8 BULLETIN 1118, U. S. DEPARTMENT OF AGRICULTURE. 
THE CAUSAL OlHiANISM. 
Citrus seal) is caused bj 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 Cladosporium 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: Ochraceous tawny to cinnamon brown (R 15). 
POTATO DEXTROSE AGAR SLANTS (FORMULA 2). 
Age 7 days: Growth moderate, about 4 millimeters in diame er, restricted, raised 
about 3 millimeters: numerous small warts becoming whitish with woolly 
hyphae. 
