RELATION OF THE AGE OF CITRUS TISSUES TO THE 
SUSCEPTIBILITY TO CITRUS CANKER 
_By H. ATHERTON LEB 
Mycologist, Bureau of Science, Manila 
FOUR PLATES AND ONE TEXT FIGURE 
INTRODUCTION 
The writer has previously shown that there is an increase in 
resistance to the plant disease citrus canker with the advance in 
maturity of citrus trees: In the present paper experiments 
are presented which indicate that local tissues of citrus plants, 
both foliage and fruit, also increase in resistance as they approach 
maturity. 
The purpose of the experiments was to inoculate fruits of 
different degrees of maturity, using identical cultures and main- 
taining identical environmental conditions, and measuring the 
amounts of canker resulting at such different stages of maturity. 
Similar experiments were carried out on foliage. 
EXPERIMENTAL RESULTS 
Preliminary experiments were begun on fruits of the pine- 
apple orange (Citrus sinensis) in the Philippines. The pine- 
apple orange variety is of Florida origin and grown to some ex- 
tent commercially in that state. Fruits on vigorous, actively 
growing orchard trees were selected in varying stages of ma- 
turity as measured by their size. The fruits were inoculated 
from the same infusion of cultures of the citrus-canker 
organism, Pseudomonas citri, and maintained under identical 
environmental conditions favorable for canker formation. The 
results are shown in Table 1. 
The data in Table 1 show that there is a very considerable 
susceptiblity for fruits of a small diameter, while large fruits 
approaching maturity were but slightly affected by canker, if at 
*Lee, H. Atherton, The increase in resistance to citrus canker with the 
advance in maturity of citrus trees, Phytopathology 11 (1921) 70. 
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