JOURNAL 



OF THE 



ROYAL HORTICULTURAL SOCIETY. 



Vol. XXXIX. 1913. 

 Part II. 



INVESTIGATIONS ON THE CONTROL OF DISEASE 

 IN PLANTS. 



By Prof. R. H. Biffen, M.A. 



[Read July 15 and 29 ; Lieut. -Col. Sir David Prain, F.R.S., V.M.H., in the 



Chair.] 



[Being the Ninth and Tenth Masters Memorial Lectures.] 



In choosing the control of disease in plants as a subject for the Masters 

 Memorial Lectures for this year I have been largely influenced by the 

 fact that it is commonly assumed, and I think rightly, that the losses 

 caused by disease are becoming increasingly serious. Apart altogether 

 from the fact that our rapidly extending knowledge of fungoid pests 

 has led to the recognition of many new diseases, the importance of 

 the subject has increased owing to those economic changes which 

 have led to the cultivation of larger areas of one and the same kind 

 of plant. In growing fruit, for instance, it is now a common practice 

 to plant considerable breadths of one variety only, instead of, as one 

 sees in the older orchards of the west of England, a mixture of different 

 varieties. Should this single variety prove susceptible to any parti- 

 cular disease, that disease has every opportunity of spreading through- 

 out the area — as silver-leaf has spread wholesale in orchards of 

 1 Victoria ' plums, for instance — whereas in mixed plantations only 

 occasional trees would be attacked. Again, crops are grown on a 

 larger scale than was formerly the case. Potatos, for instance, form 

 an almost continuous network in some counties, with the result that 

 an epidemic starting in one locality, given favourable conditions of 

 weather, can quickly spread over the whole area. Further, the 



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