244 
Journal of Agricultural Research 
Vol. XXI, No. 4 
Citrus scab has been described and illustrated in considerable detail 
in previous publications by Scribner (£, 9), Swingle and Webber (14), 
Fawcett (2), and others. Its main characteristics are corky wart-like 
projections on leaves, fruit, and small twigs, due to the attack of the 
fungus on tender, rapidly growing tissue. The portion of the leaves 
not attacked usually maintains a healthy green color close up to the edge 
of these warts or scabs. 
Considerable confusion has resulted, however, in the identity of the 
causal organism, Cladosporium citri (r, 2). It should be mentioned that 
the fungus is an unusual Caldosporium and very different from the 
ordinary type of Cladosporium such as Cladosporium herbarum Lk., for 
example. This causal relation of Cladosporium citri to scab was at 
one time questioned by Grossenbacher (5), probably because he was 
working with another Cladosporium as indicated by the characteristics 
mentioned in his paper. The author’s identification of Clodosporium 
citri as the causal organism has been confirmed by Hesler (6), Stevens (12), 
and Horne (j), as well as by his own later work (3). Moreover, Cladospo¬ 
rium citri has been found by Stevens ( 10) to be the cause of a similar 
disease known as avocado scab in Florida. Sour-orange (1 Citrus aurantium) 
(13) and lemon (1 Citrus limonia Osbeck) trees appear to be the most sus¬ 
ceptible hosts to citrus scab. Satsuma orange (1 Citrus nobilis var. unshiu 
Swingle), trifoliata orange ( Poncirus trifoliata Raf.), and pomelo 
(Citrus grandis Osbeck) are also attacked. Sweet orange (1 Citrus sinensis 
Osbeck) is nearly immune. In most of the infection experiments here 
reported, the sour orange, was employed because it was one of the most 
susceptible species. 
The experiments were made in 1917 at the Laboratory of Plant Physi- 
ology, Johns Hopkins University, in surroundings entirely free from the 
disease in question. The various temperatures employed were main¬ 
tained for the most part by means of the apparatus described by Living¬ 
ston and Fawcett (7). 1 
TEMPERATURES AT WHICH YOUNG CITRUS LEAVES WERE INFECTED 
A preliminary test on sour-orange seedlings was made on May 16, 1917, 
with 8-months-old plants, which had been potted about three weeks and 
were beginning to develop two or three new leaves each. These plants 
were placed for three days in the different temperatures indicated below 
and were inoculated by drawing over the surface of the leaves and shoots 
a camel’s-hair brush moistened with spores and bits of mycelium of the 
fungus in distilled water. 
The average temperatures of the nine different chambers were 12 0 ,16 0 , 
19 0 , 23 0 , 26.5°, 31 0 , 34.5 0 , 38.5°, and 42.5 0 C., with a fluctuation of i° 
to 2 0 . One plant was exposed in each chamber for a period of three 
days in a saturated atmosphere and then taken out and the surface water 
1 The author wishes to acknowledge the aid rendered by Dr. B. E. Livingston in these experiments. 
