398 STATE HORTICULTURAL SOCIETY. 



the cherry. De Schweinitz mentions an epidemic of black-knot which 

 destroyed the cherry trees at Bethlehem, Pa., in 1790, and in various parts 

 of the country to-day the disease seems to be almost, if not quite, as bad 

 on cherry as on plum. 



EXTERNAL CHARACTERS. 



The external appearance of the mature form of black -knot is generally 

 well known. It appears at this stage as a rough, wart-like excrescence or 

 distorted outgrowth from the bark of twigs and branches and in severe 

 cases may extend along the trunk for several feet. 



The fungus may appear on any part of the tree above ground and no 

 portion of either trunk or branches is exempt from its attacks. 



BOTANICAL CHARACTERS. 



The first outward sign of the formation of a new knot is seen in a swelling 

 of the tissue within the bark either in the fall or during the growing 

 season of the tree. The swelling increases till the bark is ruptured over 

 the surface, and thus exposed the fungus sends out numerous threads 

 (hyphse) which pi'oduce a velvety appearance and are of an olive green 

 color. Microscopic examination of the velvety surface reveals multitudes 

 of newly formed and forming spores borne on these upright threads. 



These spores (conidia) are called summer spores. When full grown 

 they drop off from the supporting threads, and when carried by winds, 

 insects, or other agencies, to another host plant, under favorable conditions 

 they may start growth and form a new center of disease from which in time 

 other trees may also be infested and thus spread the disease from tree to 

 tree and neighborhood to neighborhood, 



The wonderful provisions for the reproduction of its kind are 'not ended 

 by growing a crop of summer spores. After a time the production of 

 summer spores ceases, the velvety threads die away and the surface of the 

 knot becomes hardened and gradually changes in color to dark brown and 

 finally to black. 



Late in the fall the surface of the knot appears to be covered with pim- 

 ples visible to the naked eye. These pimples are the outside covering of 

 a tiny spherical case which may be called a spore case (perithecium). 

 A slice across the pimple surface with a sharp knife or razor will frequently 

 show the white contents of the spore case visible to the naked eye. 



The oatside rows of cells form the black, hard covering of the spore-case 

 just referred to, making a hollow sphere within which are developed the 

 winter spores. These winter spores ripen inside of long, colorless sacs 

 (asci), and each sac when matured contains eight spores (ascospores). A 

 miscroscopic examination of one of these spore-cases in the fall, discloses 

 the fact that the winter spores are not yet formed. 



It has been found that they do not develop till winter and that they 

 become mature and are capable of germination in February or March. 

 The spore-sacs rise from a mass of threads which line the inner walls of 

 the spore case; mingled with the projecting spore-sacs are numerous sterile 

 threads which do not bear spores. Eventually the spore-sacs break away 

 and the spores which they have borne escape from the spore-case and are 

 carried by the winds or other agencies to infect new localities or other parts 

 of the tree on which they have developed. 



