8 PLANT DISEASES 



forms of fruit are produced throughout the season, each 

 performing a function not capable of being done by any 

 one of the other forms. These different kinds of fruit have 

 received special names, the value of which it is important 

 to clearly understand. 



Taking as a first illustration the Vine Mildew (Plasmo- 

 para viticola^ De Bary), we find that early in the summer 

 the leaves of diseased plants are more or less covered with 

 a delicate white mildew on the under surface, which is the 

 summer fruit, or conidial form of reproduction of the fungus 

 (Fig. i, i). This fruit originates from mycelium present in 

 the tissues of the leaf, the fruiting branches emerging 

 through the stomata or breathing pores of the leaf, for the 

 purpose of dispersing their conidia in the air. This mildew, 

 when examined under the microscope, is found to consist 

 of numerous much-branched hyphae, the tip of each branch 

 bearing two or three minute reproductive bodies or conidia 

 (Fig. i, 2). Without at present entering into details, these 

 conidia are capable of germination the moment they are 

 mature ; and being produced in immense numbers and in 

 rapid succession throughout the summer months, and being 

 readily dispersed by wind, rain, insects, etc., those that 

 happen to be deposited on the surface of a damp, healthy 

 vine leaf, germinate at once, enter the tissues of the leaf, 

 form a mycelium, and in a very short period of time produce 

 the white mould on the surface of the leaf, which in turn 

 becomes a centre of disease, producing conidia ready to be 

 scattered and further extend the mischief. This explains 

 the rapid extension of a disease which has once gained a 

 foothold among plants of the same kind that are grown in 

 considerable numbers, and in contact with each other, as in 

 vineyards and field crops generally. 



