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



THE CAUSAL ORGANISM. 



Citrus scab is caused by a fungus originally isolated by Fawcett 

 (2) in 1906 and referred to by him as Cladosporium citri Massee. In 

 subsequent publications by various workers the pathogen is referred 

 to by this binomial. This organism (PL XIII) has none of the distinc- 

 tive characteristics of Cladosporimn Link. A saprophytic species 

 of Cladosporium develops almost invariably in old citrus-scab lesions 

 and becomes very conspicuous after several months of weathering 

 or after the true scab fungus has largely disappeared. Inoculation 

 tests conducted by the writer at Orlando, Fla. (Table 9), with this 

 saprophytic Cladosporium have given no evidence of any patho- 

 genicity. Since the mycological position of the causal organism has 

 not been established it will be referred to in this bulletin as the citrus- 

 scab fungus, and certain of its cultural and morphological characters 

 will be described to aid in diagnosis. 



Its growth on certain nutrient media is characteristic (PI. VII, 

 Fig. 1), so much so that it can be readily recognized from such reac- 

 tions. In general the fungus grows slowly, requiring from five to 

 eight days at room temperature for colonies to become plainly visible. 

 When scrapings are made from relatively young lesions very few 

 contaminations develop in the poured plates. Isolations from old 

 lesions which have been overrun by the common saprophytic Clad- 

 osporium are more difficult. 



One of the best media for isolation and differentiation is glycerine 

 agar. On this medium the fungus produces a distinct purplish color 

 reaction which often becomes plainly visible before growth characters 

 are advanced enough to be distinctive. The prominent growth 

 characters as they appear on the more common media, together 

 with their color reactions, are here given. For convenience in ref- 

 erence to the colors and shades Ridgway's (17) standard is used as a 

 basis of comparison, and his plates are indicated by numbers pre- 

 ceded by the symbol R. 



PROMINENT GROWTH CHARACTERS. 



MOLISCH AGAR SLANTS (FORMULA l). 



Age 7 days: Growth moderate, about 3 millimeters in diameter, restricted, raised 

 about 1 millimeter, convoluted; medium slightly penetrated. 



Age 15 days: Growth moderate, about 1 centimeter in diameter, restricted, raised 

 about ^ centimeter, occasionally center of colony more or less villous, con- 

 voluted; medium irregularly penetrated but not discolored. 



Color of fungus: Oclu-aceous tawny to cinnamon brown (R 15). 



POTATO DEXTROSE AGAR SLANTS (FORMULA 2). 



Age 7 days: Growth moderate, about 4 millimeters in diameter, restricted, raised 

 about 3 millimeters; numerous small warts becoming whitish with woolly 

 hyphse. 



