142 JOUKNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



ON A FUXGUS DISEASE OF THE CHERRY LAUREL 



(Prunus Laurocerasus, Linn.).* 



By Ernest S. Salmon, F.L.S., Hon. F.R.H.S., South-Eastern 

 Agricultural College, Wye, Kent. 



In July 1905 my attention was directed to a fungus disease affecting 

 a shrubbery in Kew Gardens formed of the cherry laurel (Prunus Lauro- 

 cerasus, Linn.). The fungus causing the disease was a white mildew, the 

 patches of which covered the leaves of the young shoots. Occasionally, 

 patches of mildew were to be seen on the young stems, near the ends of 

 the shoots ; the older leaves of all the plants remained entirely free 

 from the disease. On microscopical examination, the fungus was found 

 to be an Oidium; that is, the conidial stage of some species of the family 

 of powdery mildews known as the Erysiphacecz. The appearance of the 

 fungus, under a magnification of 400 diameters, is shown in fig. 36. 

 Tee thin cobweb-like mycelium of the fungus was composed of interwoven, 

 white, branched, septate hypha?, spread over the epidermis of the leaf. 

 Each hypha sent, at short intervals, large, roundish haustoria (h) into the 

 epidermal cells. . It was by means of these haustoria that the mildew 

 was maintaining its parasitic life, and absorbing the whole of its food 

 from the cells of its host-plant. From the hypha?, tall conidiophores, 

 often closely packed together, arose, each bearing a long moniliform 

 chain of conidia (a). Each mature conidium is oblong in shape, with 

 rounded ends, and measures 23-28 x 12-15 /u.t Minute bodies of a 

 definite shape (flattened discs, or cylindrical, or conical and hollow) occur 

 inside each conidium. The bodies are composed of a substance called 

 fibrosin, and are characteristic of the conidia of the species of the genera 

 Sphczrotheca and Podosphara. 



In order to see whether the disease was due wholly to the fungus — in 

 which case the fungus would be able to attack the cherry laurel without 

 any previous injury to the shrub — I took some conidia from the affected 

 plants in Kew Gardens, and inoculated with them some healthy young 

 shoots in a private garden at Reigate, Surrey. On examining them at the 

 end of fourteen days, I found that fuU infection had resulted, all the 

 inoculated leaves bearing small powdery patches of mildew. All the 

 young shoots of the affected cherry laurel in Kew Gardens were cut off 

 in the usual trimming of the shrubbery in July, and no trace of mildew 

 reappeared during the season. In the case of the shrubbery at Reigate, 

 also, the mildew disappeared as soon as the young shoots grew out. 



In the present instance it appeared as though the mildew is capable 

 of attacking the cherry laurel only just at the season when the young 



* From the Jodrell Laboratory, Eoyal Botanic Gardens, Kew. 



t Small oranqe-coloured larvae, which have been proved to feed exclusively on 

 conidia of the Erysiphacea: (see Journal of Bot., 1904, p. 184), occurred in consider- 

 able numbers on the mildewed leaves. 



