CITRUS SCAB : ITS CAUSE AND CONTROL. 6 
SPECIES AND VARIETIES ATTACKED. 
Among the commercial species and varieties grown under Florida 
conditions the sour orange x {Citrus aurantium) (PI. V, Fig. 2), 
lemon (C. limonia) (PI. VI. Fig. 2), calamondin (C. mitis) (PL VII. 
Fig. 2), and tangelo (a tangerine-grapefruit hybrid) (PL VI, Fig. 1) 
are extremely susceptible to citrus scab. The grapefruit and shad- 
dock (C. grandis); the King orange, tangerine, mandarin orange, 
and Satsuma orange (Tl. I. Fig. 2: PL V. Fig. 1). all of the kid-glove 
group of oranges (C. nobilis) and the citrange (a hybrid of Pondrus 
trifoliata and the tangerine) are quite susceptible to infection. The 
sweet or round oranges I C. sinensis) and kumquats (Fortunella spp.) 
are attacked rarely. The Mexican or Key lime (C. aurantifolia) and 
the Royal and Triumph grapefruits (supposed to be hybrids of the 
orange and grapefruit) appear to be immune. Data on the suscepti- 
bility to citrus scab of the less important species and varieties are 
being accumulated and will be published later. 
Since the grapefruit is the commercial type of citrus most affected 
by scab in Florida, the statements in this bulletin relate particularly 
to it unless otherwise indicated. Such grapefruit varieties as the 
Duncan. Walters, Pernambuco. Leonardi, and Foster are susceptible 
to infection in about equal degree. Hall (Silver Cluster) is much more 
susceptible and the Marsh considerably less so than the above varie- 
ties, and these two can frequently be recognized in mixed plantings 
simply by the relative proportion of scab present. The Royal and 
Triumph varieties have never been observed by the writer to be 
infected. 
DESCRIPTION OF THE DISEASE. 
On bearing grapefruit trees citrus scab usually confines itself to 
the leaves and fruits, though occasionally it is observed on the calyx 
(PL VI, Fig. 3) and pedicels of blossoms, and on tender twigs. The 
extremely susceptible species, such as the sour orange and rough 
lemon, are affected as specified above, and in addition succulent 
twigs are frequently affected. In the nursery leaves and twigs of 
the sour orange and the rough lemon may be attacked severely enough 
to reduce the annual growth to about 60 per cent of normal (PL IV). 
Grapefruit is similarly affected, but very much less so in frequency 
of occurrence and in degree of injury. 
Leaves. — Scab develops first as minute, more or less circular, 
distinctly raised protuberances, usually on the under surface of the 
leaf. In a few days the apices of these fresh lesions become cream- 
colored or pale yellow-orange and plainly evident to the unaided 
eye. As the leaf expands (Pis. I and II) these lesions become more 
conspicuous, some forming hollow, conical outgrowths, the apices 
1 The technical classification of citrus species referred to in this bulletin follows W. T. Swingle (27). 
Serial numbers (italic) in parentheses refcr to " Literature cited'' at the end of this bulletin. 
