Combining R gene and quantitative resistance increases effectiveness of cultivar resistance against Leptosphaeria maculans in Brassica napus in different environments

RESEARCH ARTICLE Combining R gene and quantitative resistance increases effectiveness of cultivar resistance against Leptosphaeria maculans in Brassica napus in different environments Yong-Ju Huang*, Georgia K. Mitrousia, Siti Nordahliawate M. Sidique¤, Aiming Qi, Bruce D. L. Fitt a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 Centre for Agriculture, Food & Environmental Management, School of Life and Medical Sciences, University of Hertfordshire, Hatfield, Hertfordshire, United Kingdom ¤ Current address: Laboratory for Pest, Disease and Microbial Biotechnology (LAPDiM), School of Food Science and Technology, Universiti Malaysia Terengganu, Kuala Terengganu, Malaysia * Abstract OPEN ACCESS Citation: Huang Y-J, Mitrousia GK, Sidique SNM, Qi A, Fitt BDL (2018) Combining R gene and quantitative resistance increases effectiveness of cultivar resistance against Leptosphaeria maculans in Brassica napus in different environments. PLoS ONE 13(5): e0197752. https://doi.org/10.1371/ journal.pone.0197752 Editor: Sabrina Sarrocco, Universita degli Studi di Pisa, ITALY Received: November 10, 2017 Accepted: May 8, 2018 Published: May 23, 2018 Copyright: © 2018 Huang et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: UK Biotechnology and Biological Sciences Research Council (BBSRC, BB I017585, M028348/1 and P00489X/1), the Innovate UK (102100 and 102641), the Agriculture, Horticulture Development Board (AHDB) (RD-2140021105), Felix Cobbold Trust and the Chadacre Agricultural Trust provided funding for this work. The funders Using cultivar resistance against pathogens is one of the most economical and environmentally friendly methods for control of crop diseases. However, cultivar resistance can be easily rendered ineffective due to changes in pathogen populations or environments. To test the hypothesis that combining R gene-mediated resistance and quantitative resistance (QR) in one cultivar can provide more effective resistance than use of either type of resistance on its own, effectiveness of resistance in eight oilseed rape (Brassica napus) cultivars with different R genes and/or QR against Leptosphaeria maculans (phoma stem canker) was investigated in 13 different environments/sites over three growing seasons (2010/2011, 2011/ 2012 and 2012/2013). Cultivar Drakkar with no R genes and no QR was used as susceptible control and for sampling L. maculans populations. Isolates of L. maculans were obtained from the 13 sites in 2010/2011 to assess frequencies of avirulent alleles of different effector genes (AvrLm1, AvrLm4 or AvrLm7) corresponding to the resistance genes (Rlm1, Rlm4 or Rlm7) used in the field experiments. Results of field experiments showed that cultivars DK Cabernet (Rlm1 + QR) and Adriana (Rlm4 + QR) had significantly less severe phoma stem canker than cultivars Capitol (Rlm1) and Bilbao (Rlm4), respectively. Results of controlled environment experiments confirmed the presence of Rlm genes and/or QR in these four cultivars. Analysis of L. maculans populations from different sites showed that the mean frequencies of AvrLm1 (10%) and AvrLm4 (41%) were less than that of AvrLm7 (100%), suggesting that Rlm1 and Rlm4 gene-mediated resistances were partially rendered ineffective while Rlm7 resistance was still effective. Cultivar Excel (Rlm7 + QR) had less severe canker than cultivar Roxet (Rlm7), but the difference between them was not significant due to influence of the effective resistance gene Rlm7. For the two cultivars with only QR, EsAstrid (QR) had less severe stem canker than NK Grandia (QR). Analysis of the relationship between severity of stem canker and weather data among the 13 sites in the three growing seasons showed that increased severity of stem canker was associated with increased rainfall during the phoma leaf spot development stage and increased temperature during the PLOS ONE | https://doi.org/10.1371/journal.pone.0197752 May 23, 2018 1 / 22 Combining R gene and quantitative resistance against Leptosphaeria maculans had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. stem canker development stage. Further analysis of cultivar response to environmental factors showed that cultivars with both an Rlm gene and QR (e.g. DK Cabernet, Adriana and Excel) were less sensitive to a change in environment than cultivars with only Rlm genes (e.g. Capitol, Bilbao) or only QR (e.g. DK Grandia). These results suggest that combining R gene and QR can provide effective, stable control of phoma stem canker in different environments. Introduction Crop resistance against fungal pathogens has a vital role to play in ensuring global food security in response to the increasing human population and the threat from devastating diseases of food crops [1,2]. Crop resistance can be used to protect food crops grown by farmers who cannot afford the use of chemical fungicides in regions where crop production is being threatened by crop diseases and climate change [3]. Even in regions where farmers can afford the use of fungicides, fungicides can easily lose their effectiveness due to the development of fungicide insensitivity in pathogen populations [1,4–6]. Therefore, the need for effective crop resistance is greater than ever. A problem with crop resistance against fungal pathogens is that it can be rendered ineffective through changes in pathogen populations [7–10]. Such changes can lead to ‘boom and bust’ cycles of crop production, when good years are followed by years with severe epidemics caused by new pathogen races. To ensure sustainable crop production, there is a need for durable crop resistance [11], defined as ‘a resistance that remains effective during its prolonged and widespread use in an environment favourable to the disease’. Effective resistance is resistance that prevents development of severe disease epidemics. Two types of resistance used to control crop diseases are quantitative resistance (QR) and R gene-mediated qualitative resistance. QR is usually controlled by several minor genes (quantitative trait loci; QTL), whereas qualitative resistance is usually controlled by single dominant R genes [12,13]. R gene-mediated resistance is race-specific; it is effective only when the avirulent allele of the corresponding effector gene is predominant in the pathogen population. Therefore, R gene-mediated resistance is not effective in protecting the crop when new pathogen races have developed, resulting in catastrophic epidemics [8,9,14]. Its effectiveness may also be influenced by environmental factors (...truncated)