Apr. 19,1924 Relation of Environmental Factors to Citrus Scab 
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development of scab. Obvipusly one could not without a close study of the 
conditions prevailing in an entire Citrus-growing region state positively that the 
whole region would be free from scab, from the weather data obtained from one 
locality in that region. The exact correlation of environment to the develop¬ 
ment of scab in a Citrus-growing area must be left to trained observers present 
in that locality. 
SUMMARY 
1. Citrus scab, caused by Cladosporium citri Massee, has been encountered 
with frequency during citrus-canker investigations made by the writers in 
Alabama during the past four years. 
2. The disease is probably indigenous to the Orient, and has been distributed 
directly or indirectly from Japan to the Citrus-growing regions where it is not 
prevalent. 
3. Citrus scab is a rather serious disease in the Gulf Coast States of Florida, 
Alabama, Mississippi, Louisiana, and Texas; in South China, Japan, and the 
West Indies. Scattering reports of citrus scab have also been made from South 
Africa, Australia, Formosa, Yucatan, Paraguay, Hawaii, India, and the Canal 
Zone. As yet, no scab has been reported from California, the Philippine Islands 
and the Citrus regions of the Mediterranean. 
4. The three essentials for successful infection of Citrus plants by Cladosporium 
citri , under controlled conditions, are the presence of free moisture, young growth, 
and temperatures between 15° and 23.5°C. (59° and 74.3°F.). The optimum 
for the best development of scab appears to be about 20°-21°C. (68°-70°F.). 
5. Under Alabama conditions, temperatures favorable to optimum infection 
usually prevail during parts of April and May. Sufficient moisture is generally at 
hand during this interval for successful infecticfn to take place. The most 
important and variable factor is the development of the early spring growth. 
6. Susceptible species or varieties of Citrus making a slight or slow 
spring growth which matures rapidly are not subject to scab. Likewise, those 
plants which do not start growth until late are also free from scab. In both 
instances it is apparently a question of escape. Plants making a large amount of 
spring growth, with slow maturation of foliage are easily infected and as a rule 
badly attacked. 
7. Any environmental factor or factors including a slight spring growth and 
rapid maturation or late starting, favors scab escape; while any environmental 
factor or factors inducing a large amount of spring growth and slow maturation, 
favors scab susceptibility. 
8. We must consider not only the internal factors influencing the develop¬ 
ment of spring growth of Citrus plants, but also the external factors which may 
inhibit or stimulate these processes. 
9. Conditions essential for an epidemic scab year in Alabama depend to a 
large extent on a late season, coupled with sufficient moisture and the develop¬ 
ment of spring growth at the time optimum temperatures for infection prevail. 
An early season is favorable to scab escape, in that first growth is about com¬ 
pleted when optimum conditions for infection are at hand. 
10. Under Alabama conditions it appears that a light or bad scab year can be 
predicted to some extent by the mean temperature prevailing in March; a tem¬ 
perature below normal indicates a bad scab season; a temperature above normal, 
a light scab year. 
11. The relation of weather and its effect on the distribution and prevalence of 
citrus scab is shown and the absence of scab in certain localities is correlated with 
either a mean temperature too high for infection or a deficiency in rainfall. 
