pest and pathogens in soil

Plant roots are a key habitat for a number of soil organisms including symbiotic bacteria, mycorrhizal fungi and root pathogens and herbivores. Different plant species and even cultivars may show different root exudates or leachates that either stimulate (susceptible crops and varieties) or inhibit (resistant crops and varieties) the germination of specific pathogenic organisms.

A combination of both plant and soil specific factors determine which organisms thrive in the rooting zone – this biologically active zone is often known as the rhizosphere. Around 4-10% of the root surface has been shown to be covered by soil microbes. 

In addition a wide range of soil micro-organisms that can cause plant disease, dominant amongst them fungi can be stimulated in the rhizosphere. The only disease of significance to grass seedlings is Fusarium culmorum. Plant diversity has been shown to contribute to plant community resistance against pathogens by fostering beneficial bacterial communities, which can out-compete the fungi. Therefore mixed species swards are expected to be at reduced risk of this wilt disease.

Survival of pathogens and infectivity with regard to the host are affected by a range of factors. In most cases the exact mechanism remains unknown and it has been observed that the more that is written about the subject the less we really know. The interaction between plant species and soil-organisms is critical – non chemical approaches to the control of soil borne disease include careful crop rotation design and increasingly the selection of resistant varieties.

There may be significant short-term effects of OM inputs within the soil food web (e.g. changes in predator/prey interactions) caused by the relative availability of energy/nutrients; and some inputs have been shown to suppress pathogenic organisms, usually by stimulating rapid microbial cycling.

During the establishment phase of new leys, little active root material is present and so pest and disease attack is concentrated on the new seedlings, which is often devastating. Direct damage to plant roots by root feeding and parasitic organisms can be considerable particularly from insect larvae (often known as grubs or maggots). The key insect pests affecting grassland, especially re-seeds, are the larvae of frit flies, chafer beetles, craneflies (also known as leatherjackets) and sitona weevils. These larvae are also an important source for food for insectivorous birds.

When establishing new leys, producing a fine firm seedbed encourages the young seedlings to establish, grow quickly and be better able to withstand attack by pests and diseases. Rolling can also close the large pores and cracks in the seedbed in which slugs lurk and when slugs are exposed on the soil surface with nowhere to hide they will fall prey to birds and various mammals. For some pests, insecticide treatments are available e.g. chlorpyrifos – see the related resources for guidance on pesticide selection and use. 

It is well known that there are naturally resistant crop varieties, including those whose resistance has been enhanced through careful breeding and selection. Some soils also seem to be naturally disease suppressive.  A wide range of interactions between soil organisms (including parasitism, direct and indirect antagonism) are thought likely to contribute to disease suppression together with interactions with soil chemical and physical properties.  Some soil micro-organisms are known to have direct inhibitory effects on various soil pathogens – though this is usually demonstrated most clearly in pot experiments, effects are much less easily shown in the field.  However, introducing a single organism to soils seldom achieves disease suppression for very long. If not already present, the new organism may not be competitive with existing microorganisms. If food sources are not abundant enough, the new organism will not have enough to eat. If soil conditions are inadequate, the introduced beneficial organism will not survive. In general, practices that enhance the overall health of the soil will tend to increase disease suppression and those that harm soil life tend to reduce it. See How do farming practice affect soil life.

Fusarium culmorum  (seedling blight)

Spores and other propagules of this disease-causing fungus are present in virtually every crumb of soil in the UK.  It has been shown that on average across a wide range of soils, sowing dates and sites on average some 20% of ryegrass seedlings are killed by seedling blight caused by this fungus.  Ryegrass is a particularly susceptible host with the disease taking effect after the rye grass has germinated, but before emergence.  Sowing shallowly (c. 1cm / half an inch) when soil is not too dry helps to limit fungal attack.

Frit fly larvae

Frit fly larvae (<1mm yellowish white grubs) are an important pest of newly sown perennial and Italian ryegrass, though clover, timothy and cocksfoot are not affected.  The main infection period is in the autumn when eggs are laid on and around the seedlings and the tiny larvae burrow into and mine within the base of the developing plants causing death or reducing vigour. Larvae present in old grassland are easily transferred to the emerging seedlings of the new vulnerable seedlings of a re-seed, especially if direct drilling, and this can occur at whatever time of year the re-seed occurs.

Leatherjackets (cranefly larvae)

Leatherjackets are the soil dwelling larvae of craneflies (daddy long-legs).  Leatherjackets have dull olive/ greyish brown tubular bodies (30 mm or just over an inch long) with no legs or any obvious head. They eat roots and attack the root system just below ground level.  Established grassland is often host to a large population of leatherjackets, which have a relatively small impact in the established sward. In fact, in an established mixed species sward, root eating larvae can help to promote the transfer of fixed N between legumes and associated species.

However, leatherjackets can cause large bare patches, even almost total failure, in a re-seed, as a wide range of both grass and clover species are affected. The larvae are often more numerous after a wet autumn which promotes their survival.  Use of a heavy roller to produce a firm seedbed without few large pores and cracks can limit their motility and hence significantly reduce any impact before the new sward is established.  Leaving a short fallow period (at least 2 weeks) between cultivation of the old grassland and sowing can also reduce the impact of leatherjackets. 

Chafer beetle larvae

Chafer beetle larvae feed on decaying plant material or plant roots, depending on the beetle species, and can cause significant damage to new leys.  The larvae (c. 20 mm, ¾ inch) have stout white bodies curved into a C shape with brown heads and three pairs of legs at the head end. There are usually less numerous and less troublesome than leatherjackets but live slightly deeper in the soil.

Sitona Weevil larvae

Sitona weevil larvae attack the root systems of mature red and white clover plants, but the adults attack seedlings as well as leaves of older plants. The adults which are the size of a match head can graze seedlings down to a stump.

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