26 
lil I.I.KTIN 1118, U. S. DEPARTMENT OF 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 
Date. 
fruit (in 
six- 
teenths 
of an 
inch). 
Total 
inocu- 
lations. 
Dropped 
or lost. 
Fruits 
remain- 
ing. 
Total 
posi- 
tive. 
Total 
nega- 
tive. 
Per- 
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 
Slight. 
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 
Apr. 30, 1919.... 
17 
40 
12 
28 
1 
27 
3.5 
Do. 
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 
Slight. 
May 17, 1920. . . . 
8 
43 
12 
31 
4 
27 
13 
Do 
May 31, 1917. . . . 
10 to 20 
25 
18 
7 
1 
6 
14 
Do. 
Tangerine: 
June 4, 1920 
4 
5 
2 
3 
! 
1 
2 
33 
Moderate. 
