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 over-wintered cruciferous crops would provide a green bridge for pests and diseases and reduce the impact of rotational strategies which try to minimise year round production. 2.2.4. Soil remediation effects The remedial effects of fertility-building crops have received some attention and are the subject of on-going research. Literature relating to this has also been considered within the DOVE project (Defra, OF0168). Brassica crop residues can produce biofumigation effects, which reduce pests and diseases (Kirkegaard et al., 1996; Morra, 1999; Rosa & Rodrigues, 1999). SAC are currently investigating use of cruciferous crop residues for control of rhizoctonia diseases in potatoes. Green manures generally stimulate soil micro-organisms and this can result in suppression of pathogens. Control of common scab (Streptomyces spp.) of potatoes with green manures is a well-known example, though there are concerns that green material could increase the risk of slug damage. One mechanism involved is antibiotic production by the bacterium Bacillus subtilis, which inhibits Steptomyces scabies (Cook & Baker, 1996). Addition of plant residues or organic matter may favour increased activity of pathogens with strong saprophytic activity. Pot experiments in the DOVE project (OF0168) indicated that interactions between pathogens and soil micro-organisms were complex, involving not only N (e.g. straw locked up N and reduced crop growth), pH changes, effects on plant growth as well as pathogen development. Most studies have examined effects on single pathogens and further work is required at the field scale to establish net benefits against a range of pests and pathogens. Bi-cropping may produce benefits through the interaction between plant species. Fertility crops that cover the lower diseased leaves of crop plants may reduce the upward splash dispersal of fungal spores such as those of Septoria tritici. There have also been effects on leek rust (Theunissen & Schelling, 1996). Soil borne diseases such as carrot cavity spot (Pythium violae) have been reduced by clover intercropping (Theunissen & Schelling, 2000). The mechanisms involved have not been established (Deadman et al., 1996), but it is possible that the intercrop modified available soil N and thereby reduced activity of the fungal pathogen. 2.2.5. Nitrogen release The amount and rate of release of N from fertility-building crops is also likely to influence disease severity. Nitrogen is often the limiting factor for microbial growth in soil and it is likely to influence both beneficial and pathogenic organisms. The N content of foliage has a strong effect in disease severity as demonstrated for yellow rust (Bryson et al., 1995) and the uptake of large quantities of N is likely to predispose plants to fungal diseases. The interaction of N with disease merits further investigation, so that the appropriate balance between crop productivity and disease risk can be established. Written by S Cuttle, M Shepherd & G Goodlass Page 30
A review of leguminous fertility-building crops, Defra project OF0316 2. Rotational aspects 2.3. The interaction of fertility-building crops and invertebrate pests 2.3.1. Introduction There are two main ways in which fertility-building crops or catch crops grown to maintain fertility can interact with invertebrate pests. Firstly, the pests may attack the fertility-building/catch crop and secondly they may carry over into the succeeding crop, resulting in poor establishment and loss of yield. In this section, the pests of fertility builders/catch crops and their potential to attack subsequent crops will be covered separately. In reviewing pests of fertility builders/catch crops, detail will be provided on basic biology and factors that should be taken into account when attempting to predict the likely risk of attack. The interaction between fertility builders/catch crops and the subsequent crop will concentrate on the risks posed by carry over of pests together with details on biology and crop husbandry, which may minimise yield loss. Gratwick (1992) provides a useful summary of crop pests in the UK and much of the information in this section is taken from that source. Control of pests and diseases of grass and forage crops (Anon., 1985) is also a useful reference. 2.3.2. Pests of fertility-building crops Clover Red and white clovers are probably the most common legumes grown as fertility-building crops, either as a crop in their own right or as part of a grass/clover ley. Pests probably play a minor role in the decline of clover in the long-term but can prevent good establishment of the crop. Aphids - Several species of aphid occur on clover. Of these the most commonly found in large numbers are the pelargonium aphid (Acysthsiphon molvae), the pea aphid (Acyrthosiphon pisum), the leaf-curling plum aphid (Brachycaudus helichrysi) and the vetch aphid (Megoura viciae). Damage due to these pests will tend to be greatest during periods of warm, humid weather. Leatherjackets and wireworms - As with grass seedlings, leatherjackets and wireworms may damage clover seedlings at establishment. The most serious damage is during spring and early summer. Pea, bean and clover weevils - The pea and bean weevil (Sitona lineatus) and the other Sitona species, are common on clover crops where both adults and larvae may be important. Adults of the narrower, pear-shaped, clover seed weevils (Apion spp.) may also be found, although they cause little damage. Adult pea and bean weevils overwinter on leguminous crops and become active in the spring and feed on the leaves causing the distinctive notching. Eggs are laid in the soil and larvae feed on the root nodules. Damage to the nodules may also allow pathogenic fungi such as Sclerotinia trifoliorum (severe clover rot) to invade the plants. Stem nematode - Stem nematode (Ditylenchus dipsaci) can be responsible for serious losses in red clover where it is known as “clover sickness”. Races or strains of the nematode exist. The red clover race damages red clover, vetch, lucerne, strawberries and onion, the white Written by S Cuttle, M Shepherd & G Goodlass Page 31
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