256 DISEASES OF CULTIVATED PLANTS 



one time male or fertilising bodies. After the dead infected 

 leaves have been lying on the ground throughout the winter, 

 ascospores are formed in cavities in the stroma. These 

 ascospores escape into the air in the spring and infect the 

 young leaves. 



The spermatia are cylindrical, hyaline, about 6-9 X i /*. 

 Ascospores needle-shaped, hyaline, often slightly curved, 

 60-80 X i '5-2 -5 ft; paraphyses very slender, tips curved. 



The disease sometimes proves to be very destructive directly 



FlG. 75. Rhytismaacerinum. i, sycamore leaf with 

 blotches caused by fungus ; 2, section through a blotch 

 bearing conidia ; 3, ascus with spores, also two para- 

 physes ; 4, an ascospore. Figs. 2-4 mag. 



and indirectly. Several large patches are frequently present 

 on a leaf, and as a rule, when the disease is present, nearly 

 every leaf is attacked, hence a considerable area of leaf 

 surface is prevented from doing its work. In addition diseased 

 leaves fall early, and as the disease, unless checked, appears 

 year after year, the tree becomes enfeebled owing to lack of 

 food, and badly matured wood, when it easily falls a prey to 

 a yet more dangerous parasite, the coral spot fungus (Nectria 

 dnnabarina\ which often follows an epidemic of leaf blotch. 

 When the diseased leaves have fallen, the tree is perfectly 

 clear of the fungus, and the only possible means of further 

 infection is due to spores alighting on the young leaves, and 



