Research

Characterisation of Potato Cyst Nematode Populations in Great Britain for Sustainable Crop Management

Abstract

The potato cyst nematodes (PCN) (Globodera rostochiensis and G. pallida) are destructive pests of potato crops in the UK. The Potato Council (PCL) have recently estimated that PCN cause annual crop losses to the value of £25.9 million. Increasing industry concern has resulted in PCN being cited as an ‘Industry Research Priority’ in the ‘PCL Research & Development Strategy: 2012–2015’. Understanding PCN populations is of paramount importance to the selection of appropriate management methods. This study aims to characterise PCN populations of Great Britain by (i) Conducting a PCN survey in England, Wales and Scotland (ii) Examining the virulence of G. pallida populations (iii) Determining the viability of the populations using a test validated within this project. There is some evidence that mixed populations of G. pallida occur together and may become more virulent over successive generations. Virulence tests will be conducted on field populations using a range of potato genotypes to gain a better understanding of PCN populations in the UK. The findings of this work will be beneficial to the development of resistant cultivars. Currently viability is not estimated in PCN population density assessments. Determining the viability of eggs within the cysts is essential for assessing the effectiveness of control treatments and making decisions on inputs. The project will explore and develop various methods for estimating viability before validating the most promising method under field conditions. Viability assessments will be conducted on survey samples in an attempt to identify factors associated with a loss of viability.

Description

The potato cyst nematodes (PCN) (Globodera rostochiensis and G. pallida) are the most destructive pests of potato crops in the UK. The Potato Council have recently estimated that PCN cause annual crop losses to the value of £25.9 million. In the most recent survey (England & Wales) of PCN (Minnis et al., 2002), G. pallida was identified as being the dominant species found on potato growing land, with 67% of the PCN positive samples being pure G. pallida. The results from the survey indicate that the species distribution of PCN in the UK has been changing and that more land has become invested with PCN. In Scotland G. rostochiensis was found to be the dominant species; 22% of ware soil samples tested positive for PCN, with 78% of those samples having G. rostochiensis present (Evans et al., 1998). The virulence status of PCN populations found within the UK has been assessed largely using PCN field populations that were collected several decades ago (Phillips and Trudgill 1998). Recent work by the James Hutton Institute has found that in Scotland populations of G. pallida can contain mixtures of genotypes which are likely to represent at least 3 distinct introductions. In England, mixtures of 2 G. pallida genotypes have also been found in recent field samples. The implications of these results with regard to virulence are not known and the potential for the formation of novel hybrids between these putative introductions has also not been evaluated. There is a need to assess the virulence of UK PCN populations alongside current field populations.

 The major resistance gene H1 is effective against G. rostochiensis Ro1 and Ro4 pathotypes and has been successfully introduced into many cultivars, effectively controlling and suppressing this species. Breeding for resistance to G. pallida, however, has proven more difficult and although monogenic resistance to G. pallida have been identified these are effective only with specific G. pallida populations: the H2 locus from Solanum multidissectum confers resistance to G. pallida pathotype Pa1 (Dunnett,1961) and the Gpa2 gene derived from S. tuberosum ssp. andigena accession CPC 1673 confers resistance to a small, distinct population of G. pallida Pa2 found in the Netherlands (Arntzen et al. 1993). Quantitative resistance has also been identified which confers high levels of resistance to G. pallida. Large effect QTLs have been successfully introgressed from S. tuberosum spp. andigena and S. vernei into potato cultivars and these are now major sources of G. pallida resistance in the JHI potato breeding programmes. The diversity of UK G. pallida field populations, however, compared to those of G. rostochiensis poses challenges for deploying resistance that is both durable and effective. In the UK only the Ro1 pathotype of G. rostochiensis has been found,  however, for G. pallida Pa1 and Pa2/3 pathotypes have been identified and possible additional “founder” populations such as the unusually virulent Luffness population have been described. Potential for selection for increased virulence over several generations (Phillips and Blok, 2008) has also been reported.

 The population density and species ratio of PCN can be estimated by a number of Plant Health Clinics around the UK. However, few if any, provide a reliable estimate of the viability of encysted eggs.   Determining the viability of eggs remaining within the cysts is essential to assess the effectiveness of control treatments and informing management decisions. Various methods have been used to assess the viability status of PCN eggs including visual inspection by a trained nematologist and using stains such as Meldola’s Blue or Iodine. Staining is difficult to standardise and involves a degree of subjectivity. Biological assays involving hatching in response to root diffusate and whole plant infectivity are time consuming and labour intensive. Recently an assay based on trehalose has been reported by van den Elsen et al. (2012). This assay is inexpensive and quick. It is based on the dissacharide trehalose which is present in high concentrations in the pervitelline fluid of PCN eggs but is released from eggs when eggshell permeability increases in response to host root diffusates. The assay, however, has not yet been shown to be quantitative over a wide sample range.

 The presence of RNA has been examined as a method for assessing viability of PCN in cysts. Lord et al. (2011) used RT-qPCR of constitutively expressed actin mRNA. They examined the rate of mRNA degradation following heat treatment of cysts and found a significant decline 24h after heat treatment. The house keeping gene glycerol 3- phosphate dehydrogenase (gpd) is another mRNA that has been assessed by Harper Adams University College though some further validation is required. Combining DNA and RNA extractions has advantages for simultaneously performing other diagnostic assays such as species identification and quantifying intraspecific variation.

 Potato cyst nematodes are identified as an ‘Industry Research Priority’ in the PCL ‘Research & Development Strategy: 2012–2015’ and specific reference is made to understanding PCN populations. Given that the last survey of PCN was undertaken nearly 15 years ago, it is critical that the current populations of this pest are fully evaluated and that measurements of viability and virulence are considered.

The aim of this project is to characterise UK PCN populations. The objectives are as follows: -

  1. Conduct a survey of UK PCN populations in order to produce a current distribution map 
  2. Using the PCN samples from the survey, examine the virulence of G. pallida populations using different sources of resistance
  3. Determine the best method for assessing PCN viability and develop a rapid and universal test
  4. Evaluate the viability test under field conditions following inputs such as nematicide application and biofumigation
  5. Using the PCN samples from the survey, examine the viability of the populations in relation to field history

This studentship will commence in October 2013.

Funding Body

Potato Council Ltd

Lead Organisation

Harper Adams University

Partners

James Hutton Institute, SASA

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