PRESSURES REQUIRED TO CAUSE STOMATAL INFEC- 
TIONS WITH THE CITRUS-CANKER ORGANISM 
By Forman T. McLEAN 
Professor of Botany, College of Agriculture, University of the Philippines 
and 
H. ATHERTON LEE 
Mycologist, Bureau of Science, Manila 
TWO TEXT FIGURES 
In the study of citrus canker, there have been found several 
species of Citrus with more or less resistance to the disease. 
Notable among such resistant species are the mandarin orange 
varieties which in the orchards of heaviest infection in Japan, 
China, and the Philippines remain almost entirely free from the 
disease. In connection with this disease resistance, interest is 
naturally stimulated to investigate the characters of the disease- 
free varieties which determine their resistance. With fungus 
diseases on various hosts, it has been shown in many instances 
that leaf-attacking fungi send mycelial threads from their spores, 
through the stomata. Leaf-attacking bacteria, such as the citrus- 
canker organism, which are motile in water, possibly can enter 
through the stomata, if the stomatal pores are filled with water. 
It seems scarcely probable that they would be carried through 
the stomata in the dry state, under usual circumstances. 
The authors? have shown in previous papers that the resis- 
tance to canker of the mandarin orange is due to some character 
of the epidermis of that species. It was further shown by the 
Senior writer * that the stomata’ of the mandarin orange, the 
resistant form, differed markedly from the structure of such a 
very susceptible species as the grapefruit. These differences 
*McLean, Forman T., and Lee, H. Atherton, The resistance to citrus 
canker of Citrus nobilis and a suggestion as to the production of resistant 
varieties in other species, Phytopath. 11 (1921) 109-114a. 
*McLean, Forman T., A study of the structure of the stomata of two 
Species of Citrus in relation to citrus canker, Bull. Torrey Bot. Club 48 
(1921) 101-106. 
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