8 BULLETIN 82, U. S. DEPARTMENT OF AGRICULTURE. 
of the potato for seed purposes. Its harmful effect on the seed has 
been emphasized by Johnson (1908, p. 453), as follows: 
Such tubers are not only much, reduced in market value for eating purposes, but 
must provide also poor seed for the next year's crop. Yet I was constantly told that 
this was the kind of seed regularly planted from year to year; and that the people 
used this seed because they had, and could get, no other. *■_**'! have no doubt 
myself that this Spongospora scab has a good deal to do with the miserable average 
yield per acre of potatoes in the west of Ireland. * * * It is in some districts of 
Ireland as injurious to potatoes as finger-and-toe is to turnips. 
The following sentence from Pethybridge's report in 1911 (p. 442) 
shows more strikingly the relation of the disease to the seed potato: 
It was found during the past season that the crop resulting from the planting of the 
"canker" form of disease in clean land gave 67.1 per cent of affected tubers, while 
the spot form produced only 54.1 per cent. 
That other countries are not considering Spongospora scab lightly 
is apparent from a farmers' bulletin, No. 110, of the Transvaal 
Department of Agriculture, issued by Evans in 1910, warning the 
grower against the use of infected seed potatoes. Evans says — 
Corky scab has caused a considerable amount of damage to the potato crop in Great 
Britain, Ireland, and Norway. It also occurs in Germany, and is particularly preva- 
lent in the west of Ireland. * * * Diseased tubers should on no account be used 
for seed purposes, for not only will the resulting crop be scabbed, but the ground will 
also be infected with the germs of the parasites. 
It must also be remembered that not only does powdery scab 
injure the crop, but the soil becomes contaminated and clean seed 
planted on this land for several years afterwards becomes infested. 
Just how long the organism can remain alive in the soil is not known, 
but that it is resistant and may live for several years is suggested 
by the structure of the spores and experiments by Pethybridgc (1911) 
showing that the spore balls can pass through an animal without 
losing then capacity for renewing the disease. Contamination of 
clean seed may take place, it is claimed, by simply being in contact 
with diseased potatoes. If such is the case, and there is good reason 
to believe it possible, clean seed may become infected through the 
use of old bags and machinery. Indeed, it is even possible for one 
field to become infested from another by the spore balls being carried 
by the wind, water, and other agencies. 
IS POWDERY SCAB A DANGEROUS MALADY? 
In considering whether powdery scab is or is not a dangerous 
disease it is well to keep in mind that any inciting agent, regardless 
of its origin or nature, that mars or defaces the tuber depreciates its 
value and ultimately its productiveness. The degree of danger pre- 
sented by this intruder is problematical, but all American plant 
pathologists who have expressed an opinion upon this point are 
agreed that powdery scab is a disease possessing characteristics that 
