This study was carried out to obtain basic data about the cause and control of shoot dieback disorder occurring on the pear trees (Pyrus serotina Rehder) in Korea. The mineral contents of normal and abnormal trees and soils have been investigated in the some sampled orchards, and trees grown in sand culture. The results were as follows;
1. Symptoms of shoot dieback in field condition consist of midrib chlorosis in July, distorted upward leaves of shoot apexes, development of lateral shoots, dying shoot downward, cracked fruits, and partly dead lateral buds.
2. Boron contents in soil of disorder and normal orchards were 0.56 and 0.14ppm, respectively, and boron contents in leaves of abnormal and normal trees were 120 and 39ppm, respectively. Boron contents in soil of disorder orchard and in leaves of abnormal trees were higher than that of normal orchard and trees.
3. In high boron treatments, shoots showed dieback and chlorosis of midrib. Lateral shoots were developed, and lateral buds were dieback. These symptoms were all the same as symptoms in the field conditions.
4. Boron contents in leaves and shoot barks of disordered trees grown in sand culture were above 122.2 and 166ppm, respectively.
5. In high boron treatments, boron contents in leaf and shoot bark were increased, while the degree of shoot dieback severity was not different among above 0.5ppm boron treatments.
6. In high boron treatments, potassium content in leaf was decreased, phosphorus content was increased, and nitrogen, calcium, and magnesium contents did not affected.
7. It may, therefore, be concluded that shoot dieback of pear trees resulted from boron toxicity.
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