RELATIVE SUSCEPTIBILITY OF SOME RUTACEOUS 
PLANTS TO ATTACK BY THE CITRUS-SCAB FUNGUS 1 
By John R. Winston, Associate Pathologist , and John J. Bowman and Walter J. 
Bach, Junior Pathologists , Fruit-Disease Investigations , Bureau of Plant Industry , 
United States Department of Agriculture 
In a former publication by the senior 
writer of this paper 2 were listed most 
of the commercially important citrus 
plants subject to scab in Florida. The 
present paper gives a more extensive 
list indicating the relative susceptibility 
to the disease as it occurs on its hosts in 
central and south Florida. 
The data presented here are based 
on: (1) Observations made in com¬ 
mercial citrus nursery and grove 
plantings during more than seven years, 
viz, 1916 to 1924; (2) studies of rare 
forms of undetermined susceptibility 
closely interplanted with sour orange 
seedlings severely affected with scab; 
and (3) inoculation tests made on 
many rutaceous plants growing on the 
grounds of the field station of the 
United States Bureau of Plant Indus¬ 
try at Orlando, Fla. The method 
used in these inoculation tests has al¬ 
ready been described. 3 
Citrus scab is caused by the fungus 
usually but erroneously referred to as 
Cladosporium citri pro tern Massee. 
Anna E. Jenkins, 4 assistant mycologist 
in the Bureau of Plant Industry, 
United States Department of Agricul¬ 
ture, has published a paper dealing with 
the morphology and taxonomy of this 
organism and has described the scab 
pathogene as Sphaceloma fawcettii n. sp. 
Observations and inoculations on 
commercial citrus varieties have been 
extensive enough to be thoroughly de¬ 
pendable; on the noncommercial varie¬ 
ties and hybrids they have been 
limited to a small number of plants, 
and for that reason the general impres¬ 
sion of relative susceptibility is given 
tentatively. Further observations may 
indicate that these hosts may be 
affected in a degree different from that 
reported herein. Many of the rutace¬ 
ous plants under study are quite small 
and have not fruited; therefore obser¬ 
vations were limited to occurrence of 
the disease on leaves and tender twigs. 
However, for those forms that have 
fruited there seems to be no difference 
in susceptibility of the fruit as com¬ 
pared with leaves and twigs. 
RELATIVE SUSCEPTIBILITY TO 
SCAB 
The results of the inoculations and 
observations on the relative suscep¬ 
tibility of many rutaceous plants to 
attack from the scab fungus are given 
in Table 1. 5 
It is interesting to note that, with 
the exception of Claucena lansium, the 
rutaceous plants attacked by scab 
belong to the subtribe Citrinae. Mem¬ 
bers of only four genera, viz, Citropsis, 
Poncirus, Fortunella, and Citrus are 
known to be affected by this disease. 
Poncirus and Fortunella until a few 
years ago were regarded as members of 
the Citrus genus. Under conditions in 
central Florida, Claucena lansium is 
attacked moderately, while Poncirus 
trifoliata is only slightly affected. 
Only once has scab been observed on 
Citropsis and likewise only once has it 
been observed on any Fortunella 
species, in spite of the fact that for 
years they have been growing close to 
scab-infected grapefruit and sour 
orange trees. This kumquat (For¬ 
tunella) infection resulted from arti¬ 
ficial inoculation. 
At no time has scab been observed 
on four species of Citrus, viz, Citrus 
ichangensis, C. medica, C. janos, and 
C. webberi. 
Several varities of lemon ( Citrus 
limonia) are extremely liable to attack 
by scab, while the sweet lemon and the 
dwarf Chinese lemon have thus far 
been observed to be distinctly less 
susceptible to severe injury. 
The Kusaie and Sour Rangpur lime 
(C. aurantifolia) are severely attacked 
by scab; the Tahiti lime has been found 
1 Received for publication Sept. 24, 1924; issued August, 1925. 
2 Winston, J. R. citrus scab: its cause and control. U. S. Dept. Agr. Bui. 1118, 39 p., ulus. 1923. 
* Winston, J. R. Op. cit., p. 23. 
4 Jenkins, A. E. the citrus scab fungus. Phytopathology 15: 99-104, illus. 1925. 
fi The botanical classification of rutacoous species and hybrids referred to in this paper follows W. T. 
Swingle in his articles in Bailey, L. H. the standard cyclopedia of horticulture. 6v. New York. 
1914. Entry under genus names. 
Vol. XXX, No. 11 
June 1, 1925 
Key No. G-497 
Journal of Agricultural Research, 
Washington, D. C. 
( 1087 ) 
