144 JOUENAL OF THE EOYAL HOKTICULTUKAL SOCIETY. 



spots. The same was the case with the attacked stems when they grew 

 out. The phenomenon was even more marked in the case of some 

 plants of P. Laurocerasus var. camellia folia in another part of Kew 

 Gardens which became attacked about the same time by the present 

 mildew. 



All the appearances suggested that the mildew with which we are 

 now dealing was not fully adapted to living on the plant it was attacking. 

 Instead of merely taxing the leaf-cells, and establishing that peculiar 

 symbiotic connection between parasite and host which is characteristic 

 of the present group of fungi as a whole (3, p. 146), the mildew of 

 P. Laurocerasus had caused the death of many of the cells attacked, 

 and stimulated the plant to the production of periderm over the injured 

 places on the leaf and stem. 



On looking through the literature on the subject, it was apparent that 

 very little was known about the present disease. It has been recorded 

 twice from the Continent, but in neither case has the exact determination 

 of the fungus been satisfactorily made. 



In 1879 Bertolini (1) first mentioned the fungus, which he found 

 growing on the fruits of P. Laurocerasus,* in a park near Bologna, Italy, 

 in June 1877, and again in 1878. It was noticed that only the fruits 

 which were growing on the shrubs planted on excessively damp soil, or 

 on branches in deep shade, were attacked by the mildew. In the drier 

 localities close by in the same park the bushes remained unattacked. 

 Bertolini gave a very brief description of the fungus, which he called 

 Oidium Passerinii. 



In 1880 Boumeguere (2) published an article entitled " Appearance 

 in France of a New Mould, Oidium Passerinii, Bert, f., the Conidial 

 Stage of a New Erysiphe, E. Bertolini." Here an account is given of a 

 mildew which appeared suddenly in July 1880, on the famous cherry 

 laurel shrubberies in the public garden at Tarbes (Hautes-Pyrenees). 

 The mildew spread with great rapidity through the shrubbery, due, it 

 was believed, to certain favourable conditions, viz. the excess of 

 humidity caused by the heavy rains that fell in July and August. " All 

 the leaves, absolutely all, both in the sunlight and in the shade, became 

 covered with large patches of mildew, which also spread over the fruits." 

 The mildew, at the time of its appearance, was identified as Bertolini's 

 Oidium Passerinii. On visiting the same shrubberies a month later, 

 Boumeguere found that the Oidium had disappeared. On certain leaves, 

 however, a few minute blackish perithecia were discovered ; each con- 

 taining four asci with from five to seven brown spores. This Boumeguere 

 believed to be the perfect, or winter, stage of the fungus. It is almost 

 certain, however, that Roumeguere was in error in connecting this peri- 

 thecial form with the Oidium (4). Dried examples of the conidial stage 

 from the shrubbery at Tarbes were sent out in Rourneguere's Exsiccati 

 Fung. gall. No. 963, under the name of Oidium Lauro-Cerasi. I have 

 examined the example under this number contained in the Kew Herbarium, 

 and found the fungus to be identical with the mildew which has lately 

 appeared on shrubs of P. Laurocerasus in Kew Gardens. 



* An error occurs in Sacc. Syll. Fung. iv. p. 43, where the fungus is recorded " in 

 foliis J/auri lusitanica." 



