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Broom's Barn Plant Clinic - Causes of brown patches on sugar beet foliage
Causes of brown patches on sugar beet foliage
Turning brown is part of the natural ageing process of the leaf, and so does not necessily indicate a problem. However, there are many reasons why a leaf may turn brown prematurely and this page examines some of those causes.
Click on images to
view enlarged versions | Links are provided within the text to appropriate sections of the BBRO Growers Guide. |
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Beet leaf miner,
the larva of the mangold fly, eats the tissue between the upper and lower leaf surfaces. Where it has eaten, the leaf first appears pale and later brown. It is a common pest but most attacks are of little consequence economically. However, occasionally the adult flies arrive in the beet fields, and lay eggs on the leaves, early enough for control measures to be required. Also, there is a risk that normally-safe herbicides can damage plants that are already damaged by beet leaf miner.
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Yellowing resulting from infection by any of the viruses that cause
Virus Yellows
(the picture is of Beet Mild Yellowing Virus) is followed by the appearance of brown spots on the leaf as it is invaded by secondary pathogens such as Alternaria. |
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In some years bacterial leaf spot is never seen but in warm wet summer weather outbreaks may occur. However, they are transient and of no importance. The brown spots are often associated with yellowing. |
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In some years in some fields
downy mildew
can be a serious disease. In late spring / early summer the undersides of the heart leaves develop a purple-grey covering of the fungus, then later turn brown and die. A small proportion of plants succumb to the disease and those that survive may be multi-crowned and develop bright yellow leaves in the summer. |
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Ramularia
is common but control measures are rarely required. The first stage in the development of the disease is the appearance of isolated brown spots, but eventually these may become so numerous that they coalesce to form large brown patches. Cercopsora is similar but rare in the U.K. |
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Rust
appears in the crop during from July onwards, and if attacks are severe control measures may be warranted. It produces red-brown spore-bearing pustules, about 1mm acoss, on the leaves. Brownish marks remain after the spores have been shed. |
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Magnesium deficiency
often appears early in the season and begins as a pale yellowing around the edge of the leaf. As the yellowing spreads inwards it becomes more intense and the tissue soon turns brown due to invasion by secondary fungal pathogens such as Alternaria. With Mg deficiency there is a high ratio of brown to yellow tissue on the leaf. It can be induced by problems affecting the roots. |
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The first indication of
manganese deficiency is the appearance of faint yellow blotches on the leaves. This symptom often appears in the crop during rapid growth in spring and often develops no further. However, in severe cases the yellowing becomes more intense and eventually the blotches merge and turn brown. The leaf margins may turn inwards. Most soils have sufficient manganese but in certain conditions, such as high pH, it may become inaccessible to the plants. |
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Although boron is only required in small quantities by plants
boron deficiency can have severe effects on growth and yield. The deficiency has several symptoms. The leaves and the leaf stalks may develop cracks and the tissue around these cracks turns brown. Alternatively some leaves may turn yellow. Tap roots may become rotten at the crown - the problem is often termed 'heart rot'. |
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Frost is often the first cause of brown patches apppearing on leaves during a season and can cause serious damage to young seedlings. The symptoms vary. On some occasions the cotyledons may turn yellow, then brown, at the tips and, in extreme cases, be completely destroyed. Often the stemns are 'burnt' by a frost layer on the soil surface. |
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Lightning occasionally strikes in fields, even if there are trees nearby. Some plants at the centre of the strike die, whereas others may be damaged but survive. The electrical current runs across the soil surface and scorches the foliage and upper part of the tap root. In addition, the tap root is boiled because the water in the soil heats. Often the tip of the tap root dies and the surviving upper part sends out numerous new lateral roots. Brown patches and streaks develop on the leaves and petioles. |
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Leaves that wilt during hot dry weather may touch the ground and be scorched. The scorching commonly occurs around the edges of leaves, as in the picture, but when wilting is severe and many leaves are on the ground, leaves may be partly protected by the leaves below them and then it is possible for just the inner areas of the leaf to be scorched. |
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Top dressings of fertiliser are usually washed into the soil by the rain shortly after application. However, if there is no heavy rain, any fertiliser that rests against or on a seedling may scorch the foliage as it slowly disintegrates. The pattern of damage is scattered plants |
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Damage by tractors and other farm machinery passing through the crop is unsightly but if they avoid running over the crown of the tap root it is usually fairly light. The cause of the damage should be obvious. |
roundup spray drift |
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 are very variable and can include discolouration of the leaves. 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 and 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 |
