87 
fruit is not so highly valued as that of some of the more 
susceptible varieties. At the same time it ought to be 
quite possible to produce, by crossing, a variety which 
combines the qualities of disease-resistance and those most 
acceptable in the fruit. 
Many diseases of leaves belong to the class spoken of 
as mildews and are caused by fungi belonging to the 
family Evysiphacee. The Rose mildew, caused by 
Sphaerotheca pannosa, is one of the most familiar of these 
diseases and is typical of the class. The diseased leaves 
become covered with a white, powdery growth of the 
fungus, which causes them to curl up and die. The 
fungus grows mainly on the surface of the leaves, 
swollen branches from the filaments acting as sucker-like 
organs of attachment, while other branches penetrate the 
outer walls of the epidermal cells and swell out within the 
surface cells in the form of bladder-like haustoria. These 
absorb food material from the cells occupied as well ‘as 
from the adjoining cells. |The substances thus absorbed 
from the living leaf are passed on to the vegetative fila- 
ments of the fungus outside which is thus enabled to grow 
and multiply. Erect threads arise from the creeping fila- 
ments on the. surface and bear single chains of colourless, 
thin-walled, oval spores or conidia. Myriads of these are 
produced on the surface of mildewed leaves and give to 
the latter the characteristic powdery appearance. Being 
extremely’ light they are easily spread by the wind to 
healthy leaves and produce new infections throughout the 
summer. Later in the season, the production of conidia 
gradually gives place to another means of spreading. The 
fungus on the shrivelling leaves and also on the twigs 
assumes a brown colour and gives rise to minute dark 
bodies about the size of a pin head. This is the winter or 
resting stage of the fungus. Each of the minute dark 
bodies or perithecia is furnished with a thick wall made 
up of a number of closely interwoven fungal threads: and 
within this resistant coat a club-shaped spore-case (ascus) 
containing eight oval spores, gradually develops. When 
mature the perithecium splits across and the ascus is 
squeezed out. The latter then opens at the apex and the 
oval spores are forcibly ejected. These are able to infect 
leaves with the mildew and serve to start the disease afresh 
each spring. As has been stated above, many destructive 
mildews are caused by fungi having a life history closely 
