Boron deficiency

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Characteristics and occurrence

Sweetpotato seems to be more susceptible to boron (B) deficiency than many other crops. Boron deficiency has long been recognised as a significant problem of commercial sweetpotato crops in the USA, particularly as a cause of poor root quality. Cases have been recorded in many other countries, in a range of environments. 


Boron deficiency affects actively growing tissue, both of the shoot and the roots. The first sign is usually a thickening of young leaves. The leaves and stem near the shoot tip are brittle and break easily when bent. Other symptoms are variable, depending on the cultivar and growing conditions. Young leaves are usually paler than the older leaves  but the extent and pattern of chlorosis is variable: it may be uniform, or diffusely interveinal, fading gradually with distance from the main veins. In other cases it may be a less distinct interveinal mottle. Leaves may become puckered (slightly raised in interveinal zones) and the tip and lateral lobes may curl down. Petioles may twist. Internodes may be shortened, producing a compact habit around the apex. In some cases, the leaf veins may appear thickened, bumpy or even callused. Deeply lobed cultivars may display reduced development of the lateral lobes.

At greater severity, B deficiency causes death of the growing points. The dominant tip is usually the first affected, but the axillary buds also become necrotic.

Fibrous roots of B-deficient sweetpotato plants become short, stumpy and highly branched, producing the coral-like structures typical of B deficiency in many plant species.

Storage roots are often short and blunt-ended, and may have an increased incidence of splitting.  Splits formed early in development tend to look healed and overgrown. The skin may be rough and may wrinkle towards the ends. The cut root exudes less white sap than normal, and the flesh may be mottled or corky in places. Orange-fleshed roots are generally paler than normal. The flavour is less sweet, or even bitter. Some cultivars may display irregular thickening of roots, resulting in dumbbell and spindle shapes. Other disorders reported to be associated with B deficiency are “internal brown spot” and “sweetpotato blister”. Internal brown spot is characterised by the occurrence of brown necrotic areas in the flesh, irregularly distributed but more prevalent in the cambial zone, close to the surface of the root. Sweetpotato blister refers to raised dark spots which develop on the skin of some cultivars following a period of storage, and may be sparsely scattered or numerous, covering a large proportion of the root surface.

Storage root development may be severely reduced or completely inhibited by severe B deficiency.

Possible confusion with other symptoms

Symptoms on foliage might resemble those of false spider mite, zinc deficiency or mild iron deficiency.

Blister-like bumps and cracking of storage roots can be symptoms of root-knot nematode.  Rapid changes in soil moisture can also cause cracking.  Cold soil or storage temperatures might also cause the absence of latex sap and mottled, poor-textured flesh.

Calcium deficiency may also result in the death of shoot and root tips. However, in the case of Ca deficiency, young leaves below the shoot tip are not thickened but develop a necrosis which spreads from the lateral margins.

Diagnostic soil and plant tissue tests

A critical concentration of 40 mg B/kg in the 7th to 9th youngest leaf blades was determined in solution culture studies using cv. Wanmun. This seems to coincide well with field observations on a wide range of cultivars.

With regard to soil analyses, the relationship between hot water extractable B and B deficiency is modified by a number of factors including soil texture, pH and available Ca. There is no published information on soil B in relation to B deficiency in sweetpotato. Critical concentrations quoted in the literature for a wide range of crops mostly lie in the range 0.3-0.5 mg/kg hot water extractable B, but may be up to 1 mg/kg for susceptible species such as beet, and may be higher in alkaline soils than in acid soils.


Cultural control

Application of borax or other borates (eg. “Solubor”) to the soil before planting. Recommended rates are 1-1.5 kg B/ha on sandy, acid soils, or up to 4 kg B/ha on clayey, alkaline soils. Overfertilisation may result in B toxicity, so it is best to aim for the lowest effective rate, which may be further reduced on subsequent crops.

Foliar application of B is often recommended for other crops, as it allows lower dose rates with less risk of accumulation in the soil. However, it is unclear whether foliar treatment is effective for sweetpotato. Boron is not readily transported within the plant from vines to roots, and while the tops may appear healthy after foliar spraying, symptoms may persist on the storage roots. The mixed reports from this method may depend on how much of the B is subsequently washed onto the soil.


Bourke, R.M. 1983. Crop micronutrient deficiencies in Papua New Guinea. Technical Report 83/3, Department of Primary Industry, Papua New Guinea.

Bradford, G.R. 1966. Boron. In: Chapman, H.D. (ed.) Diagnostic criteria for plants and soils. University of California, Riverside. pp 33-61.

Miller, C.H. and Nielsen, L.W. 1970. Sweet potato blister, a disease associated with boron deficiency. Journal of the American Society for Horticultural Science 95, 685-686.

Nusbaum, C.J. 1946. Internal brown spot, a boron deficiency disease of sweet potato. Phytopathology 36, 164-167.

O’Sullivan, J.N., Asher, C.J. and Blamey, F.P.C.1997. Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

O’Sullivan, J., Loader, L., Asher, C., Blamey, P. 1997b. Troubleshooting nutritional problems in a new industry: sweet potato in North Queensland. Proceedings of the First Australian New Crops Conference, Gatton, July 1996. Rural Industries Research and Development Corporation, Australia.

Pillai, N.G., Mohankumar, B., Kabeerathumma, S. and Nair, P.G. 1986. Deficiency symptoms of micronutrients in sweet potato (Ipomoea batatas L.). Journal of Root Crops 12 (2), 91-95.

von Stieglitz, C.R. and Chippendale, F. 1955. Nutritional Disorders of Plants. Queensland Department of Agriculture and Stock.

Willis, L.G. 1943. Apply borax to improve quality of sweet potatoes. North Carolina Agricultural Experiment Station Special Circular 1, 1943.



Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Compact growth, mottled chlorosis, mild puckering and downward curling of young leaves on a B-deficient plant in PNG (J. O'Sullivan).

Deformity of a young leaf, including thickening, reduction in lobes and irregular margins (J. O'Sullivan).

Boron-deficient leaves generally become thickened and brittle, and tend to curl downwards.  Tip death is common (J. O'Sullivan).

Symptoms on a B-deficient crop in PNG, including small tubers with splits, vine tip death and small, thickened, chlorotic young leaves (R.M. Bourke).

A vine showing short internodes to the right of marker, and lengthened internodes on growth since boron was applied (L. Loader).

Short, blunt-ended roots from a B-deficient crop (left) and normal, spindle-shaped roots developing on a younger crop to which boron was applied (L. Loader).

Healed and overgrown cracks on B deficient storage roots (C. Asher).


Blisters on storage roots caused by boron deficiency (C. Clark, APS).


Short, highly branched lateral roots with thickened, clublike ends are characteristic of severe boron deficiency (J. O'Sullivan).