72 PLANT DISEASES 



duction of an average crop of fruit the following year is 

 very doubtful. Soon after the appearance of the pale 

 patches on the upper surface of the leaf, corresponding 

 areas on the under surface will be found covered with a 

 very delicate white mildew. This is the fruit of the 

 fungus, which has sprung from the mycelium present in 

 the tissues of the leaf, and has come to the surface through 

 the stomata or breathing pores, to form its conidia or re- 

 productive bodies in the air, where they have an oppor- 

 tunity of being carried by wind, rain, etc., on to the surface 

 of other leaves, where, if conditions are favourable, they 

 germinate within a few hours, entering the tissues of the 

 leaf, and thus starting a new centre of disease. As the 

 conidia are produced in immense numbers, even on a 

 single leaf, it can be seen how rapidly the disease may 

 spread after its first appearance. 



The conidiophores are repeatedly branched near the 

 tip, the conidia are oval, and produced in twos or threes 

 at the tips of the branches. On germination the conidia 

 produce zoospores, which soon become stationary and 

 germinate. A second form of conidial reproduction is 

 sometimes produced late in the season, differing from the 

 one already described in the much larger and fewer 

 conidia, which are borne on very short, sparingly branched 

 conidiophores. During the autumn the winter form of 

 fruit or resting-spores are produced in great numbers in 

 the tissues of diseased leaves. The tips of certain branches 

 of mycelium present in the leaves become swollen, and 

 after a sexual process of fertilisation assume a spherical 

 form, develop a thick, coloured wall, and remain in an 

 unchanged condition in the tissues of dead and fallen 

 leaves. If the leaves decay, the resting-spores are not 



