Cover crops species and blends with either biofumigant, trapping or allelopathic mechanisms can be utilised in an integrated nematode management with fungal endophytes to supress stubby root (Trichodorus spp/Paratrichodorus spp) and needle nematodes (Longidorus spp) that cause docking disorder in sugar beet (Beta vulgaris)
Sugar beet (Beta vulgaris) is the second most important sugar crop in the world after sugarcane (Ahmad et al., 2017). Sugar is the primary product derived from sugar beet and the biproduct, pulp is used as livestock feed. The crop covers 3.7% of total area under crops and supplies 55% of sugar consumed in the United Kingdom (Tzilivakis et al., 2005). Sugar beet is an economically important crop in East Anglia and the East Midlands areas of England, occupying in the region of 105,000 hectares; .15 M t of sugar were produced from the 2018/19 harvest. Plant parasitic nematodes PPNs) are important pests for many crops globally and result in a c. 14% loss of production annually, which equates to losses of £132 billion. The sugar beet crop is no exception, being subject to infection by a variety of different PPN species, including those categorised as free-living. The most common nematodes affecting sugar beet production are beet cyst nematode (Heterodera schachtii), root knot nematodes (Meloidogyne hapla and Meloidogyne chitwoodi) and stubby root nematodes Paratrichodorus and Trichodorus species (Hafez, 1998).
In the UK, stubby root (Trichodorus and Paratrichodorus spp.) and needle nematodes (Longidorus spp.) are the most economically important genera of free-living PPN. In alkaline sandy soils at Docking, Norfolk, and elsewhere in eastern England, sugar beet and other crops suffer a patchy stunting called ‘ Docking disorder ’, where multiple tap roots are formed (fanging). Trichodorus spp and Longidorus spp were found to be more abundant in the spots where this disorder was detected as compared to where sugar beet was healthy Seven species of stubby root nematode i.e. T. primitivus (de Man), T. viruliferus Hooper, T. similis Seinhorst, T. cylindricus ,T. teres Hooper and T. anemones and two species of needle nematode i.e. Longidorus attenuatus and Longidorus elongatus have been reported in field where the docking disorder occurs by Whitehead and Hopper (1970). T. primitivus (de Man) and T. cylindricus have been reported as the most abundant species in the genus Trichodorus, while L. attenuatus is most predominant Longidorus spp in East England.(Whitehead & Hooper, 1970) Meanwhile, feeding by Longidorus spp. can cause diffuse patches of plants of uneven size, sometimes with nitrogen and manganese deficiency. Swellings or galls may occur at the root tips and this is often accompanied by local necrosis around the point where feeding took place (Evans, 2007; Whitehead & Hooper, 1970).
Nematode activity and subsequent damage occur when the soil conditions are moist during seedling establishment and as soil moisture decrease and temperature increase, the nematodes move deeper into the soil. The intensity of damage caused by nematodes is dependent on the nematode densities in the soil. Densities exceeding 100/litre of soil may affect the crop (Evans, 2007), densities of up to 4500/litre of Trichodorus cylindricus have been recorded after a crop of sugar beet (Whitehead & Hooper, 1970).The stubby root nematodes and the needle nematodes are also responsible of transmitting several plant viruses in crops i.e. nepovirus and tobra viruses respectively (Brown et al., 1995).
Currently, there is much speculation over the impact of cover cropping on free-living PPN species. Several seed suppliers claim that their cultivars will reduce nematodes, but there is also some anecdotal feedback to suggest the contrary. It is possible that the confusion is due to generalisation and a lack of knowledge about specific nematode-plant interactions. However, there is little, if any, research conducted with UK sugar beet crops to support appropriate use of cover crops for this purpose. The stubby root nematodes (Trichodorus and Paratrichodorus spp.) and the needle nematode (Longidorus spp.) have a wide host range, therefore the research will focus on development of multiple nematode management strategies that can be recommended in an integrated nematode management programme to sugar beet growers. The research will explore the use of cover crop species and blends with different mode of actions. Cover crops may reduce nematode population densities through being poor hosts (trap crops), biofumigants (certain brassica species) or releasing toxic compounds from their roots (allelopathy) (Ntalli & Caboni, 2017). Such mechanisms of suppression require thorough investigation.
Integrating endophytes with cover crop species will also be investigated. Fungal endophytes colonise the plant tissue and initiate a beneficial relationship that does not negatively affect the plant. They have been successfully used as biological control agents for nematode management as they protect the crop either through production of bioactive compounds/secondary metabolites (antibiosis), that can either directly kill the nematode, repel, paralyse, interfere with host finding ability, interfere with nurse cell development or through competition for resources (Poveda et al., 2020). Studies have also shown a fungal endophyte, Epichloë (syn. Neotyphodium) spp. helped in inducing resistance in perennial ryegrass where colonised plants experienced increased plant growth, reproduction and resistance to various biotic and abiotic stress factors (Esqueda et al., 2017). Exploration of integrated management of cover crops and endophytes will be key in development of a strategy that has multiple mode of actions against mixed populations of free-living parasitic nematodes occurring in sugar beet fields.
British Beet Research Organisation, Joordens-Zaden, RAGT Seeds and The Lugden Hill Charity
Harper Adams University