POWDERY SCAB OF POTATOES. 
13 
Table II. — Yield of diseased 'potatoes ivhen soil ivas 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 
Pounds. 
99 
74 
102.5 
1.00 
97 
94 
106 
Per cent. 
30.3 
6 
Each tuber planted in a handful of wet sawdust 
34 
Extra sulphate of potash added, 1 hundredweight to statute acre 
38 
8 
51 
9 
do 
52.5 
10 
Extra muriate of potash added, 1 hundredweight to statute acre 
52.1 
11 
Flowers of sulphur applied, 6| hundredweight to statute acre. 
23.6 
Pethyb ridge 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 can not 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. 
