Combining partial host resistance with bacterial biocontrol agents improves outcomes for tomatoes infected with Ralstonia pseudosolanacearum

Highlights

• Three Pseudomonas strains effectively suppressed bacterial wilt in tomato seedlings.

• Biocontrol efficacy was 4 times higher in a partially resistant vs. susceptible line.

• Host resistance genotype is an important component of biocontrol efficacy.

• A mixture of six biocontrol strains was less effective than the three best strains.

• 5In vitro antagonism and in vivo biocontrol efficacy were not correlated.

Abstract

We selected six strains with in vitro antagonism against 15 South Asian strains of Ralstonia pseudosolanacearum from among 54 previously characterized Pseudomonas and other bacterial biocontrol strains and evaluated the value of integration of these biocontrol agents with partial host resistance of tomato in management of bacterial wilt. Biocontrol strains were almost four times more effective in suppressing bacterial wilt in IRAT L3, a partially resistant tomato line, than in L390, a highly susceptible line. Pseudomonas brassicacearum 93D8 and Wood 1R, and P. fluorescens Clinto 1 were the most effective strains with biocontrol efficacy of 68, 47 and 56% respectively, in IRAT L3. Bacterial wilt incidence was suppressed in IRAT L3 in four of four experiments by P. brassicacearum 93D8, and in three of four experiments by P. protegens Clinto 1 and P. brassicacearum Wood 1R. However, in L390, biocontrol efficacy of P. brassicacearum 93D8 and Wood 1R, and P. fluorescens Clinto 1 was 7, 15 and 12%, respectively. There was no correlation between in vitro antagonism of the biocontrol strains against R. pseudosolanacearum and in vivo biocontrol efficacy. These results highlight the value of integration of biocontrol agents with host resistance in management of bacterial wilt.

Introduction

Bacterial wilt, caused by members of the Ralstonia solanacearum species complex (RSSC), is one of the most important soil-borne plant diseases of the tropics and sub-tropics, as well as certain warm temperate regions of the world (Hayward, 2005). The pathogen can infect over 250 plant species belonging to 54 families, including both monocots and dicots (Hayward, 1991; Wicker et al., 2007). The RSSC is highly diverse, and results of recent functional and genomic analyses split the complex into three species: R. solanacearum, R. pseudosolanacearum and R. syzygii (Safni et al., 2014; Prior et al., 2016). South Asian (phylotype I) and African (phylotype III) strains are classified as R. pseudosolanacearum, while phylotype II strains retain the name R. solanacearum. Bacterial wilt is difficult to manage due to the genetic diversity and aggressiveness of the pathogen, its ability to survive in varied and adverse environmental conditions, its modes of dissemination, and the large number of weed hosts (Ramesh and Phadke, 2012; Saddler, 2005). Crop loss due to bacterial wilt has been estimated to be more than $950 million globally in the potato industry alone (Walker and Collion, 1999). Bacterial wilt has been a severe problem in South Asia. Wilt incidence across all commercially grown tomato cultivars was reported to range from 9 to 39% in Karnataka state of India (Vanitha et al., 2009). Crop loss in some parts of India and Nepal reached up to 91 and 100%, respectively, and about 14% on average in Bangladesh (Elphinstone, 2005; Gurung and Vaidya, 1997).

Different practices have been employed to manage bacterial wilt (Yuliar et al., 2015). Chemical bactericides such as copper compounds and antibiotics have limited impact (Hartman and Elphinstone, 1994), and copper compounds have been shown to induce the viable but non-culturable state (VBNC), a survival mechanism of R. solanacearum species complex members (Grey and Steck, 2001). Acibenzolar-S-methyl, a plant resistance inducer without biocidal activity, significantly reduced bacterial wilt incidence when applied to partially resistant tomato varieties but not susceptible varieties (Pradhanang et al., 2005). Transgenic tomato lines expressing the ELONGATION FACTOR TU RECEPTOR (EFR) gene exhibited significant reductions in bacterial wilt incidence in the field and reduced stem colonization by the pathogen (Kunwar et al., 2018). However, the lack of acceptance of transgenic food crops currently restricts the commercial application of this approach. Crop rotation (Adhikari and Basnyat, 1998), soil amendment with various antimicrobial compounds (Dhital et al., 1997; Hong et al., 2011; Lee et al., 2012) or organic fertilizers (Liu et al., 2015, 2018), biological soil disinfection (Blok et al., 2000; Messiha et al., 2007b), soil solarization (Anith et al., 2000; Chellemi et al., 1997), soil fumigation (Chellemi et al., 1997), and microwave disinfection of seeding materials (Kumar et al., 2005) provide variable degrees of protection from bacterial wilt.

The use of resistant cultivars is one of the most effective and practical means of bacterial wilt management (Huet, 2014; Rivard et al., 2012). Tomato lines CLN2020C, CLN2026D, All Rounder, Swarakhsha, Rakshak, Trishul, and Arka Alok, and eggplant lines Kata Begun, Marich Begun and Uttar are a few of the bacterial wilt resistant cultivars used in South Asia (Dutta and Rahman, 2012; Rahman et al., 2011; Timila and Joshi, 2007). However, full deployment of this approach may be hampered by partial resistance, a lack of desired horticultural traits in resistant lines (Wang et al., 1998), instability, and location and strain specificity of resistance (Lebeau et al., 2011; Lin et al., 2008; Rivard et al., 2012). Grafting desirable varieties of solanaceous fruiting vegetables onto R. pseudosolanacearum-resistant rootstocks, including wild Solanum species, has been shown to reduce wilt incidence and increase yields (Rivard et al., 2012; Singh et al., 2015). Despite the additional cost of producing grafted seedlings, adoption of this tactic in certain production systems such as high tunnels is increasing (Miller et al., 2005; Rivard et al., 2012).

Biological control of plant diseases has yet to be widely adopted in solanaceous fruiting vegetables, despite its desirability as a potentially sustainable disease management practice. Biological control agents (BCAs) employ several mechanisms including antagonism, competition and induction of host resistance to suppress plant diseases (McSpadden Gardener, 2004; Pal and McSpadden Gardener, 2006; Pieterse and Wees, 2015). Some BCAs are also known to degrade signals that activate pathogens (Molina et al., 2003). Strains of Pseudomonas, Burkholderia, Bacillus, Streptomyces, Actinomyces, Acinetobacter, Enterobacter, Escherichia, Erwinia, Stenotrophomonas, Serratia, Ralstonia, several phlD + rhizobacteria, and ectomycorrhizal fungi have been studied for their efficacy against bacterial wilt (Messiha et al., 2007a; Ramadasappa et al., 2012; Ramesh and Phadke, 2012; Saddler, 2005; Wei et al., 2013; Xue et al., 2009). Performance of these biocontrol agents depends highly on biotic and abiotic factors such as environmental conditions, soil type, soil microbiota, host resistance genotype (Barretti et al., 2012) and disease pressure (Gerbore et al., 2014).

Bacterial wilt can be prevented in disease-free areas by adopting strict exclusion practices, however no single management practice provides satisfactory results where the disease is endemic (Lopez and Biosca, 2005; Saddler, 2005). The majority of hosts highly resistant to bacterial wilt lack desired agronomic traits, and the performance of biological control agents is often poor when disease pressure is high. Therefore, the first objective of this study was to investigate a collection of previously characterized bacterial biocontrol agents (Aly, 2009; Mavrodi et al., 2012; McSpadden Gardener et al., 2005; Raudales et al., 2009) for their antagonistic activities against R. pseudosolanacearum. The second objective was to test the value of integration of selected antagonistic bacteria with partial host resistance in management of this disease.