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Abstract

The loss of nematicides poses a real and immediate threat to UK potato growers. Our project offers a sustainable method of crop protection (optimised partial biofumigation) that could be applied to a wide range of pests and pathogens, while reducing the risk of soil damage and improving soil health

Description

Biofumigation using brassica cover crops is a new crop protection method for UK potato growers, although suppression of cyst nematodes using biofumigation was first recorded in 1925. The technique is based upon the cultivation of brassica cover crops such as Brassica juncea (Indian mustard), which produce high quantities of glucosinolates (GSLs) in their foliage, stems and roots. When brassica tissue is cut and bruised, GSLs and myrosinase are liberated from the damaged cells; in intact/undamaged tissue these compounds are compartmentalised in separate cells. In the presence of water, myrosinase degrades the inert GSLs into glucose and a variety of volatile compounds including thiocyanates, nitriles, epithionitriles, oxazolidines and isothiocyanates (ITCs). Isothiocyanates are toxic to a variety of pests and pathogens and are widely considered to be the key component of biofumigation due to their lower volatility.

Potato cyst nematodes (Globodera pallida and G. rostochiensis) are regarded as the most important pests of UK potato crops. Diminishing chemical control options, such as the recent loss of Vydate (oxamyl), and the lack of commercially resistant varieties for G. pallida, hasten the need for alternative crop protection strategies. The movement towards regenerative agriculture has popularised cover cropping for improving soil health and fertility. Indeed, cover crops are an integral component of farming policies, such as the Environmental Land Management Scheme to be launched in 2022.   

There has been a substantial body of work undertaken on the use of biofumigation for potato cyst nematode (PCN) management in recent years. In vitro studies have provided convincing evidence that isothiocyanates such as 2-propenyl and 2-phenylethyl are nematicidal and inhibit hatching. Lord et al. (2011) demonstrated a 95% reduction in the viability of encysted eggs of G. pallida when lines of B. juncea were chopped and incorporated into pots of PCN infested soil and covered with polyethylene. Additionally, Ngala et al. (2014) and Watts et al. (2015) observed reductions in the viability of encysted PCN eggs following biofumigation in field experiments. A unique finding made by Ngala et al. (2014) was the reduction in viable PCN eggs observed pre-incorporation of brassica biomass. This effect was observed in two years of field experiments and also in a separate AHDB funded project (Back et al., 2019; Project 114R476). While glucosinolates can be released in root leachates (McCully et al., 2008), myrosinase is not thought to be leached from the roots. Instead, soil microorganisms such as bacteria and fungi have been shown to secrete the enzyme. It is likely that these microbes utilise the glucose component of GSLs while in turn facilitating the release of ITCs. This effect was termed ‘partial biofumigation’ by Motisi et al., (2009) who observed a reduction in Rhizoctonia root rot of sugar beet in field plots where B. juncea was grown and then removed. Ngala et al. (2015) conducted several glasshouse experiments to investigate partial biofumigation and demonstrated rapid degradation of 2-propenyl GSL in unsterilised field soil in comparison to the same soil that was heat sterilised. Additionally, a Fluorescein Diacetate Assay (FDA) revealed increased microbial activity in pots where B. juncea and Raphanus sativus (oilseed radish) were grown pre- and post incorporation when compared to fallow controls. Unsterilised soils planted with R. sativus resulted in a 30-35% reduction in the eggs of G. pallida.  A full understanding of partial biofumigation could lead to an improved approach to traditional biofumigation that uses lower inputs, is potentially less damaging to soils due to minimal cultivations and results in greater suppression of pests and pathogens.  Whilst this study would focus on PCN, the concept and approach are transferable to a wide range of soil-borne pests and pathogens in different crops.

Objectives/brief methodology: 1) Develop methodology for identifying and assessing the activity of myrosinase producing microorganisms (MPMs) in soil - Here we will use a range of approaches such as chromatography, molecular diagnostics and bioinformatics to assess changes in myrosinase activity and MPMs within the soil microbiome. 2) Evaluate soil amendments for elevating densities of MPMs within microbial communities (priming partial biofumigation) - Inputs, such as biproducts from mustard processing or other brassica crop waste products, will be applied to increase the activity of MPMs. 3) Conduct glasshouse experiments to compare partial biofumigation from different brassica species - Variation in GSL profile and root architecture needs to be considered. 4) Develop methods for upregulating GSL release from root exudates - Controlled environment experiments will determine the effect of elicitors such as methyl jasmonate, which have been shown to increase GSL secretion from roots. 5) Explore techniques for optimising root development for increased GSL release - Approaches such as use of plant growth regulators or arbuscular mycorrhizal fungi will be assessed. 6) Evaluate the performance of the optimised partial biofumigation system on PCN suppression under field conditions - Field experiments will be conducted to compare the impact of the above modifications on PCN suppression