Apr. 19,1924 Relation of Environmental Factors to Citrus Scab 
249 
CONDITIONS DETERMINING THE YEARLY PREVALENCE OF 
CITRUS SCAB 
It is a well-known fact that there are epidemic and nonepidemic scab seasons. 
Thus, in Alabama, the seasons of 1914, 1915, and 1920 were noted as very bad 
scab years. The season of 1918 was probably the lightest scab year recorded 
and yet scab was plentiful on some plants in the isolation field as early as March 
8. Some scab was present during the years 1916, 1917, and 1919, but these 
can be classed as nonepidemic years. 
Although scab is present during the month of March in some seasons, the 
weather conditions prevailing during April and May determine largely the 
relative prevalence of scab. To determine the conditions necessary for an epi¬ 
demic or nonepidemic year, the seasons of 1916 and 1918 are cbntrasted with 
those of 1915 and 1920 (fig. 1). 
To facilitate the discussion, it can be said that most plants were in full leaf or were 
starting growth by the week ending March 4, during 1918. In other words, the 
mean temperatures for February and March were above the normal for these 
months, so growth responded very early in the season. Note also that the 
prevailing temperatures did not approach the optimum for infection until well 
along in April, by which time the plants had about completed their first spring 
growth, and so escaped the disease. Let us contrast these conditions with those 
of 1915. The monthly mean temperature for March was almost 7° below the 
normal. No temperatures suitable for growth and subsequent infection occurred 
until the middle of April. The following week, just as the young growth was 
developing the weekly mean temperature went above the temperature for opti¬ 
mum infection and remained more or less at this point for several weeks. During 
the first two weeks in May almost 4 inches of rainfall occurred on eight different 
days. The three conditions essential for infection were thus well supplied so 
that the year 1915 is noted as the worst scab year on record in Alabama. 
Table II.— Monthly mean temperature y precipitation , and number of rainy days 
at various localities 
MONTHLY MEAN TEMPERATURE 
Station 
Jan. 
Feb. 
Mar. 
Apr. 
May 
June 
July 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
An¬ 
nual 
Manila, P. I. 
77.0 
77.2 
80.2 
82.9 
83.5 
82.2 
80.8 
80.8 
80.6 
80.4 
79.0 
77.4 
80.2 
Los Angeles, Calif... 
51.0 
52.7 
60.1 
60.4 
65.2 
70.6 
76.3 
76.4 
72.1 
64.2 
58.2 
53.2 
63.0 
Mobile, Ala. 
49.8 
53.2 
59.1 
66.0 
75.6 
79.1 
80.5 
79.7 
76.5 
61.1 
57.5 
51.5 
66.1 
Nagasaki, Japan. 
42.4 
42.4 
48.7 
57.9 
64.4 
71.1 
78.3 
80.0 
74.5 
64.6 
54.9 
46.0 
60.4 
MONTHLY MEAN PRECIPITATION 
Manila, P. I. 
1.10 
0.39 
0.67 
1.18 
4.06 
9.80 
15.32 
13.78 
14.53 
7.48 
5.16 
2.28 
75.87 
Los Angeles, Calif... 
2.01 
1.98 
2.34 
0.70 
0.35 
0.05 
0.02 
0.14 
0.14. 
0.54 
0.77 
1.57 
10.59 
Mobile, Ala. 
4.86 
5.06 
7.17 
4.35 
4.00 
5.95 
7.04 
6.81 
5.02 
3.18 
3.74 
4.57 
62.04 
Nagasaki, Japan. 
2.96 
3.03 
5.04 
8.12 
7.44 
11.10 
10.16 
7.73 
7.09 
5.52 
3.58 
3.64 
74.29 
NUMBER OF RAINY DAYS 
Manila, P. I. 
Los Angeles, Calif... 
Mobile, Ala_ 
Nagasaki, Japan. 
5 ! 
3 
3 
4 
9 
16 
21 
21 
20 
16 
12 
8 
138 
6 
6 
7 
4 
3 
1 
0 
0 
1 
3 
2 
6 
40 
11 
10 
10 
8 
8 
12 
15 
14 
10 
6 
7 
7 
121 
16 1 
13 
14 
15 
14 
16 
14 
13 
13 
11 
12 
16 
165 
