Broom's Barn Applied Crop Sciences

Causes of poor patches in sugar beet fields

It is common to see patches in a beet field where the plants do not seem as healthy, or as large, as in the rest of the field, or where there are simply fewer plants. Often the reasons for the poor areas are obvious (e.g. compaction on headland or gateway) but sometimes diagnosis is difficult. Examination of roots, leaves and soil may help in identifying the problem.

Development of a diagnostic key based on symptoms in the field would in practice be difficult because 1) symptoms of many problems vary or, in the case of some types of herbicide damage, may be unknown 2) some problems may cause other, more conspicuous problems to develop. For example, pest or disease damage to roots may induce nutrient deficiencies. Diagnosis of the more visible symptoms alone may lead to the underlying cause being ignored.

Click on images to view enlarged versions

Weather and soil conditions

Poor soil structure is a common cause of stunted growth. Soil that has consolidated due to heavy rain, and then hardened during hot dry weather, becomes difficult for roots to penetrate. Some fields have only a shallow topsoil layer in which roots can grow easily, before they reach a harder-to-penetrate layer of clay or chalk below. Often a layer of hard soil can be detected by digging down with a spade or trowel.

Poor growth due to compacted headlands and variable seedbeds

If soil structure is the problem, many of the stunted plants will have abnormally shaped tap roots, with branches growing horizontally. Affected plants may be unable to take up adequate nutrients from the soil and as a result show a nutrient deficiency.

Waterlogged soil or a straw layer in the soil can also be difficult for roots to penetrate and may have a similar effect on root growth. Waterlogging can induce wet rot (see later) and nutrient deficiencies. Differences in soil type or drainage may also account for growth differences, particularly during drought. Beet on light soil are more likely to wilt in dry weather. The type of subsoil, as well as the top soil, influences growth. During drought, poor patches may develop over gravel subsoils.

Diseases

Virus Yellows is the name given to a disease caused by one of several viruses and spread by aphids, particularly Myzus persicae. With Beet Mild Yellowing Virus (BMYV) leaves develop a golden yellow coloration and become thickened and brittle. It is introduced into fields by winged aphids. Wingless aphids, in the colonies started by the winged ones, then spread the disease within the crop. Consequently, the disease may appear as numerous small roundish yellow patches in beet fields, although in severe cases all plants within a field may be infected. The severity of the disease varies greatly from year to year and field to field.

The other viruses that cause virus yellows are Beet Chlorosis Virus (BChV) and Beet yellows Virus (BYV). BChV is a similar virus to BMYV and produces similar symptoms. The yellowing produced by BYV is more lemon coloured, less golden, although commonly there are red spots. (Note that plants that have survived attack by downy mildew - see below - often display yellowing closely resembling virus yellows.)

The first sign of downy mildew 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, most of which eventually blacken and die. Cracks may then develop in the tap root crown. Some plants succumb to the disease, but the leaves on those that survive usually turn yellow. The yellowing is very similar to that produced by virus yellows (see above). Although it typically infects several plants within a small area, these plants may not be adjacent to each other and are often somewhat scattered.

Rhizoctonia is a fungus that attacks the roots. It is very common, particularly on unhealthy plants. Normally it seems to be a secondary problem but occasionally, particularly on light soils, it seems to be more aggressive. Then it can be found amongst the lateral roots and over the tap roots also. Occasionally it is associated with nutrient deficiencies and then there is a problem deciding which is the primary and which is the secondary problem. In severe cases, plants are badly stunted with misshapen tap roots, and quite often, a profusion of lateral roots. The patches may be stretched out along the direction of soil cultivations etc.

Rhizomania is a disease caused by a virus (Beet Necrotic Yellow Vein Virus). The virus is spread by a soil fungus, Polymyxa betae, which is widespread throughout UK soils. Infection is often revealed by the appearance within fields of patches of plants with pale foliage. This paleness is caused by N deficiency, which is induced by damage to the roots. Badly affected plants may also have a profusion of lateral roots and the vascular tissue at the tip of the tap root is distinctly brown.

Violet root rot, a fungal disease of roots, commonly affects areas within fields. Affected plants wilt more readily than healthy ones - as in picture opposite. It is easily identified during the early stages of infection as the roots are distinctly purple. Later, however, the infection turns black due to invasion by secondary rots such as Fusarium. Wet Rot, which causes the bottom of the tap root to go rotten, occurs in waterlogged areas of fields.

Pests

Most pest damage is patchily distributed within fields. For example, damage by capsid bugs is usually more common adjacent to the hedgerows and grassy banks where they overwintered. Other insect pests are also often more numerous near hedgerows because the adults were deposited there by the wind. However, this section describes pest damage that typically causes patches of poor plant growth, and where the cause may not be too obvious.

Beet cyst nematode. If patches of stunted unhealthy-looking beet appear in fields from June onwards, the roots should be checked for beet cyst nematode, particularly on light soils. Diagnosis involves searching for the cysts (mature female nematodes full of eggs). These are visible to the naked eye, and usually attached to the lateral roots. They are white initially though turn brown later when the adult nematode dies. Affected plants often have a profusion of lateral roots (see picture). Plants may wilt easily and show signs of a nutrient (e.g. magnesium) deficiency due to damage to the roots.

Free living nematodes of Longidorus and Trichodorus species attack roots and stunt the growth of plants. The tap roots are liable to become 'fangy' (branched) and the lateral roots short and stubby. The damage may be widespread within fields, with most plants affected, but typically badly stunted plants are intermingled with large, apparently healthy ones - see picture.

Several species of aphids feed on sugar beet but in the UK only the black bean aphid, Aphis fabae, forms large colonies, sometimes with thousands of aphids per plant. Such large numbers can severely set back growth of plants and even kill them. However, any large increase in aphid numbers is closely followed by a build up in the population of their predators, such as ladybirds and hoverfly larvae. Also, during wet weather they are attacked by fungi. Consequently, aphid populations crash quickly. Usually they do so before much damage is done, but occasionally their populations build up to serious levels. The cause of the damage may not be obvious once most traces of the aphid colony disappear.

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

Nutrient deficiencies

Patches of plants within a crop showing nutrient deficiencies may reflect differences in the distribution of the nutrient concerned in the soil (which in turn may relate to inadequate or uneven fertiliser application), but not necessarily. They may result from a problem which is affecting the ability of the roots to take up sufficient nutrients (e.g. soil compaction, nematode attack).

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 cyst nematodes, or soil compaction. As both these problems typically occur in patches in the field, it follows that Mg deficiency can do the same.

Manganese deficiency is also common in sugar beet and is also referred to as Speckled Yellows. The deficiency often occurs on soils with a high pH, and may be induced by liming. It is often patchy within fields that are of variable soil type. Phosphorus deficiency causes leaves to become reddened or purplish. It is often seen on headlands where the soil is compacted and roots are unable to develop properly. It may also appear when the roots are damaged, e.g. by pests, herbicides or acid soil.

Nitrogen deficiency causes leaves to be pale, verging towards yellow in extreme cases. However, as with some other deficiencies, it is not always caused by a deficiency of the element in the soil but may result from damage to the roots by pests or diseases. For example, it is one of the symptoms of rhizomania.

Herbicides

Accidental herbicide applications 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, 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 very brittle.

Premature death after
tank contamination

Residues of herbicides applied to previous crops can 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 relating the damage in the field to past herbicide applications, or to soil cultivations which may have redistributed the herbicide in the field. Therefore, if damage is patchy, it may follow a pattern.