SOURCES OF APPLE BITTER-EOT INFECTIONS. 15 



more so than the Winesap. The York Imperial and Yellow Newtown 

 are very nearly immune. While on the two last-named varieties the 

 fungus may begin the development of a canker, the reaction of the 

 host is such that the growth of the fungus is soon checked and the 

 incipient canker itself is later obliterated by the formation of callus. 

 As these two varieties are the ones chiefl}'' found in Virginia orchards 

 and as their fruits, especially those of the Yellow Newtown, are sus- 

 ceptible, the writer believes that these inoculation experiments ex- 

 plain why bitter-rot cankers have not been found in that State. 



It is also worthj^ of note that in these experiments the inoculations 

 were eventually more successful on older branches than on 1-year-old 

 twigs and water sprouts, though the growth was usually slower on 

 the older branches. This is in accord with the writer's observations 

 on naturally formed bitter-rot cankers. 



The inoculations made in Jul}^ were more uniformly successful 

 than those made in June, doubtless owing to the fact that in July 

 the temperature was much higher and the growth of the host more 

 restricted. 



These experiments and the writer's observations indicate that 

 cankers in which the growth of the fungus is comparatively slow 

 are apt to be much longer lived than those in which the fungus de- 

 velops rapidly. This is particularly true in the case of the Givens, 

 a variety which harbors more cankers than any with which the writer 

 has come in contact. Cankers on Givens branches grow very slowly, 

 but are exceedingly persistent. 



Young vigorous-growing twigs are able, after the hot season which 

 favors the bitter-rot fungus is over, to continue growth and by the 

 formation of a callus layer to utterly isolate the mj'celium of the fun- 

 gus in the tissues which it has killed. This explains why the writer 

 has not been able to find bitter-rot cankers on young branches. 



It seems probable that another reason why cankers on branches of 

 the Givens variety are so persistent is that this variety begins growth 

 in the spring nearly two weeks later than most varieties. It is in 

 the spring that the most vigorous growth of the apple occurs, and 

 hence it is at this time of year than the branch is in the best condi- 

 tion to isolate the cankered areas and develop callus. The fungus, 

 on the contrary, is not able to start growth until the weather be- 

 comes hot. This conclusion is drawn from the inoculation experi- 

 ments of 1914 and 1915. 



On January 9, 1917, three cankered branches of each variety in- 

 oculated in the 1916 experiments were cut off, brought into the 

 laboratory, and placed in moist chamber for 48 hours. Masses of 

 spores were produced from all cankers except those from the York 

 Imperial and Yellow Newton trees. Cankers on the branches of 



