68 MANUAL OF FRUIT DISEASES 



disease was then held. Several years elapsed before it was 

 recognized that another stage of G. Juniperi-mrginianos oc- 

 curred on the apple. Prior to this discovery the stage on the 

 apple was referred to as Rcestelia Pyrata. After the hetercecious 

 habit of the fungus was established, the last name was dis- 

 carded and the former name was, and is now, used to designate 

 it in all of its stages on the cedar and the apple. 



In the spring basidiospores, or sporidia, are blown from the 

 gelatinous horns of the cedar-apple to the young leaves and 

 fruits of the apple. In some cases they may be carried several 

 miles, but most of them do damage only at a mile or less. 

 Under favorable conditions the sporidia germinate. Low 

 temperatures, such as prevail in April and May, are most 

 favorable to the germination of the sporidia. The germtubes 

 penetrate the apple-leaf through the cuticle on the upper sur- 

 face. The mycelium develops locally within the leaf, occupying 

 the spaces between the cells and sending haustoria into the 

 cells. As a result of the stimulative action of the mycelium, 

 the leaf is hypertrophied, due to an excessive enlargement and 

 multiplication of the cells in the lower part of the leaf (spongy 

 parenchyma). These cells become elongated and the inter- 

 cellular spaces of the spongy parenchyma are obliterated. 

 The apple-fruit tissues are at first enlarged, but finally growth 

 of the fruit at the point of attack is stopped, which fact explains 

 the marked dwarfing and reduction in size. About one month 

 after infection, the mycelium forms the first fruiting bodies of 

 the season. These are on the upper surface of the leaf or on 

 the fruit and are called pycnia (spermagonia) . Spores are 

 developed within the pycnia and are called pycnospores (sper- 

 matia) ; their function is unknown. They do not reproduce 

 the fungus. Within a short time after the spermagonia appear, 

 the cup-shaped bodies develop on the lower surface of the leaf 

 opposite the spermagonia; these are known as secia. About 

 the middle of July these open and their spores, called secio- 



