26 



BULLETIN 1118, U. S. DEPARTMENT 0¥ AGRICULTURE. 



These inoculation experiments were conducted on properties where 

 more or less citrus scab occurs normally, consequently a small pro- 

 portion of scab developed in the water checks, but not enough to 

 invalidate the general conclusions from these experiments. 



Inoculum from various strains of the citrus-scab fungus was used 

 on each host plant listed. There is no evidence that any one strain 

 of the scab fungus is more virulent than another. 



The smallest sizes of citrus fruits in Table 8 represent stages imme- 

 diately after the falling of the petals. The evidence is that suscep- 

 tibility is highest at this time and decreases progressively until a 

 diameter of about three-fourths of an inch is reached for grapefruit 

 when they attain immunity. 



The leaves of all species tested are found to be most susceptible 

 to infection as they emerge from the bud. They become progres- 

 sively resistant and reach a stage of immunity when they have devel- 

 oped to one-half inch in width (PL VIII, Fig. 2) . 



The Royal grapefruit, both leaves and fruits, appears to be immune 

 to scab infection. 



The scab fungus after growing on artificial media four years is 

 as pathogenic as recently isolated strains. 



Table 8. — Water-blank checks on grapefruit, Satsuma orange, and tangerine fruits. 





Size of 



















truit (in 



Total 





Fruits 



Total 



Total 



Per- 





Date. 



six- 

 teenths 

 of an 

 inch). 



inocu- 

 lations. 



Dropped 

 or lost. 



remain- 

 ing. 



posi- 

 tive. 



nega- 

 tive. 



centage 

 posi- 

 tive. 



Degree of infection. 



Grapefruit: 



















Apr. 10,1920.... 



4 to 5 



19 



4 



15 



4 



11 



26 



Moderate. 



Apr. 23, 1920.... 



5 



20 



7 



13 



1 



12 



8 



Do. 



May 2, 1917 



2 to 20 



5 







5 







5 









Apr. 3, 1919 



6 



40 



28 



12 



3 



9 



25 



Do. 



Apr. 10, 1919. . . . 



6 to 8 



40 



20 



20 







20 









Apr. 20, 1918.... 



6 to 8 



20 



6 



14 



2 



12 



14 



Slight. 



Apr. 15, 1918. . . . 



8 



25 



9 



16 



2 



14 



12.5 



Do. 



May 13,1920.... 



8 



25 



5 



20 



1 



19 



5 



Moderate. 



Apr. 17, 1919.... 



8 to 10 



40 



9 



31 



4 



27 



12.9 



SUght. 



Apr. 19, 1919.... 



8 to 12 



39 



10 



29 







29 







Many yellow. 



May 9, 1917 



8 to 16 



30 



5 



25 



4 



21 



16 



Moderate. 



June 11,1920.... 



10 to 24 



25 



6 



19 







19 









Apr. 25, 1919.... 



11 to 14 



40 



9 



31 



2 



29 



6.4 



Slight, probably not 

 scab. 



May 8, 1920 



12 to 16 



47 



10 



37 







37 









May 1, 1919 



15 



40 



11 



29 



3 



26 



10.3 



Do. 



May 8, 1919 



17 



40 



12 



28 



1 



27 



3.5 



Do. 



Apr. 30, 1919.... 



16 to 20 



48 



20 



28 







28 









May 15,1919.... 



28 



40 



4 



36 







36 









Satsuma orange: 



















May 24, 1917.... 



4 to 8 



36 



11 



25 



5 



20 



20 



Moderate 



June 4, 1920 



6 to 8 



10 



1 



9 







9 









May 24, 1917.... 



6 to 8 



36 



11 



25 



5 



20 



20 



SUght. 



May 17, 1920. . . . 



8 



43 



12 



31 



4 



27 



13 



Do 



May 31, 1917. . . . 



10 to 20 



25 



IS 



7 



1 



6 



14 



Do. 



Tangerine: 



June 4, 1920 



















4 



5 



2 



3 



1 



2 



33 



Moderate. 



