22 BULLETIN 1118, U. S. DEPARTMENT OF AGRICULTURE. 



on tlie widespread impression that for a number of years this disease 

 was confined to the sour orange and rough lemon, later ''adapted" 

 itself to grapefruit, and still later to the round orange. This alleged 

 adaptation on the part of the scab fungus has been explained in 

 several ways. Some claim the grapefruit and round orange, as a 

 result of cultural practices, are rapidly losing their original immunity; 

 others are of the opinion that the scab fungus is gradually adapting 

 itself to the more resistant species; still others would explain the 

 condition on the ground that very virulent strains of the causal 

 organism capable of infecting grapefruit and orange have resulted 

 as natural mutations; and a few do no more than make the prediction 

 that within a short time the orange will be as seriously affected as is 

 now the case with grapefruit. 



In the case of citrus scab there appears to be no sound basis for 

 such claims in spite of the fact that such a phenomenon has been 

 noted for other pathogens. Within a few years after citrus scab was 

 first recorded in America, Underwood {30) reported the commercial 

 species as being susceptible to infection in essentially the same 

 proportion as is the case to-day. His report was made before the 

 grapefruit as a fruit, or rough lemon as a root stock, began to attract 

 very much attention from commercial growers. Since then these 

 species have gained a great deal of popularity, resulting in the set- 

 ting out of millions of trees susceptible to infection from scab. This 

 has increased immeasurably the number of foci of infection and also 

 the mathematical chance for infection of all species. 



If the scab fungus is adapting itself to the more resistant species, 

 this transition is going on very slowly, so much so that it can not be 

 measured even when viewed over a period of 30 years. The causal 

 organism isolated from round orange, grapefruit, and avocado has 

 shown itself by actual inoculation tests to be neither more nor less 

 pathogenic to grapefruit than was the same organism isolated from 

 sour orange and grapefruit. Cultural studies of the causal organism 

 isolated from various hosts also failed to reveal any difference in the 

 fungus, and field studies over a period of five years fail to give the 

 slightest indication that the round orange is losing its original re- 

 sistance. There is no valid reason for presuming that at least within 

 the life of trees now planted the orange will be as susceptible to infec- 

 tion as is grapefruit at present. There is no evidence that an inher- 

 ently more virulent strain of the scab fungus is developing. 



INOCULATION EXPERIMENTS. 



In order to determine accurately the period of development dur- 

 ing which fruit and foliage are likely to become infected and the 

 relative susceptibilities of some citrus species to scab, a rather ex- 

 tensive series of inoculation experiments was conducted on young 



