10 



BULLETIN" 1118, U. S. DEPARTMENT OF AGRICULTURE. 



In addition to these media, the fungus has been grown on corn- 

 meal agar, prune agar, nitrogen-free agar; on clover stems, bean pods, 

 and orange stems; and on celery, carrot, tomato, bean, peach, apple, 

 and prune broths. 



DEVELOPMENT OF SPORES. 



True conidia (PI. XIII, Fig. 9) are borne very sparingly in culture, 

 so much so that they are not considered in this bulletin. Secondary 

 colonies developing in test tubes are rather unusual. As cultures age 

 the convoluted mycelium rapidly becomes thick-walled and develops 

 tangled chains of more or less regularly constricted hyphse. These 

 convoluted chains are easily broken apart into many single-celled 

 thick-walled bodies, apparently chlamydospores, which possess the 

 power of germination (PI. XIII, Figs. 1 to 7) and which are most 

 likely responsible for the greater part of the infections induced by arti- 

 ficial inoculations from cultures. 



TEMPERATURE RELATIONS. 



In order to determine the effect of temperature upon the radial 

 growth of the citrus-scab fungus on artificial media a relatively large 

 number of 'cultures were held for 42 days at various temperatures. 

 In each tube a bit of fungus tissue not more than 0.5 of a millimeter 

 in diameter was planted as near as possible in the center of the slant 

 surface. The media used were potato-dextrose agar, glycerin agar, 

 beef agar, and Molisch agar. The strains used were isolated from 

 various susceptible citrus varieties, but no appreciable variation could 

 be noted between the several cultures. The average final diameter 

 of colonies on the several media is the basis of comparison of growth 

 as here recorded in Table 1. 

 Table 1. — Growth of citrus-scab fungus attained in ^2 days at various temperatures. 





Colonies. 



Temperatures (° C). 









5 



15 



20 



18 to 25 

 (room 

 range). 



28 



36 



45 



Diameter . . 



miUimeters. . 



1 



2 



8 



10 



12 



7 



2 







It will be seen that around 20° C. (68° F.) is probably the optimum 

 temperature for the development of the fungus in culture. This in a 

 measure may account for the relatively large proportion of citrus- 

 scab infection which occurs during the cool weather of spring. Nev- 

 ertheless an abundance of infection occurs in the hot summer months 

 as well. 



OVERWINTERING. 



In order to determine whether or not the citrus-scab fungus lives 

 over winter on fruit or leaves, or both, a number of isolation tests 

 were made at various times during the past four years. Invariably 



