CITRUS SCAB : ITS CAUSE AND CONTROL. 3 



SPECIES AND VARIETIES ATTACKED. 



Among the commercial species and varieties grown under Florida 

 conditions the sour orange^ {Citrus aurantium) (PL 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 (PL I, Fig. 2; PL V, Fig. 1), all of the kid-glove 

 group of oranges (C. nobilis) and the citrange (a hybrid of Poncirus 

 trifoliata and the tangerine) are quite susceptible to infection. The 

 sweet or round oranges (C. sinensis) and kumquats {Foi^tunella 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 Koyal 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 citras species referred to in this bulletin follows W. T. Swingle (S7). 

 Serial numbers (italic) in parentheses refer to "Literature cited" at the end of this bulletin. 



