78 
Recent investigations, especially in Ireland and 
America, have done much to make it clear how most 
epidemics of “Late-Blight” originate. We have seen 
that the potato tubers very frequently become infected by 
the fungus. If badly diseased tubers are used for seed 
it has been found that they either wholly decay in the 
ground or occasionally send up a few perfectly healthy 
shoots. Recently, however, very careful field experiments 
have shown that if only slightly diseased tubers are 
planted a much larger proportion of them send up shoots, 
and under certain weather conditions some of these become 
diseased. It has also been proved that the disease arises 
in such young shoots by the growth into them of the 
fungus from the slightly diseased tuber. Dr. Melhus has 
confirmed De Bary’s earlier observations that such 
diseased shoots occasionally reach the surface where 
conidia of the fungus are produced. These conidia are 
then carried to the leaves of adjoining plants, which very 
soon show typical disease spots. Upon these, more conidia 
arise and so starting with a single diseased shoot as a 
centre the disease spreads over the whole field in a few 
warm moist days. 
In taking measures to prevent the Late-Blight disease, 
it is necessary,‘as in other plant diseases caused by fungi, 
to consider the means by which the fungus spreads during 
the season, and also the manner in which it is carried 
over to the following year. It has been shown that this 
disease spreads very rapidly throughout the summer, by 
means of air-borne conidia that infect the leaves on which 
they alight. If the germination of conidia can be pre- 
vented then the spread of the disease will be controlled. 
This may be accomplished by spraying with a fungicide, 
that is, a poisonous substance which is harmful to germi- 
nating spores. It has been found by experience that one 
of the most powerful mixtures for preventing infection by 
filaments from germinating spores of fungi is Bordeaux 
mixture. This consists of a solution of Blue stone, that 
is copper sulphate, to which slaked lime or lime water is 
added. The lime is added to prevent the copper sulphate 
injuring the leaves of the plants treated, and also to make 
the mixture form a fine film on the surface of the leaves. 
Bordeaux mixture should always be freshly prepared, 
and the home-made article is much better than any adver- 
tised preparations. It is a fairly simple matter to make 
