THE LEAF BLOTCH DISEASE OF HORSE-CHESTNUT 



413 





CAUSE OF THE DISEASE 



Leaf blotch is caused by a fungous pathogene, Guignardia ALsculi 

 (Peck) Stewart. This parasite obtains its food and nourishment by 

 means of minute vegetative, root-like strands, called mycelium, which 

 penetrate the leaves of the horse-chestnut and kill the tissues invaded. 



LIFE HISTORY 



With the presence of the fungus in the leaf, the mycelium extends 

 in all directions and kills the surrounding tissues, producing the character- 

 istic leaf blotch lesions. Soon 

 gnarls of the mycelial threads 

 appear at various points near 

 the upper surface of the leaf, 

 and these result in the forma- 

 tion of fruiting bodies of the 

 fungus. These fruiting bodies 

 are known as pycnidia (Fig. 

 85), and are the character- 

 istic minute specks already 

 mentioned as appearing on the 

 upper surface of the affected 

 area. 



Within the pycnidium are 

 borne a large number of spores, 

 which are extruded from the opening in the top of the pycnidium. The 

 spores, being very minute, are easily carried to other leaves by wind and 



rain, and are thus dis- 

 seminated to a con- 

 siderable distance. 

 Falling on a leaf, the 

 spore germinates, if 

 moisture is present, by 

 sending out a slender 

 germ tube (Fig. 86, A), 

 which grows into the 

 tissue of the leaf and 

 develops a mycelium 

 with many branches, 

 causing a disorganiza- 

 tion and killing of the tissue in the affected area. When a number of spores 

 infect the same leaf, a large part or all of the tissue becomes affected and 

 the leaf is no longer able to function in the manufacture of food for the tree. 



FlG. 85. CROSS SECTION OF A HORSE-CHESTNUT 

 LEAF THROUGH A PYCNIDIUM OF THE FUNGUS 

 (MUCH MAGNIFIED) 



The spores are shown inside the pycnidium. They escape 

 through the opening in the top 



FlG. 86. SPORE FORMS OF THE LEAF BLOTCH FUNGUS 



A, pycnospores, some of which have germinated; B, ascus, containing 

 eight ascospores; C, ascospores, some of which have germinated 



