296 Journal of Agricultural Research voi. xiii, No. s 



center of the upper surface of the spots soon changes to a light yellow- 

 ish brown with a reddish brown border. Around this border for i to 2 

 mm. the leaf becomes light green. In some cases the spots may be 

 almost white. The color depends apparently on the amount of sunlight 

 as well as on the humidity. On the lower surface the spots remain 

 lighter colored and do not develop the dark border. Mature lesions 

 are of two distinct types; one a small spot, the other a large mottled spot, 

 depending on the abundance of infection and on weather conditions. 

 The small spots are produced most abundantly when infection is followed 

 by dry weather. They range from 0.2 mm. to 3 or 4 mm. in diameter, 

 the majority being i to 2 mm. (PI. 21, A, B). At first circular in outline, 

 they may become irregular or markedly angular, being limited by the 

 veins of the leaf. As already mentioned, there may be considerable 

 variation in the color of the spots, in some cases the reddish brown 

 border being almost entirely absent. 



When infection is followed by moist weather, and especially if several 

 spores germinate near each other in a large drop of water, a different 

 type of lesion is produced. These lesions are larger, ranging from 4 to 

 15 mm. in diameter (PI. 22, A, B). They vary from white to light brown, 

 even in the same spot, with reddish-brown areas along the veins. This 

 gives them a characteristic mottled appearance. These large spots 

 may originate in various ways. In some cases the infection appears 

 over the large spot all at once; in others it first appears as several small 

 lesions which later unite; while in others the infection may later extend 

 outward from a central point. In the last-named case there may be a 

 dark-brown center of 3 to 5 mm. with a lighter area around it. On the 

 lower surface of the leaf the mottled appearance is much less distinct, the 

 spots usually being light yellowish with occasionally a slight tinge of 

 brown. During hard rains the centers of these spots may be broken 

 out, leaving holes in the leaves. In severe cases the leaf may be so badly 

 infected that it turns yellow or brown and dies. 



The writer has not seen infection on stems, petioles, or fruit in the 

 field, but occasional longitudinal lesions on stems and petioles have 

 been produced by artificial inoculation in the greenhouse. These spots 

 are slightly sunken, 0.5 to i mm. wide by 3 to 5 mm. long, with a very 

 light yellowish-brown center and darker border. 



ISOLATION OF THE CAUSAL ORGANISM 



Material collected both in 191 5 and 191 6 was invariably found to have 

 associated with the spots a fungus belonging to the Dematiaceae-Dic- 

 tyosporae group of the Hyphomycetes. 



Since the fungus produces rather large spores on a surface mycelium, 

 spore dilution in plate cultures was found to be the easiest method of 

 obtaining it in pure culture. Young spots free from dirt on which there 

 was abundant spore formation were inverted under a binocular micro- 



