10 BULLETIN 534, TJ. S. DEPARTMENT OF AGRICULTURE. 



This orchard, consisting of trees of Missouri, a very susceptible 

 variety, was, prior to 1913, badly infected with blotch. In fact, pre- 

 vious to 1913 the orchard, mainly because of blotch, had not matured 

 a crop. After three years of thorough spraying, done at the proper 

 time, the trees are almost free from twig cankers and the disease 

 is nearly eliminated from the orchard. 



Spraying, then, not only prevents the infection of fruits during the 

 current year, but tends also to lessen the number of possible infec- 

 tions during succeeding years. Furthermore, since, as the writer (4) 

 has shown, the bitter-rot fungus may live through the winter in blotch 

 cankers, the elimination of the latter may be an aid in the control of 

 bitter-rot in orchards in which that disease is present. 



SUMMARY. 



(1) Apple blotch, a serious disease of the more southern apple- 

 growing sections of the United States, affects the twigs, fruit, and 

 leaves of the apple. It has been shown by previous investigators 

 (Waite, Clinton, Sheldon, and Scott and Rorer) to be caused by the 

 fungus Phyllosticta solitaria, which, as Sheldon and Scott and Rorer 

 discovered, winters over in twig cankers and infects the young fruit, 

 leaves, and twigs during the following year. Neither Scott and Rorer 

 nor Lewis considered mummied fruit of the previous year an impor- 

 tant source of infection. 



(2) The writer has made successful cross inoculations on fruit, 

 leaves, and twigs from pure cultures of the fungus obtained from 

 naturally diseased fruit and twigs, thus confirming the inoculation 

 work of Scott and Rorer, which, however, was not done by the use of 

 pure cultures. 



The reason so few infections occur late in the season is due to the 

 increased resistance of the host, in addition to the fact that there is a 

 gradual decrease in the number of spores produced by the causal 

 fungus. A large number of mummied fruits were examined at in- 

 tervals throughout the spring, but no spores were found. Hence 

 it is concluded that mummies are not an important source of infection. 



Wet weather favors blotch, but in orchards in which twig cankers 

 are abundant the disease is not checked effectively by dry weather. 



(3) The disease is controlled by three sprayings with 3-4-50 

 Bordeaux mixture at intervals of three weeks, the first of which 

 should be completed about three weeks after the blossom petals have 

 fallen. Summer-strength lime-sulphur solution may be substituted 

 for Bordeaux mixture where the disease is not severe, thus lessening 

 the risk of injury. The proper time for the first application has been 

 determined both by spraying experiments and by spore germination 

 tests in the laboratory. This spraying schedule differs only slightly 

 from that originally worked out by Scott and Rorer. 



