Boron deficiency (cork) in pome fruit
Boron is required only in very small amounts by apples and pears, but when it is deficient, disorders can occur. Waterlogged and very dry soil can lead to boron deficiency, as roots are unable to take up nutrients. Granite soil is generally more likely to be boron-deficient than other soil types. Symptoms can develop on fruit, twigs and leaves.
Symptoms
Fruit
Boron deficiency can appear as either internal or external cork. Internal cork is the more common form of deficiency in NSW. It is thought to occur when the deficiency occurs later than 8 weeks after petal fall. Dry climatic conditions are usually the trigger in NSW.
Fruit affected by internal cork often show no external symptoms, but when the fruit is cut open, small, round water-soaked spots will be throughout the flesh. The spots dry out and turn very dark from the centre outwards, finally forming brown spongy lesions, often with a greenish halo. Lesions in the core region sometimes run together, dry out and form cavities. The brown tissue normally occurs closer to the core than the skin. As the corky tissue dies, the fruit becomes misshapen.
External cork is more likely to occur if the deficiency occurs early in the growing season. It causes irregular depressions to appear as the fruit matures. Raised brown or reddish spots can occur. These later crack, and the skin dries up and scales off, leaving a rough corky layer. Dark brown to greenish-black lesions with a green border occur on or just beneath the skin, mostly in the depressions. These can later become dry and corky. When fruit is cut open, corky lesions are scattered throughout the flesh.
In apples, the fruit symptoms of boron deficiency can often be confused with calcium deficiency, especially where cracking has not developed. However, the 2 disorders can be distinguished by the features in Table 1.
Table 1. Comparing fruit symptoms of boron and calcium deficiency.
| Boron deficiency (cork) | Calcium deficiency (bitter pit) |
|---|---|
Brown tissue near the core | Brown tissue appears closer to the skin |
Cracks can develop in the fruit | No cracking |
Symptoms do not worsen in storage | Symptoms often appear worse or develop after storage |
Twig and foliage symptoms
If the deficiency occurs mid-season, dieback can occur in late summer. Leaves on current-season twigs become distorted and turn yellow with red veins. Small brown necrotic areas develop at the leaf tips and margins. The twigs can die from the tips.
If mid-season deficiency is not treated before dormancy, dieback can develop in spring. In spring dieback, buds fail to develop. Affected twigs die back and an abnormal number of small branches might develop from below the dead portion. Dieback can also be a symptom of problems other than boron deficiency.
Boron deficiency can also lead to rosette symptoms in leaves. Leaves become dwarfed, thickened and brittle and are found on very shortened twigs. Zinc deficiency can also cause leaf rosettes, but the presence of corky lesions in fruit will distinguish between the two.
Control
Sprays should be applied in late spring when there is ample foliage to absorb the nutrients. It should not be used when the fruit is small and tender.
Boron can be applied after harvest, but it must be applied when leaves are still green. Boron can also be supplied as part of the orchard solid fertiliser program.
Refer to Apple and pear nutrition for more detailed information.
Warning
Excessive application of boron to trees is dangerous. Do not exceed the recommended dosage rate and do not apply both spray and soil dressing in the same season.
