A. S. HoRNE 199 



By far the most important case of curl recently recorded in Britain 

 has occurred in connection with the President potato, a variety imported 

 from Germany and Holland on several occasions during the last few 

 years. On land in the neighbourhood of Dunbar, the President yielded 

 an enormous crop — fifteen tons to the acre — and produced only a small 

 percentage of bad plants. But in certain other localities, this variety 

 produced a high percentage of dwarfed plants with foliage of a light 

 green colour or tinged with yellow or pink and leaves rolled upward 

 and frequently much blotched as already described by the writer^. 

 Quanjer^, who has investigated curl in certain varieties in Holland, states 

 that the phloem strands in the stem of leaf-roll plants are shrunken and 

 lignified so that the translocation of food material to the tubers is 

 interfered with. The bad plants produced by the President in Britain 

 produced an exceedingly small yield of very small tubers. 



Curl has often been attributed to fungi and notably Fusariura, 

 Verticillium, and Macrosporium. Thus Appel originally believed that 

 it was due to Fusarium and this view obtained for quite a considerable 

 time and is held still by some authorities, notably Koch and Kormanth. 



But abundant cases of leaf-roll have been recorded, as already 

 pointed out by Sorauer, including the present case of the President, 

 where no fungi are present. Macrosporium was present in the blotched 

 foliage of the President, in the potato field in 1910, but since blotched 

 foliage of a similar nature was produced in localities where Macrosporium 

 was entirely absent from the plants, the disease could not be attributed 

 to this or any other fungus. 



It was next necessary to establish to what extent, if any, the condi- 

 tion of the foliage in bad plants might be brought about by injurious 

 insects. Accordingly a joint enquiry into the subject was instituted 

 by Professor H. Maxwell Lefroy and the writer, and yielded results 

 which show that although the insect factor is of more importance in 

 relation to diseased foliage than has been hitherto supposed, neverthe- 

 less this particular malady is not caused by insects. 



But considerable light was thrown on the probable nature of the 

 disease during the study of the plants raised from seed of the President 

 for the purpose of insect infestations by Professor Lefroy and the writer 

 at the Royal Horticultural Society's gardens, Wisley (1912-1913-1914), 

 the Chelsea Physic Garden (1911-1912), and at Messrs Sutton's trial 

 grounds, Reading (1912), and evidence was obtained which points 



1 A. S. Home in Jo^ir. Roy. Hort. Soc. xxxvi, p. 618 (1911). 



2 H. M. Quanjer in Med. van de Rijks Hoogere Land-, Tuinen Bosch., Deel vi (1913). 



