It is well known that the young and tender leaves of orange trees are more susceptibly infested by citrus canker (caused by Xanthomonus citri (Hasse) Dowson) than old and hard ones. In this paper, the author wishes to show the most susceptible stage of young leaves on this disease and the time of control by means of fungicides in relation to leaf ages.
At first, stomatal inoculation tests were made upon various ages of young leaves of orange trees in order to distinguish the most susceptible age of leaves.
The developmental stage of young leaves was classified in four grades as follows; age No.1, leaf length<1.5cm, age No.2, leaf length 1.5cm-3.0cm, age No.3, leaf length>3.0cm and the leaf is still soft and juicy, age No.4, the leaf is beginning to harden.
The stomatal inoculation test showed that the leaf age No.2 and No.3 were most susceptible.
Next, the most effective time and the number of times of fungicides spraying for the control of citrus canker were examined during the period of young leaves in summer.
These experiments were designed as once, twice and three times of spraying during the period of leaf growth, and once and twice of spraying were arranged each ages of young leaves respectively.
These experiments showed that the most effective control took place in the three times of spraying which ranged all ages of leaves. In the experiments on twice of spraying, more effective control took place by the spraying in age No.3 and No.4 than the others. It seems that the most effective time of control for citrus canker during the growth of leaves may be the leaf age No.3 and No.4.
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