146 DISEASES OF CULTIVATED PLANTS 



Podosphaera tridactyla (De Bary) forms a white mildew on 

 cultivated plum leaves, it also infests other wild and culti- 

 vated species of Prunus^ etc. 



Perithecia subglobose, having a tuft of appendages growing 

 nearly erect from its apical portion, tips of appendages 3-5 

 times forked, tips of ultimate branchlets knobbed. The 

 solitary subglobose ascus contains 8 spores, averaging 

 20-30 x 12-15 p. 



Salmon considers this species to be a variety of P. oxy- 

 acantJiae. 



SPHAEROTHECA (LEV.) 



Perithecia having the vague, floccose appendages springing 

 from its base and interwoven with the mycelium ; ascus 

 solitary, 8-spored. 



American gooseberry mildew (Sphaerotheca mors-uvae, 

 Berk.) has long been known as a serious pest in the United 

 States, its destructiveness being so pronounced that in many 

 districts the cultivation of gooseberry bushes has been dis- 

 carded, and this is more especially true of the introduced 

 European varieties. It also occurs on wild varieties of 

 gooseberry in the United States. I recorded the first appear- 

 ance of this pest in Europe, in the Gardeners' Chronicle, 

 Aug. 25, 1900, fig. 39. The specimens were sent to Kew 

 for determination by Mr. F. W. Moore, F.L.S., Keeper of 

 the Royal Botanic Gardens, Glasnevin, Dublin, and came 

 from the county Antrim. It is assumed that the disease 

 was in some way imported to Europe from the United 

 States, but there is no direct evidence on this point. Since 

 the above date the disease has spread rapidly in Ireland, 

 and at the present day it is rampant in several districts in 

 England, and also in most of the countries of northern Europe. 



In this country the disease usually appears about the end 

 of May, on the expanding leaf-buds and leaves, and rapidly 

 passes on to the young fruit and shoots. The fungus first 

 appears as a very delicate cobweb-like film, which gradually 

 becomes more compact, of a pure white, and soon becomes 

 mealy owing to the presence of numerous conidia, which arise 

 from the creeping mycelium as upright chains. This is the 

 summer form of fruit, and is produced in abundance 

 throughout the season. It is this summer fruit that enables 

 the disease to spread with such rapidity. The conidia are 



