A review of leguminous fertility-building crops, with particular ...

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A review of leguminous fertility-building crops, Defra project OF0316 2. Rotational aspects 2.2. Disease implications of fertility-building crops Consideration of disease interactions and implications with fertility-building crops needs to include: • Diseases affecting the fertility-building crops directly. • Diseases affecting fertility-building crops which will also affect other crops in rotation. • Consequences of incorporation of the fertility-building crop into soil: suppressive or antagonistic effects on soil- or air-borne pathogens. • Poor performance of the fertility-building crop has implications on the subsequent cash crop. Two types of cropping are considered: • Mono-cropping where specific fertility-building crops are grown exclusively for a period of time. • Bi-cropping or companion cropping systems where the fertility-building crop is grown with another arable or vegetable crop (but not necessarily completely synchronous with it). In these systems, fertility-building crops are grown usually as an understorey with arable or vegetable crops. Many of the principles applied to monocrops will still be valid, but there may be other direct benefits from the ‘physical’ presence of foliage of the companion crop, which interferes with disease activity. 2.2.1. Diseases affecting the fertility-building crops directly. Diseases of forage and grass crops have been documented (O’Rourke, 1976) and, in some circumstances, specific problems may lead to failure or impaired productivity of fertilitybuilding crops. However, disease resistant cultivars are often available and can be used to overcome problems or reduce risks of problems. Local knowledge of diseases in the crops selected for fertility-building is important, as is the potential for disease spread from related crop species nearby. For example, stubble turnips are very prone to dark leaf spot (Alternaria brassicae), which is common in other forage and vegetable brassicas and oilseed rape. 2.2.2. Diseases transfer from fertility-building crops The legacy of disease problems left by fertility-building crops is of major concern to growers. The extent of problems will be influenced strongly by previous cropping and its legacy of the soil pathogen population on the field or farm. Thus, farms with a history of legume cropping are likely to carry high populations of pathogens affecting legumes, whilst few legume pathogens may be present where legumes have not been grown for many years. Very intensive legume cropping may leave high soil populations of fungal pathogens which will only decline slowly. Root and foot rot pathogens (e,g Fusarium spp., Pythium spp., Phoma medicaginis) are economically important in arable and vegetable cropping, with field evidence indicating that there are interactions between different legume types such as Pisum, Phaseolus and Vicia spp.. The intensity of total legume types and the intensity of total legume cropping influences disease severity (Biddle, 1984). Ideally, there should be at least a five-year interval between legume crops (Biddle, 1984). The influence of forage legume cropping on the disease risk to subsequent legume crops requires further investigation. Written by S Cuttle, M Shepherd & G Goodlass Page 28
A review of leguminous fertility-building crops, Defra project OF0316 2. Rotational aspects It is clear that disease problems in forage legumes are aggravated by soil and nutrient stress factors and such problems must be minimised for successful cropping. Use of legume fertility-building crops may not be compatible with legume cash crops such as vining or dry harvested peas and dwarf beans if farms have a history of soil-borne disease problems (Biddle & Whalley, 1996). The occurrence of sclerotinia rot (Sclerotinia trifoliorum) in winter beans in the UK appears to be linked to a history of red clover cropping in the 1950’s (ADAS, unpublished data). This disease is common in winter beans but severe attacks occur only spasmodically. Clover rot is caused by the same pathogen and problems also occur in lucerne (O’Rourke, 1976). Clover varieties are now available with improved resistance to clover rot and this should reduce the risk of severe problems in future. The fungus survives for many years when buried in soil by means of large resting bodies (sclerotia). Red clover has been more susceptible than other forage legumes and it may still create problems for winter beans in the same rotation. Spring beans are affected by a related fungal pathogen, Sclerotinia sclerotiorum, and should be less at risk from the clover rot pathogen (though they may be more seriously affected by the foot rot pathogens). The fungus can be introduced with the seed and healthy seed is a fundamental requirement for organic systems. Some spread may occur through air-borne spores so the occurrence of the disease on the farm may necessitate a change to non-legume fertility-building crops if there is high soil-borne inoculum. The impact of soil-borne legume diseases is strongly influenced by soil compaction and poor drainage, both of which aggravate root disease problems. Sites which are prone to such problems shopuld not be considered for legume cropping. Fertility-building crops of legumes are likely to be a source of virus diseases to nearby legume crops and occasionally other crops (Hagendorn, 1984). For example, pea enation mosaic virus overwinters in clover and vetch and is spread by the pea aphids to a range of legume species in the spring. Similarly the occurrence of severe violet root in organic sugar beet (Gladders, pers. comm) is believed to be related to clover cropping within the rotation. Violet root rot is known to survive better if perennial weeds are present and clover roots may be having the same effect. 2.2.3. Suppressive or antagonistic effects on soil- or air-borne pathogens Cruciferous fertility-building crops can be affected by clubroot disease (Plasmodiophora brassicae) and problems are likely to occur where there is existing soil-borne inoculum. The disease is favoured by acid soils and maintaining a neutral or slightly alkaline pH (7.0- 7.3) is an important part of its management. Wet conditions are required for infection and the disease is widespread in livestock areas, which usually have higher rainfall and a history of growing susceptible brassicas (e.g. swedes and turnips) for stockfeed. The spores of the clubroot fungus can survive passage through animals and infested manures from stock fed on affected root crops can introduce the disease. Cruciferous crops are likely to share a range of soil-borne pathogens, which can interfere with seedling establishment and crop productivity. Rotations with five years between cruciferous crops are generally advocated though in some areas such as South Lincs, vegetable brassicas have been grown successfully in almost continuous production. If fertility-building crops are of short duration, it may be possible to use them frequently, particularly if the farm has a naturally high pH and no history of clubroot. However, use of Written by S Cuttle, M Shepherd & G Goodlass Page 29
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