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Blackleg is a seedling disease caused by several soil fungi, principally Aphanomyces cochlioides. The stem and tap root of the seedlings turn black and stringy, and many seedlings die. The disease is more prevalent in warm wet weather. Consequently, late sown crops (including re-sowings) are more at risk. Some seedlings survive attack by the fungus but the tap roots may be badly misshapen. |
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Rhizoctonia is a soil fungus that can infect the roots of sugar beet plants. Under the microscope brown hyphae of the fungus can be seen creeping amongst the lateral roots, and sometimes over the tap roots. Severely infected plants are stunted, although as the fungus appears to thrive better on plants that have been set back initially in some other way (e.g. by drought, poor soil structure) it is often difficult to determine the extent to which Rhizoctonia is responsible for the effect on growth. Sometimes the lateral roots of infected plants are profuse and appear reasonably healthy (as in the picture) but, more often, they have a dark unhealthy appearance. |
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Rhizomania is the name given to the disease caused by beet necrotic yellow vein virus. The virus is spread by Polymyxa betae, a commonly-occurring soil fungus. The disease was first discovered in the UK in 1989, and now over 400 farms are known to be affected. Rhizomania means 'root madness', a reference to the profusion of lateral roots that results from infection by the virus - see picture. The foliage is pale due to nitrogen deficiency (caused by the damage to the root system) and often, when the tap root is cut open, the tip is brown . In severe infections there may be yellow streaks running along some leaf veins. |
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Scab is a common disease caused by the fungus Streptomyces scabies. It produces dark bown round or oval marks on the tap root surface. If the marks are numerous, as in the picture, the infection looks unsightly. However, the fungus is not thought to affect the growth of the plants and is therefore unimportant. |
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The most common cause of galls or tumours on the roots is the bacterium Agrobacterium tumefaciens and the disease it produces is usually referred to as crown gall. However, only occasional plants are affected and, so, the disease is of no importance. |
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Violet root rot is caused by a soil fungus, Helicobasidium brebissonii. Characteristically, it is a dark purple rot on the tap root surface. The rot often, though not always, starts near the tip of the tap root and spreads upwards, and may eventually cover most of the tap root surface. However, normally the rot does not penetrate deep into the tap root tissue though infection by secondary rots such as Fusarium (see below), which can affect the entire root, may follow. If it becomes a problem growers should consider reducing the frequency of host crops (inc. beet, potatoes, carrots) in the rotation. |
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Fusarium is a secondary pathogen, i.e. the tissue must have some other type of damage in order for the fungus to invade. The initial damage may be caused, for example, by violet root rot, cracking during rapid growth, downy mildew, boron deficiency or herbicides. It appears as a white film on the root surface but it can affect the entire root. It spreads very quickly in clamps and, partly for this reason, growers must take harvested beet to the factory as soon as possible after lifting. |
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Wet rot occurs (unsurprisingly) in soil which has been waterlogged for some time, possibly as a result of compaction or because it is at the bottom of a slope. It typically spreads from the tip of the tap root upwards. If the plant survives, the rotten tissue eventually disappears, leaving an odd-shaped tap root. It is caused by the fungus Phytophthora megasperma. |
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Cracking occurs in the root crowns of many plants. Boron deficiency is one cause of this and it is suspected that downy mildew can cause it. However, the problem is very widespread and it seems that it often occurs simply as a result of rapid growth in late summer. Corky tissue develops over the cracks preventing invasion of pathogens. However, sometimes the cracks do not heal sufficiently quickly and pathogenic fungi (see Fusarium section above) and bacteria invade, causing the root to rot. |
