Eur J Plant Pathol (2015) 143:527–541 DOI 10.1007/s10658-015-0696-6 Virulence differences among Sclerotinia sclerotiorum isolates determines host cotyledon resistance responses in Brassicaceae genotypes Xin Tian Ge & Ming Pei You & Martin J. Barbetti Accepted: 19 June 2015 /Published online: 1 July 2015 # Koninklijke Nederlandse Planteziektenkundige Vereniging 2015 Abstract Differences in Sclerotinia rot (SR) disease se- (e.g., B. napus Charlton against the ‘Cabbage’ and verity, caused by two categorized pathotypes and one MBRS1 isolates; B. napus Oscar against the WW4 iso- more recent isolate of S. sclerotiorum andmeasuredin late). These findings highlight the value from using terms of cotyledon lesion diameter, were studied across pathotypes of different physiological specialization in diverse Brassicaceae hosts to characterize host response screening programs to identify host resistance that is du- and pathogen virulence. There were significant differences rable across multiple pathotypes. Distinct host resistance (P ≤0.001) between genotypes, isolates and a significant symptom types were reported for the first time on some genotype x isolate interaction. The mean diameter of genotypes against isolate WW4; including a distinct yel- cotyledon lesions ranged from 5 mm in the most resistant low halo observed around lesions on B. napus RQ001, genotypes (e.g., Brassica juncea Ringot I and Seeta) to≥ indicative of leaf senescence involved in programmed cell 13.6 mm in the most susceptible genotypes (e.g., death (PCD); a distinct dark brown margin observed B. tournefortii Wild turnip #1 and #2, Sisymbrium irio around lesions on R. sativus, indicative of a hypersensitive London rocket Wild #1 and #2, and B. nigra 4381). response (HR); and the HR ‘flecking’ on Sinapis alba Responses, in at least one experiment for some Concerta and B. juncea Seeta. That WW4 was the most B. juncea (e.g., Seeta, Ringot I) and Raphanus sativus pathogenic isolate for genotypes such as B. juncea (e.g., Colonel) genotypes, were generally highly resistant Hetianyoucai and B. napus Oscar that showed high level irrespective of the isolate used, making them ideal sources resistance to the ‘Cabbage’ isolate and intermediate resis- of resistance to exploit for developing new varieties with tance to MBRS-1, dispels previously held views that more effective resistance to SR across multiple pathotypes WW4 was a largely avirulent pathotype of little conse- of this pathogen. In contrast, some other genotypes quence. Rather, isolate WW4 offers unique opportunities showed significant isolate dependency, with high levels to investigate HR and PCD host resistance responses to of resistance against one isolate (e.g., B. napus Charlton S. sclerotiorum in Brassicaceae. against the WW4 isolate; B. napus Oscar against the ‘ ’ Cabbage isolate) but quite susceptible to other isolates Keywords Sclerotinia sclerotiorum . Sclerotinia rot . : : Brassicaceae Crucifer Raphanus Brassica Radish X. T. Ge M. P. You M. J. Barbetti Oilseed rape . Mustard . Host resistance School of Plant Biology, Faculty of Science, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Introduction M. P. You : M. J. Barbetti (*) The UWA Institute of Agriculture, Faculty of Science, The University of Western Australia, Crawley, WA6009, Australia Sclerotinia sclerotiorum (Lib) de Bary causes devastat- e-mail: [email protected] ing and severe yield-limiting diseases worldwide for 528 Eur J Plant Pathol (2015) 143:527–541 many agriculturally and horticulturally important spe- identification of hosts with durable resistance may only cies, in particular Brassicaceae (Saharan et al. 2008). be possible if several pathotypes are used to screen for This includes sclerotinia rot (SR), particularly on oilseed resistance (Garg et al. 2010a;Geetal.2012; Uloth et al. rape (Brassica napus L.) and mustard (B. juncea (L.) 2013, 2014). Studying the interaction of a range of Czern.) (Barbetti and Khangura. R 2000; Delourme cruciferous host species to multiple pathotypes of et al. 2011; Singh et al. 2011), and in vegetable crops S. sclerotiorum is also important because it not only such as cabbage (B. oleracea L. var. capitata (L.) Alef.), enhances our understanding of resistance and increases Chinese cabbage (B. rapa L. var. pekinensis (Lour.) the possibility of identifying resistant hosts to individual Kitam.), radish (Raphanus sativus L.), broccoli pathotypes, but more importantly, allows opportunities (B. oleracea L. var. italica Plenck), turnip (B. rapa L. to identify hosts with resistance against multiple var. glabra Regel), swede or rutabaga (B. napus L. var. pathotypes (Ge et al. 2012;Gargetal.2010a;Uloth naprobrassica (L.) Reichenb.), and kale (B. oleracea L. et al. 2013, 2014). While various Brassicaceae geno- var. acephala Bailey) (Kim et al. 2003; Pedras and types have been studied for resistance to S. sclerotiorum, Ahiahonu 2004). The fact that S. sclerotiorum exhibits fewer studies have assessed resistance when challenged little host specificity to Brassicaceae, and infects almost with multiple isolates. For example, while Kim and Cho all species, has constrained selection for locating disease (2003) studied pathogenicity in cruciferous crops using resistance such as the hypersensitive response (HR) and six randomly picked S. sclerotiorum isolates, they found programmed cell death (PCD) among Brassicaceae no significant differences in the susceptibility of the host species (McCartney 2000; Purdy 1979). to these different isolates. In contrast, Ge et al. (2012) In addition to the lack of host specificity by showed that 53 isolates could be clustered into 8 distinct S. sclerotiorum, infection with different S. sclerotiorum pathotypes, while Clarkson et al. (2008, 2013)showed pathotypes or isolates can result in range of virulence the wide diversity of S. sclerotiorum from agricultural and/or pathogenicity, including variations in disease crops and/or wild hosts. Such studies suggest that progress and severity (Garg et al. 2010a;Geetal. utilising randomly picked and uncharacterized isolates 2012). The molecular basis of this variation is uncertain may not be the most effective approach for screening and may come from various S. sclerotiorum genetic for resistance as this can confound interpretation of determinants (Harel et al. 2006; Jurick and Rollins the host resistance reactions, even where the host 2007; Rahmanpour et al. 2010). Many studies report reactions are similar across isolates. Screening multi- tremendous pathogenic and phylogenetic variation in ple hosts using multiple pathotypes is the best ap- S. sclerotiorum populations. For example, in Ontario proach for efficient screening for resistance against Canada, studies have shown that field populations of S. sclerotiorum. S. sclerotiorum on oilseed rape are genetically hetero- Although absolute or near-absolute resistance to geneous (Kohn et al. 1990, 1991). In south-east S. sclerotiorum is considered by some to be not feasible Australia, S. sclerotiorum isolates collected from oilseed (e.g., Saharan et al. 2008), it is not impossible as dem- rape crops also showed high levels of genotypic diver- onstrated by Garg et al. (2010b). In contrast, screen- sity (Sexton et al. 2006); and within a particular geo- ing for varying levels of partial resistance, while of graphic region, oilseed rape can host different higher occurrence (e.g., Li et al. 2006, 2007, 2009; S. sclerotiorum pathotypes (Ge et al. 2012). High vari- Uloth et al. 2013, 2014), requires a screening method ation in aggressiveness between isolates of that is consistent in terms of host resistance across S. sclerotiorum has also been observed on common bean multiple experiments (Garg et al. 2010b;Lietal. (Phaseolus vulgaris L.) (Otto-Hanson et al. 2011)and 2006; Nelson et al. 1991; Prajapati et al. 2005;Uloth on sunflower (Helianthus annuus L.) (Ekins et al. 2007). et al. 2013). Resistance screening against As S. sclerotiorum frequently exhibits little host S. sclerotiorum in B. napus and/or B. juncea commonly specificity, identification of resistant genotypes may involves utilization of a field stem inoculation test orig- best be achieved by screening genotypes from diverse inally developed by Buchwaldt et al. (2005) that, with cruciferous species rather than a single species (Saharan some modification, has proved very successful in iden- et al. 2008; Uloth et al. 2013, 2014). In addition, as tifying resistance that will be effective in commercial polymorphisms in S. sclerotiorum may be responsible crops (e.g., Li et al. 2006, 2007, 2009;Gargetal.2010b; for pathogenic and phylogenetic differences, Ge et al. 2012; Uloth et al. 2014). While field Eur J Plant Pathol (2015) 143:527–541 529 evaluations of SR were generally considered the most whether selected isolates representing distinct pathotype appropriate and reliable technique for evaluating stem groups is a more efficient and effective means for resis- resistance against S. sclerotiorum, expression of host tance screening than utilizing a single isolate; and, 3) to resistance can be highly variable, particularly where determine whether host resistance expression to fluctuating environmental conditions influence the re- S. sclerotiorum is expressed as programed cell death sponse of individual Brassicaceae (Uloth et al. 2015a) (PCD) and HR on some Brassicaceae. and similarly for infection and disease development in other crops, for example lettuce (Young et al. 2004) genotypes. For this reason,