Boron Deficiency in Trees and Cover Crops

In birch trees that are boron deficient, the development of the lamina is restricted, resulting in an uneven growth and giving the leaves a blistery surface. The leaves are normally dark green in color but a few chlorotic and necrotic spots may appear on the older leaves.

Reduced extension growth and the development of small leaves are associated with boron deficiency in Eastern cottonwood.

Similar symptoms of boron deficiency have been recorded on a number of eucalyptus species (Eucalyptus grandis, E. citriodora, E. cloeziana, E. torelliana, E. saligna, E. resinifera, E. tereticornis, and E. alba). However, there are indications that species differ in their requirement for boron. For example, E. grandis appears more susceptible to boron deficiency than E. cloeziana.

The first typical symptom is the crinkling and discoloration of the young, unfolding leaves. The buds, which are brittle, die and the lower leaves in the upper crown often become discolored and fall. In some species the leaves become a reddish purple, but in others a yellowing takes place. Normally the discoloration progresses down the tree in advance of the dieback. A bark necrosis can later be expected to start at the buds and progress down the stems, resulting in a gradual dieback.

Boron deficiency is known to reduce the frost hardiness of eucalyptus.

Irregularly shaped red or purple spots can be expected on the upper surface and water-soaked spots on the under surface in holly that is boron deficient.

Young leaves are pale green and deformed when kauri is boron deficient. Apical growth is distorted.

If leguminous cover crops are boron deficient, growth is stunted. Short thick bines are produced which do not spread over the soil surface. Additionally, the leaves are:

• Very small
• Thick
• Brittle
• Misshapen

The veins are frequently prominent. Axillary meristems will develop to a limited extent resulting in “prostrate clump” growth habit.

When mullberries are boron deficient, the young leaves show broken veins and have cracked petioles. Growing points eventually die.

Most species of pine exhibit similar symptoms of boron deficiency including:

• The cessation of growth of the main leader
• Terminal dieback is associated in some species with resin exudation
• Crooked leader growth has been reported in several species of pine

The most characteristic symptom is the cessation of apical growth and the repeated death of the leading shoot. In P. radiata and P. taeda the growing points may become necrotic and the stem apex swell. Young needles adjacent to the apical bud may die and resin exude from the bud. In these two species, the juvenile needles may be bluish-green color and the mature needles show a tendency to fuse.

Crooked leader growth has been especially reported on P. caribaea, P. khasya, and P. patula. P. khasya and P. patula seem less susceptible to boron deficiency than P. radiata and P. caribaea. In P. strobus the primary needles become light blue-green, with yellow/orange tips.

Boron deficiency in rubber trees would be expected only in soils of extremely low boron status as the rubber tree is efficient in absorbing boron. It is particularly sensitive to excess boron supply.

Boron-deficient leaves are distorted, reduced in size, and somewhat brittle. The leaf deformation does not follow any consistent pattern and there is no loss of color. On young unbranched trees, the first sign of boron deficiency is found in the younger, upper stories of leaves on the plant, which will not be separated by any discrete internode. The individual stories cannot be distinguished, resulting in the “bottle brush” appearance of the stem. When boron deficiency is severe, the apical meristem may die and axillary meristems develop prematurely.

Symptoms of boron deficiency in wattle usually first appear on 2 year old trees during the dry season. The first signs are:

• Leaf chlorosis
• Cambial necrosis
• Drying and death of the growing point on the main shoot

The branches also die back. Defoliation and death then spreads steadily downward and inward from the apical growing points. If the rains start before the tree is dead there may be a partial recovery, but further attacks can be expected in subsequent dry seasons, and ultimately the tree dies.