244 
Journal of Agricultural Research voi. xxviii, no. 3 
INFLUENCE OF ENVIRONMENTAL FACTORS UNDER FIELD 
CONDITIONS 
ON THE HOST PLANTS 
Citrus scab is usually prevalent during the development of the first spring 
growth. Under controlled conditions the maximum temperature for successful 
infection is about 75° F. Under Alabama conditions a mean weekly tempera¬ 
ture of 75° F. and above is reached and the first spring growth of the plants 
is usually completed by the first of June. For the purpose of our discussion, 
then, we are interested primarily in the behavior of the plants during the in¬ 
terval required for the development of the first growth period. 
It might be of interest in this connection to correlate the appearance of new 
growth of the more common Citrus plants under Alabama conditions, with the 
prevailing temperatures recorded at Mobile for the first five months of the 
years 1914 to 1920. 
In Table I are listed the weekly mean temperatures from January 1 through 
June 10 for these years. In figure 1 the weekly mean temperatures for 1918 
and 1916 are plotted with the two late seasons of 1915 and 1920 for comparison. 
In 1915 no weekly mean temperatures suitable for active growth of Citrus 
plants were recorded until the week of April 9-15, when the mean rose to 67° F. 
and proceeded rapidly upward. This temperature was sufficient to force all 
plants into activity at practically the same time. 
During the early part of 1916 the weekly mean temperature rose to 59° F. 
and above on three separate occasions, each of which was sufficient to force 
active growth of grapefruit plants but not Satsuma or trifoliate orange. In 
each case, however, these temperatures were followed by killing frosts. It was 
not until the week of March 19-25 that the weekly mean temperature was 
high enough to force new growth. The mean for this week was 68° F. which 
was high enough to force all Citrus plants into active growth. 
While January, 1918, was conspicuous for its low temperatures, weekly mean 
temperatures for forcing and continuing growth of grapefruit plants occurred 
from the middle of February on. By the last of February the weekly mean 
temperatures were high enough for Satsuma and trifoliate orange to develop. 
Notes on the plants made in the isolation field on March 8 showed that the 
majority, which survived the low temperatures of January, were in full leaf 
or starting growth. In fact, scab was observed on a number of plants on 
this date. Without any question, the season of 1918 was the earliest under 
discussion. 
During the 1920 season temperatures high enough to cause active growth of 
grapefruit plants and swelling of the trifoliate orange and Satsuma buds occurred 
during periods in January and February. In fact, grapefruit plants were almost 
in full leaf when killed back by the freezes of February 15 and 16. The lowest 
temperatures of the season occurred during the first week in March. All new 
growth which occurred prior to that time was killed back. With the advent of 
higher weekly mean temperatures it was some time before new growth started. 
The late season of 1920, then, was due to the destructive frosts the first week in 
March. The development of the uninjured buds was also delayed until suit¬ 
able temperatures were again at hand. To summarize, most of the plants were 
in full leaf or were starting growth the week ending February 4 in 1918, March 
25 in 1916, April 1 in 1920, and April 15 in 1915. 
From the above notes it can be clearly seen that the dormancy of Citrus 
plants during the early part of the year is variable and depends to a large extent 
on the prevailing temperatures. The seasons of 1914, 1915, and 1920 can be 
