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Abstract
Die-back and other symptoms consistent with those of boron toxicity developed in spring 1980 and 1981 in 70 and 39% respectively of Sundrop apricot trees that had been given about 2.4 g per tree of B as borax when they were planted in a commercial block in 1978. Trees of the same variety planted in adjacent rows in 1979 which had not received B grew well. Analyses of 13 major and minor elements in buds, leaves, bark, and fruits from both groups of trees showed no consistent differences between the 2 groups of trees except in tissue B levels. B concentrations were highest in buds, with a similar pattern of change occurring during the dormant period in both healthy and unhealthy trees. In spring 1981, buds on unhealthy trees which failed to break contained 40-100 μg/g dry matter (DM) (average 82.3 μg/g DM) B, whereas buds from similar positions on healthy trees not receiving B contained 23-43 μg/g DM (average 37.0 μg/g DM) B. Replant Moorpark apricot trees in another commercial orchard, given greater amounts of B over several years, had developed similar and more pronounced symptoms of ill-health than the Sundrop apricot trees; tissue B concentrations were also higher. The older trees in the planting were less affected. Some symptoms observed on the unhealthy trees in spring were similar to those reported to be caused by the bacterium Pseudomonas syringae pv. syringae van Hall. We speculate briefly on the possible significance of this finding. An experimental planting of Moorpark apricot trees given 3.4 g of B as borax during planting failed to produce leaves in spring, whereas trees not given B produced leaves and grew normally.