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Recent investigations, especially m 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 m 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 

 m 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 bv 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 freshlv prepared^ 

 and the home-made article is much better than any adver- 

 tised preparations. It is a fairly simple matter to make 



