14 APPLE BLOTCH IN SOUTHERN ORCHARDS. 



In September, 1906, the writers found apple-twig cankers caused 

 by a fungus which appeared to be the same as the apple-blotch fun- 

 gus. The fungus from these twig cankers and that from the spots 

 on the fruit grown in the same kind of medium under the same con- 

 ditions were so similar in all respects that one could not be distin- 

 guished from the other. Moreover, spores from both agreed in color, 

 shape, and size. It seemed evident, therefore, that the two diseases 

 were caused by the same fungus. For like reasons it was suspected 

 that the fungus causing the leaf spots and petiole cankers found asso- 

 ciated with the twig cankers was also the same, (PL III, figs. 2, 3, 

 and 4.) Definite proof of this was obtained by inoculation experi- 

 ments made in the following year. 



In the spring of 1907 at the time the first fruit infections were 

 taking place spores were found oozing in great quantities from 

 pycnidia on the twig cankers, strongly indicating that these cankers 

 formed the principal spurce of infection for the first outbreak on 

 the fruit. 



Under favorable conditions the spores which are carried from the 

 cankers to the young fruits, twigs, and leaves may germinate in fif- 

 teen hours, giving rise to one or two germ tubes, which soon pene- 

 trate the epidermis and form a much-branched mycelium. In the 

 fruit the growth of the fungus is slow and restricted to a few layers 

 of cells just beneath the skin. The mycelium spreads out from the 

 point of infection in a somewhat stellate manner ; the invaded tissues 

 turn brown, producing the characteristic blotches as seen on the sur- 

 face. In a short time the fungus begins to produce pycnidia, or small 

 receptacles in which spores are borne. . (See PI. Ill, fig. 1.) These 

 pycnidia develop beneath the epidermis and appear as small, black, 

 raised points. T\ Tien mature, they rupture the epidermis and emit 

 the spores, which are ab jointed from short sporophores, through a 

 small opening called an ostiolum. 



The spores are ovoid to elliptical ovoid, hyaline, unicellular, and 

 measure 8 to 10 by 5.5 to 6.5 /*. They are enveloped in a gelatinous 

 sheath, which is frequently prolonged at the apical end into a thread- 

 like appendage. (See PL III, fig. 4.) These appendages are as a 

 rule rather inconspicuous, usually difficult to discern on spores from 

 dried material, but readily seen on those that have just reached ma- 

 turity. They vary in length, but are generally about twice as long as 

 the spore. They are apparently of the same nature as the appendages 

 which occur on the conidia of Guignardia vaccinii Shear and G. 

 MdiveUii (Ell.) V. & E. as described by Shear. The writers have 



° U. S. Department of Agriculture, Bureau of Plant Industry, Bulletin 110. 

 p. 15, 1907. 



144 



