Broom's Barn Applied Crop Sciences

Causes of yellowing and other discoloration of sugar beet foliage

Leaves become yellow as part of the natural ageing process (see Senescence below), and so this does not necessily indicate a problem. However, there are many reasons why a leaf may yellow prematurely and this page examines some of those causes.

Links are provided within the text to appropriate sections of the BBRO Growers Guide. This is situated on the UK Sugar Beet Portal for which you need to register.

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Nutrient deficiencies

'Sugar Beet Nutrition - an introduction' on the UK Sugar Beet Portal provides useful information about the nutritional requirements of the crop

Magnesium deficiency is a common cause of yellowing of sugar beet. Initially it appears as a faint yellowing around the edge of the leaf. As it creeps in towards the centre, between the veins, it becomes more intense and is closely followed by the death of the leaf tissue as this is invaded by secondary pathogenic fungi, e.g. Alternaria. The deficiency can result from damage to the roots by nematodes, or soil compaction.

Manganese deficiency commonly occurs in sugar beet and is also referred to as Speckled Yellows. The symptoms are often faint and transient. Occasionally they become more severe and in such cases the 'speckles' coalesce to form, eventually, large brown patches. The deficiency often occurs on soils with a high pH.

Phosphorus deficiency is distinctive as it causes leaves to develop not only yellow patches, but purple ones also, particularly around the edge. The symptoms are often seen on headlands where the soil is compacted and roots are unable to develop properly. They may also appear when the roots are damaged, e.g. by pests, disease, herbicides or acidity.

Nitrogen deficiency causes leaves to be pale, almost verging towards yellow in some cases (see leaf on left in picture). However, as with some other deficiencies, it is not necessarily caused by a deficiency of the element in the soil but may result from damage to the roots by pests or diseases. Such pale leaves are one of the symptoms of rhizomania for example.

When severe, boron deficiency causes leaves to become yellow. The yellowing may be rather similar to that caused by other problems, e.g. virus yellows. However, other symptoms (cracking of the leaves, brown streaks and cracks on the petioles, cracking and rot of the tap root crown) may also develop. Often, not all the symptoms are present but usually more than one occurs, enabling diagnosis.

Iron deficiency is uncommon but is occasionally seen on chalk soils, because the nutrient becomes unavailable to the plant at high soil pHs. The full symptoms (completely yellow leaves) rarely develop because as the roots grow down they usually start to take up sufficient iron. During this phase the veins become etched in green as they carry the iron, which is required for chlorophyll formation, into the leaves. The mottling thus produced is similar to that caused by manganese deficiency, but is rather brighter

Diseases

Beet Mild Yellowing Virus (BMYV) (left) Initial symptoms of infection include diffuse yellow patches on fully-expanded leaves, which eventually expand and merge. The leaves develop a golden yellow coloration and become thickened and brittle. It is the most common of the viruses that cause the disease called Virus Yellows. The viruses are spread by aphids, particularly Myzus persicae, and incidence of the disease varies greatly from year to year and field to field. In severe cases all plants within a field may be infected.

Beet Chlorosis Virus (BChV) is also part of the Virus Yellows complex. It is a similar virus to BMYV, but less common. The yellowing it produces is paler than that caused by BMYV.

(Also note that plants that have survived attack by downy mildew (see below) often display yellowing closely resembling virus yellows)

Beet Yellows Virus (BYV) is another cause of Virus Yellows. It produces similar symptoms to the above but the yellowing does not normally have the distinct orange tinge produced by well-developed BMYV infections. However, as the disease progresses, dark red spots and patches develop. It occurs less frequently than BMYV but causes greater yield loss.

Beet Mosaic Virus (BtMV) is not quite as common as BMYV (above) and is not so damaging. Nevertheless early infection can reduce root yield of a plant by almost 10%. The discolouration is normally pale green, though late in the season it can be rather yellowish. The pattern of discoloration is of a mosaic of small pale patches amongst tissue with the normal green colour.

Tobacco Rattle Virus is spread by Trichodorus spp. nematodes and is rarely widespread in a field. The yellowing is normally rather bright and it is sometimes said that infected foliage appears to have been splashed with paint. Uncommon.

Bacterial Leaf Spot. This disease is normally uncommon but may become widespread in warm damp weather. It is of minor importance only. The yellowing is associated with black spots.

Powdery mildew rivals virus yellows as the most important disease of sugar beet in the UK. It appears in crops from late July onwards. The fungus, which is favoured by warm, dry conditions, becomes visible as a light grey film on the upper surface of older leaves. Eventually the entire foliage may become smothered. Control of the disease is almost always worthwhile.

Downy Mildew. The first sign of this disease is the pale distorted heart leaves in late spring / early summer. On close inspection, a purplish grey fungus can be seen on the underside of these leaves. Some eventually blacken and die, and cracks may then develop in the tap root crown. Some plants succumb to the disease. The leaves of those that survive may turn yellow and then the symptoms resemble those of virus yellows (see Beet Mild Yellowing Virus above).

Pests

Beet leaf miner. The mangold fly lays its white eggs (usually in pairs) on the underside of beet leaves. On hatching the larva, called the beet leaf miner, burrows into the leaf and feeds on the tissue between the upper and lower surfaces. The areas where it has fed are initially pale, but later turn brown. The pest is common but infestations of a crop rarely reach seriously damaging proportions

Capsid bug damage. Capsids have toxic saliva and leaf growth outwards from the point of feeding on the midrib or a vein becomes yellowed and wrinkled. The intensity of the yellowing is very variable. The tissue around the feeding point often turns brown, and sap oozing from this wound may develop a sooty mould. The damage is reasonably common in most years and is generally more severe near hedgerows.

Red Spider mite damage. Red spider mites occasionally cause damage to sugar beet, though as with many pests damage varies greatly from year to year or field to field. To determine whether yellowing in a crop is due to mites, look on the underside of the leaves for the pest and for the webbing that it produces. There are no approved acaricides for use on sugar beet in the UK.

Herbicides

Premature death after
tank contamination

Accidental herbicide applications - herbicides have an important role to play in the culture of sugar beet but they can damage crops when they are applied accidentally (e.g. spray drift from nearby field, tank contaminations) or under the wrong conditions.

The symptoms of herbicide damage are very variable but can include discoloration of leaves, which could be the prelude to their premature death. Some herbicides turn the heart leaves white (see bottom picture opposite), others cause the tissue around the major veins of the leaves to become yellow. Direct applications of sulphonyl urea herbicides can cause leaves to become yellow and brittle.

DFF tank contamination

s.u. residue

Residues of herbicides applied to previous crops can also cause damage, although fewer persistent residual herbicides are now used in beet rotations than previously. Diagnosing herbicide residues as the problem is difficult, particularly if the effects on growth are only slight. It usually involves looking for a pattern to the damage in the field and relating this to past herbicide applications or to soil cultivations which may have redistributed the herbicide in the field.

s.u. residue

Miscellaneous

Senescence is the most common of all the causes of yellowing, and is usually easily recognisable. Generally only the older leaves are affected, though it is possible that a problem such as disease may cause leaves (or even whole plants) to die prematurely. The discoloration is usually more even than in other types of yellowing. However, occasionally it can be difficult to distinguish from, say, virus yellows or from scorch of wilted leaves that have rested on the soil surface during hot dry weather.

A chimaera is the presence on an organism of a part that is genetically different from the rest. A chimaera that occasionally occurs on sugar beet plants is areas of white or pale yellow tissue, resulting from lack of chlorophyll, on some leaves. Only isolated plants, at the most, are affected.

During the summer the petioles and midribs of leaves may become red or purple. This appears to be a drought stress response - the plants that are wilting most tend to have the reddest petioles.

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