110 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



CONTRIBUTIONS FROM THE WISLEY LABORATORY. 



XXVIII. — The Control of Peach Leaf Curl. 



By A. S. Horne, D.Sc, F.G.S., F.L.S. 



During the last few years the outdoor peaches and nectarines at Wisley 

 have suffered considerably from leaf curl due to the fungus Exoascus 

 deformans (Berkeley) Fuckel, and to such an extent that only a small 

 percentage of leaves remained entirely unaffected. Exoascus curl is 

 an ancient scourge and much has been written about it, nevertheless it is 

 still prevalent in Europe and Asia, but South Australia, New Zealand, 

 and the United States claim to have brought it under control. 



The symptoms are well known and various figures have appeared 

 in this Journal and elsewhere.* The disease appears in spring, soon 

 after the foliage buds unfold, affecting the very young leaves, which 

 exhibit a tendency to curling and crumpling and are tinged in the 

 affected areas with pink. As the leaves become mature the curling 

 and crumpling are more pronounced, and the tissue becomes fleshy and 

 the coloration of darker hue. The affected leaves fall prematurely. 

 As a result the tree is rendered unsightly, and, more important still, 

 the process of nutrition is seriously hampered owing to the injury to and 

 subsequent loss of so many leaves ; this results in poor productivity, 

 fruit remaining small and failing to ripen even when the conditions 

 are good for ripening. G. Arnaud records that the parasite also 

 attacks the young branches, flowers, and fruit. The diseased branches 

 are deformed and often curved. When the flowers or fruit are attacked 

 they die and fall ; rarely the fruit grows for a time and develops 

 deformities. 



In winter, symptoms of fungous activity appear in the young growth, 

 brown patches being present here and there on the young laterals ; 

 these patches increase in size and local death ensues, upon which the 

 whole length of the lateral beyond this point withers and perishes. 

 In this way the fungus is the cause of many dead ends, which have to be 

 removed in spring. 



According to Massee, the mycelium of Exoascus hibernates in the 

 bark, medullary rays, and pith of young shoots, and extends along 

 the growing parts, from which it passes into the leaf buds and infects 

 the leaves each succeeding season, its activity synchronizing with the 

 renewal of plant vigour in spring. 



Arnaud, whilst admitting the possibility of this mode of behaviour, 

 is strongly of opinion that this manner of infection is not of importance, 



* See e.g., Journal R.H.S. 28, p. 16 ; Gardeners' Chronicle, July 9, 1887, 

 fig. 13, and May 13, 1905, fdg. 121, Massee's Text Book of Plant Diseases, 

 and elsewhere. Excellent figures by G. Arnaud are to be found in La Revue 

 de Phytopathologie, i. No. 2, May 5, 191 3. 



