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Joumal of Agricultural Research v 0 i. xxvm, No. 3 
At the most, only the months of March, April, and May would be suitable for 
scab infection judging from the temperature factor alone. However, we should 
note that for these three months an average of approximately 5 inches of rain 
occurs during about ten days each month. 
At Nagasaki, Japan, the monthly mean temperatures for the first five months 
of the year are from 8° to 11° lower than at Mobile. In other words, the season 
at Nagasaki is from one to one and one-half months later. Scab should be 
prevalent in Japan during the last of April, May, June, and part of July. Opti¬ 
mum conditions for scab infection should prevail during the month of June. 
During these months from 7 to 11 inches of rain occur during approximately 15 
days each month. Scab is more serious in Japan than in Mobile district because 
of the slower start made by the plants in the spring, the larger amount of pre¬ 
cipitation, and the greater number of rainy days. 
After following through the temperature curve for Los Angeles, Calif., we 
would expect that as far as this one factor was concerned scab would be more 
serious than in either Japan or Alabama. Note that the curve follows that of 
Mobile for the first three months and then crosses over and runs along that for 
Nagasaki through July. We should expect then from the temperature stand¬ 
point alone to have conditions favorable for scab during the months from March 
through July. However, one need but look at the amount of precipitation to 
determine why scab can not propagate itself in California. This is a further 
indication that generous precipitation over most of the year is necessary for the 
development and propagation of scab. 
The absence of scab in a locality may be due either to a mean temperature 
too high for the pathogene to infect the host or to a deficiency in rainfall. In 
the Philippine Islands the high mean temperature and possibly the effects of the 
dry season are a sufficient barrier to prevent the spread and development of scab, 
while in California the large deficiency in precipitation alone is sufficient. 
The factors necessary for this disease to develop in any locality are a suitable 
temperature, sufficient moisture, and young growth. Wherever these condi¬ 
tions are fulfilled as in the Gulf Coast States and Japan, scab becomes a serious 
disease. No doubt, as has been pointed out before, we may have epidemic and 
nonepidemic years, depending on the prevailing weather, but some scab is present 
each season. 
DISCUSSION 
In the foregoing pages the relation of environment to the various phases of 
the development of citrus scab, from the standpoint of experimental results 
obtained in the laboratory and greenhouse and from observations made in the 
field, are discussed. The temperature to which the pathogene and the plants 
were submitted under controlled conditions have been constant and not fluctu¬ 
ating as occurs in nature. Observations made under field conditions in south 
Alabama, where the Citrus-growing area is small and rather compact, when cor¬ 
related with results obtained under controlled conditions are in close uniformity 
and the interpretations placed on these correlations can be made specific. Note 
also that we have confined ourselves to the relation of the scab during the devel¬ 
opment of the first spring growth. It is usually only during this period that 
scab does any serious damage. Occasionally scab appears in the fore part of 
June, late in August, or September on the young growth, when there are 
periods of several days during which the mean temperatures and other factors 
suitable for infections are favorable. 
In the discussion of the effects of weather on the distribution and prevalence 
of scab in the other Citrus-growing regions we have tried to point out in a general 
way that there are certain types of weather which either favor or inhibit the 
