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Description of Symptoms. The disease appears in the 

 spring shortly after the leaves begin to emerge from the bud. 

 The leaf-blade becomes thickened and puckered along the 

 midrib, causing the leaf to become curled and twisted. The 

 diseased part remains yellowish in colour with a tinge of red 

 in it. As the leaves become older, this curling and crumpling 

 of their surface becomes more pronounced, then* substance 

 becomes fleshy and the coloration darker; finally the upper 

 surfaces of the diseased leaves become covered with a delicate 

 " bloom " due to the fungus passing into its spore-bearing 

 stage. Affected leaves finally die and drop from the tree, and 

 in severe cases the entire tree may become defoliated. New 

 sets of leaves, however, usually develop and replace those that 

 have fallen. 



The fungus not only attacks the leaves but also invades the 

 young shoots and, more rarely, the flowers and fruits. Young 

 shoots infected with the fungus become swollen and twisted 

 and the diseased leaves usually form a tuft on a stunted shoot, 

 owing to the internodes failing to elongate. 



In winter, symptoms of fungal activity are visible on the 

 young growth, brown patches being here and there present; 

 these patches increase in size, until finally the whole length 

 of the lateral beyond this point withers and a number of dead 

 ends are left. 



The injury caused by the disease consists not only in the 

 distortion of the leaves and premature defoliation but in the 

 dropping of the fruit at an early stage and in the strain on 

 the tree due to the development of a second crop of leaves. 

 In the case of nursery stock, consecutive attacks for three or 

 four seasons usually kill the tree or stunt its growth to such 

 an extent that it is practically valueless. 



Cause of the Disease. Leaf-curl is caused by an attack 

 of the fungus Exoascus deformans. The fungus enters the 

 young leaves early in the spring when the buds are just 

 commencing to expand. The mycelium (or system of fine 

 fungus threads) develops between the cells of the leaf, robbing 

 them of nourishment, destroying the green colouring matter 

 and causing the leaf to become deformed. 



After a time the fungus mycelium forms a layer just 

 beneath the skin of the leaf and from this layer a number of 

 spore-sacs (termed asci) are developed; it is their presence 

 which causes the " bloom " upon a diseased leaf. Within 

 each of these spore- sacs eight spores are produced and these 

 spores usually bud off a number of secondary spores, all of 

 which are capable of germination and reproducing the disease. 

 The spores are produced in great abundance upon the diseased 

 leaves during the spring and early summer. 



Commencement of Attack in Spring. It was long 

 believed that the fungus mycelium hibernated in the tissues 



