SOURCES OF THE EARLY INFECTIONS OF APPLE 

 BITTER-ROT ' 



By John W. Roberts, 



Pathologist, Fruit-Disease I nvestigntions , 



Bureau of Plant Industry 



INTRODUCTION 



Notwithstanding the excellent work which has been done by previous 

 investigators, the problem of determining the sources from which the 

 early infections of the apple bitter-rot fungus (Glomerella cinyiUata) may 

 arise has never been worked out with completeness sufficient to account 

 for the heavy initial damage sometimes caused by this disease. Often 

 bitter- rot will break out suddenly, and every apple {Mahis spp.) in an 

 orchard will be affected in an incredibly short time. In the Ozark 

 region of Arkansas during the season of 191 4 the crops of six of the 

 orchards under the writer's observation were within two weeks almost 

 entirely ruined by bitter-rot. Nearly all the apples on these trees were 

 infected comparatively early in the season — i. e., about July i — a large 

 majority of them having from 50 to 100 points of infection. The sud- 

 denness of the appearance of the disease and the almost simultaneous 

 infection of the fruit over the whole orchard strongly indicated that 

 practically all the rotten spots w-ere caused by spores which had washed 

 down from primary sources of infection. In one orchard soon after the 

 disease broke out nearly every apple was found to be affected with the 

 small blister-like spots characteristic of the early stages of the disease 

 (PI. VII, fig. 1). These spots had not yet developed far enough to 

 produce acervuli and were evidently due to infection by spores from over- 

 wintering or primary sources. 



The later infections are, of course, easily accounted for because the 

 fungus forms acervuli in the rot areas of the earlier infections and from 

 these the spores may be washed by rain or carried by insects to sound 

 apples, which, if conditions are favorable, may become diseased and in 

 turn become sources of infection. Thus, we may have primary sources 

 of infection, which may continue to act as such throughout the season, 

 and secondary sources of infection, consisting of the diseased fruits of 

 the current season. 



The primary sources of infection therefore become of great importance 

 in the control of the disease, especially when they are present in great 



1 A brief but incomplete report of these investigations was made before the Northwest Arkansas Fruit 

 Growers Society in July, 1914. (Roberts, J. W. The sources of apple bitter-rot infection, and control. 

 In Ozark Fruit and Farms, v. 5, no. 2, p. 3. August, 1914.) 



Journal of Agricultural Research, Vol. IV. No. i 



Dept. of Agriculture, Washington, D. C. -^P""- '5. '9iS 



G— 43 



(59) 



