8; 



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 m the fruit. 



Many diseases of leaves belong to the class spoken of 

 as mildews arid are caused by fungi belonging to the 

 family ErysifkacecE. The Rose mildew, caused by 

 Sfhaerotheca fannosa^ is one of the most familiar of these 

 diseases and is typical of the class. The diseased leaves 

 become covered wdth a white, pow^dery growth of the 

 fungus, which causes them to curl up and che. 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 w^hich 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 w4th 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 



