150 DISEASES OF CULTIVATED PLANTS 



Rose mildew. This scourge of the rosarian is the work of 

 Sphaerotheca pannosa (Lev.), it is also abundant on wild roses. 

 It attacks the leaves, young shoots, and flower-buds. On the 

 leaves it forms a very delicate white mildew, which in course 

 of time becomes mealy from the presence of conidia. On the 

 shoots, calyx, and fruit the mycelium on the other hand forms 

 thick, felty patches of a dingy white colour, that persist till 

 late in the season. Perithecia do not appear to be formed on 

 the sparse mycelium present on the leaves, but occur on the 

 felted mycelium present on the shoots and fruit, where they 

 are quite immersed in the felt, and are nut very conspicuous. 

 It is to be noted that the conidial form of reproduction is very 

 scantily produced on the felted type of mycelium, suggesting 

 a kind of differentiation in function between the thin, 

 evanescent mycelium present on the foliage, and the felted 

 kind met with on the shoots, calyx, and fruit. The former 

 is most concerned in producing conidia, which serve for the 

 extension of the species in space, whereas the felted mycelium 

 bears the ascigerous or winter form of fruit, whose function is 

 to continue the species in time, or to secure its continuance 

 from year to year. 



As a rule during ordinary seasons there are two distinct 

 waves of disease during the year. The first, which is 

 generally slight, occurs soon after the leaves are fully 

 expanded. The second wave occurs after midsummer, when 

 the young wood has made considerable growth, and the 

 flowers have begun to appear. This is the critical period, 

 for, as already stated, the ascigerous fruit, which alone can 

 perpetuate the disease the following season, is produced 

 on the young wood and fruit. The spring wave of disease, 

 although it may injure the individual plant, cannot perpetuate 

 the disease, as the mildew is at that stage confined to the 

 foliage, and does not produce winter fruit. 



Diseased leaves curl and fall after producing chains of 

 conidia only. Perithecia are produced on the felted mycelium 

 present on the young wood and fruit, appendages short, 

 colourless, ascus solitary, subglobose containing eight ellip- 

 tical spores, 20-27 x I2_I 5 l L - 



Dusting with flowers-of-sulphur, mixed one-third its volume 

 of quicklime, checks the disease, as also does spraying with 

 sulphide of potassium. I find, however, that spraying with 

 sulphuric acid one part in 1500 parts of water is a very 

 certain cure, both under glass and in the open. It is very 



