Articles
EFFECTS OF DIFFERENT LEVELS OF MANGANESE AND BORON UPON THE NUTRIENT STATUS AND THE INCIDENCE OF INTERNAL BARK NECROSIS IN APPLE TREES
The first symptoms of internal bark necrosis (I.B.N.) appeared at the end of the first growing season in a few trees treated with 15 ppm of manganese in solution.
The mean manganese content in mid-shoot leaves sampled at the same time from that treatment was 561 ppm and the leaf manganese content of the affected trees ranged from 538 to 865 ppm.
The mean leaf manganese content of the trees treated with 1 and 8 ppm of manganese was 161 and 364, respectively, with no evidence of I.B.N. symptoms.
In the second growing season all the trees treated with 15 ppm of manganese were affected by I.B.N. and most of the trees treated with 8 ppm of manganese in solution showed some symptoms of I.B.N.
A low level of boron (0.01 ppm) in the nutrient solution, as compared with 0.5 ppm, enhanced the incidence and increased the severity of I.B.N. induced by the above indicated high levels of manganese.
At the end of the first growing season, the leaf boron content in the low boron (0.01 ppm) treatments was 21 ppm in mid-shoot leaves and 15 ppm in terminal leaves, and in the normal boron (0.5 ppm) treatments 37 and 41 ppm, respectively; the leaf boron content of the affected trees showed a high variation, ranging from 18 to 38 ppm of boron in mid-shoot leaves and from 7 to 30 ppm in terminal leaves.
No significant difference in the intensity of I.B.N. was found between the trees grafted on two different rootstocks.
It was concluded that manganese toxicity is the main causal factor of I.B.N. and the low level of boron supply may be a modifying factor.