ON A FUNGI'S DISEASE OF THE CHERRY LAUREL. 145 



Although no perithecial stage of the fungus occurred on the affected 

 shrubs in Kew Gardens, I believe we are now, on account of the following 

 facts, in a position to determine to what species of the Erysiphacea the 

 present mildew belongs. After the mildew had been growing for some 

 weeks on the young shoots of the cherry laurel, I noticed on a few stems, 

 near the youngest leaves, the formation of small, white, barren mycelial 

 patches composed of interwoven thick-walled hyphae. Such persistent 

 pannose patches of mycelium are characteristic of the rose-mildew 

 Spharotheca pannqsa (Waflr.), Lev. (3). Comparison of the conidia and 

 conidiophores of the rose-mildew on roses with those of the mildew on 

 the cherry laurel further established the identity of the two. 



An interesting question suggests itself in connection with the present 

 form of S. pannosa on the cherry laurel. Is it that this shrub is sus- 

 ceptible at a certain stage of growth or under certain cultural or climatic 

 conditions to the attacks of the rose-mildew, or is it rather that we have 

 in this case a species of Sphcerotheca the individuals of which now and 

 again are capable of infecting new hosts ? S. pannosa, as is well known, 

 is extremely common on wild and cultivated species of roses, and on these 

 plants the fungus forms perithecia freely, at least in some countries. 

 Besides these hosts, it not uncommonly attacks the peach (Prunus persica). 

 On this plant, however, it very rarely advances beyond the conidial 

 (Oidium) stage.* Very rarely, too, according to Starback (6), it is found 

 in the conidial stage on the apricot (P. armeniaca), and I have in one 

 case seen the conidial stage on the almond (P. Amygdalus).* 



It appears, then, that at the present time the rose-mildew (S. pan- 

 nosa), whilst flourishing on species of the genus Bosa and proving itself 

 fully adapted to live on these hosts, is capable of attacking occasionally 

 species of the genus Prunus — commonly the peach, and very rarely the 

 apricot, the almond, and the cherry laurel. 



Should the present disease prove troublesome in the future in shrub- 

 beries of cherry laurel, recourse should be had to sulphur as a fungicide. 

 It should be applied either in the form of the dry powder known as 

 ''flowers of sulphur," or as potassium sulphide ("liver of sulphur"). 

 The " flowers of sulphur " should be well dusted over the affected leaves, 

 and the application should be made on a still day during sunshine. The 

 potassium sulphide is to be used as a spray, at the rate of 1 oz. to 

 2 gallons of water, and the application should be made about every ten 

 days until the fungus has disappeared. 



Bibliography. 



L Bektoleki. A. : Xuovo Oidium del Lauroceraso (Nuovo Giorn. Bot. Ital. xi. 

 389-394, 1879). 



2. Roumegueee. C. : Appar. en France (Tune niucedinee nouvelle : Y Oidium 

 Passcrini, Bertol. tils, etat conidien d'un Enjsiplic nouveau, YE. Bertolini (Rev. 

 Mycolog. ii. 174-176, 1880). 



* 1 have seen the pannose mycelium characteristic of the perithecial stage in 

 one case only out of the many hundreds I have examined. 



t This is the example sent out in Roumeguere, Fung, select, exsicc. Xo. 4484. 

 under the name of Manilla Candida (Wallr.), Oidium fructigenum Pers. var. folii- 

 colum Westend. 



L 



