14 Introduction to Fungi [CH. 
to year. A fungus may become so abundant in certain 
seasons as to cause considerable loss; such epidemics 
are usually due either to very favourable conditions 
for the fungus or to the introduction of a foreign 
fungus. A cucumber grower estimated his loss from 
a fungus disease as £1000 in one season. The potato 
famine in Ireland in 1845 was the result of an enormous 
reduction of the potato crop by the potato disease 
fungus, and occurred in a season when conditions were 
favourable to its growth. 
The American gooseberry mildew as its name 
implies came to us from America. It first appeared in 
1900 and for a few years did comparatively little 
damage, so that no means were taken to destroy it 
and to prevent re-importation. It: has now spread 
over all the gooseberry districts in England and causes 
so much damage as to threaten the existence of those 
varieties which are most liable to its attacks. If means 
(however drastic) had been taken at first to destroy 
all infected bushes and also to prevent re-importation, 
this disease might never have established itself here. 
Many countries adopt special precautions against 
the introduction of diseases. In South Africa potatoes 
are imported from this country only when accompanied 
by a certificate from the Board of Agriculture that 
they are free from certain diseases, that these diseases 
are not present in the area in which they were grown, 
and that they were packed in clean new packages. 
On arrival in South Africa they are examined by experts 
and are not admitted if found to be diseased. In this 
way the black scab and other diseases of potatoes are 
kept out of South Africa, thereby preventing consider- 
able loss to the potato crop. 
