366 Journal of Agricultural Research voi. xxu. No. 7 



DESCRIPTION OF THE DISEASE 



The infected parts on the unripe fruit appear as dark-colored raised 

 areas with fringed margins and are somewhat roughened by the presence 

 of small blisters and depressions (PI. 34, B). As in the case of apple 

 blotch, the skin often becomes ruptured as the fruit increases in size. 



On the ripe fruit the blotched parts appear as irregular browned areas 3 

 to 6 mm. in diameter and consist of an aggregation of from 4 to 20 sunken 

 spots, each separate spot being i mm. or less in diameter. At this stage 

 the spots have a peculiar light blue cast owing to the "bloom" of the 

 ripe plum covering the browned epidermis. The diseased area is rather 

 superficial, extending only slightly below the epidermis. The affected 

 tissues become hardened and somewhat leathery and show no tendency 

 to decay. 



Small, glistening pycnidia are produced in considerable numbers even 

 in the younger spots. Quite commonly there are 25 to 30 scattered 

 promiscuously about in each blotched area. Infection evidently takes 

 place when the fruits are very young, since the spots found May 29 were 

 well formed and bore pycnidia with mature spores. Judging from the 

 writer's inoculation experiments, infection probably took place five to 

 six weeks earlier, or about the middle of April. 



On account of its characteristic appearance on the fruit, the disease has 

 been given the common name of "plum blotch." 



On the upper surface of the leaf blades (PI. 34, A), the spots are angu- 

 lar, rather small (about 0.5 mm. across), brown when young, but later 

 becoming gray or silvery in color. They may be numerous, as many 

 as 200 sometimes appearing on a single leaf. Usually only a single 

 pycnidium is present in each spot, except where two or more spots have 

 coalesced to form a single large spot. Affected areas are also found on 

 the petioles and on the veins of the lower surface, especially on the 

 midrib. On these the diseased areas are much larger than on the upper 

 surface of the blade and are black and sunken. Pycnidia, bearing 

 spores, are present in great abundance. 



,, Pycnidia, apparently identical with those found on the fruit and 

 leaves, were found also in small light-colored, often slightly sunken areas 

 on the twigs; but, as spores were lacking, positive identification could 

 not be made. It is possible that these pycnidia had discharged their 

 spores early in the spring and had brought about the early infections on 

 the fruit. 



CAUSE OF PLUM BLOTCH 



By comparison with type specimens, the organism involved in the pro- 

 duction of plum blotch has been found to be identical with the fungus 

 described by Heald and Wolf ^ as Phyllosticta congesta. Heald and 



1 Heaid, F. D., and Wolf, F. A. new spEaEs op texas fungi. /wMycologiav. 3, no. i, p. 8. 191 1. 



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