CITRUS SCAB : ITS CAUSE A^^D C0:NTR0L. 7 



Bordeaux mixture gave satisfactory control (25) and sulphur dusts 

 no control of this disease. 



Grossenbacher (7) lists susceptible species in Florida and discusses 

 conditions favoring citrus-scab outbreaks. His attempts to pro- 

 duce the disease artificially failed, undoubtedly because he used cul- 

 tures of the saprophytic Cladosporium sp. which develops on old 

 citrus-scab lesions rather than the active pathogen first isolated by 

 Fawcett. Bordeaux mixture proved to be much more effective than 

 lime-sulphur solution against citrus scab. Spraying after the middle 

 of May was found to be too late for effective citrus-scab control. He 

 advises one application of Bordeaux mixture in the height of the 

 bloom, followed in a week or 10 days by an application of lime-sul- 

 phur solution and still another application of lime-sulphur solution 

 two or three weeks later. 



Fawcett (5) reports further confirmative inoculation experiments 

 in producing typical scab lesions by using the true scab organism. 

 He further discusses the confusion of this organism with the common 

 saprophytic Cladosporimn. 



Lee (11) reports finding citrus-scab lesions on leaves of Citrus 

 nohilis collected by Maximo wicz in 1863 at Nagasaki and deposited 

 in the herbarium of the Hongkong Botanic Gardens. This observa- 

 tion coupled with the fact that scab is distributed throughout the 

 citrus-growing section of South China led him to conclude that this 

 disease may be indigenous to the Orient. 



Stevens {22) found in avocado-scab lesions an organism in every 

 morphological and cultural character similar to the citrus-scab 

 fungus. With the avocado-scab fimgus he produced typical scab 

 lesions on the avocado as weU as on the citrus, but the fungus isolated 

 from lesions on citrus failed to infect the avocado. In a later publi- 

 cation {23) he predicts that scab will eventually be as severe on the 

 round orange as it is now on the grapefruit. 



Fawcett {6) reports the results of experiments to determine the 

 effect of constant temperatures on infection of sour-orange leaves 

 and on the vegetative growth and spore development of the citrus- 

 scab fungus. Infection on rapidly growing parts occurred between 

 16° and 23° C, while detached leaves floated in water were infected 

 at temperature ranging from 16° to 27.5° C. Vegetative growth was 

 most rapid at 21° C. Spores developed at all temperatures from 

 13.5° to 27.5° but not at 32° C. At 21° C. the spore development 

 was more abundant than at lower or higher temperatures. In the 

 light of these experiments Fawcett thinks the relatively low tem- 

 peratures of spring tend to aid and the high temperatures of summer 

 in Florida to retard or inhibit infection. 



