in the amount of scab due to the methods of soil 

 treatment above indicated, they cannot be looked upon from the practical standpoint 

 as sufficient, and a suitable, cheap soil disinfectant is still a great desideratum for this, 

 as well as for other purposes. 



liming the soil, as is practiced for clubroot of cabbage, a parasite 

 related to Spongospora, has proved an aid to the fungus rather than a 

 check to its development. This makes it clear that it does not 

 behave like clubroot of cabbage, as suggested by Massee. The effect 

 of lime on the development of Spongospora has been pointed out by 

 Home (1911) and Pethybridge (1911). 



It is, of course, obvious, as Pethybridge suggests, that there is as 

 yet no method of controlling this disease when it once gets into the 

 soil. In view of this fact, it is plain that potatoes should not be 

 grown for some years on a piece of land that has produced a crop 

 infected with Spongospora scab. Just how many years the fungus is 

 able to remain alive in the soil is not known and is a question that 

 merits investigation. The nature of the spore balls suggests that the 

 disease may well be able to live in the soil for several years. It 

 should be said also in this connection that if more was known as to the 

 germination of the spore balls, it might be possible to predict their 

 longevity. 



