Some plant pathogens can infect an extremely diverse range of hosts (Newman & Derbyshire, 2020). Among these is the necrotrophic fungus Sclerotinia sclerotiorum, which stands out for its ability to cause significant disease in numerous crops. Disease caused by this pathogen is given various names, though one of the most common seems to be sclerotinia stem rot. S. sclerotiorum infects crops in almost all agricultural production zones ranging from dry bean (Phaseolus vulgaris) in the tropics (Lehner et al., 2017) to canola (Brassica napus

Sclerotinia sclerotiorum is a fungal pathogen that causes stem rot in oilseed rape (Brassica napus). Previously, B. napus accessions with partial stem resistance to a Canadian S. sclerotiorum isolate (#321) were identified using a stem test in which flowering plants were inoculated with mycelium plugs. The present study examined the partial stem resistance of four of these accessions, PAK54, PAK93, DC21 and K22, following inoculation with Australian isolates. Mycelial compatibility groups and intergenic spacer (IGS) region haplotypes were identified among 71 isolates from Australian oilseed rape and lupin fields. Eleven genetically diverse isolates showed differences in aggressiveness when inoculated onto nine oilseed rape varieties and one Chinese accession. Isolates CU8.24, CU10.17 and CU11.19 were selected based on genetic diversity, growth rate in vitro and high aggressiveness in the initial screen and subsequently inoculated onto the four B. napus accessions. These accessions developed significantly smaller lesions compared with the susceptible control varieties ('AV Garnet' and 'Westar'), with the average frequency of soft and collapsed lesions being less than 20% in PAK54, DC21 and K22, 29% in PAK93 and greater than 88% in the susceptible controls. Microscopic examination revealed that hyphae were typically confined to the stem cortex in the smallest lesions, but could be found in the stem pith in larger lesions. These results show that B. napus accessions PAK54, PAK93, DC21 and K22 can be used in Australia for development of varieties with partial stem resistance to S. sclerotiorum.Résumé: Sclerotinia sclerotiorum est un agent pathogène fongique qui cause la pourriture sclérotique chez le colza oléagineux (Brassica napus). Auparavant, les accessions de B. napus possédant une résistance partielle à la pourriture causée par l'isolat canadien de S. sclerotiorum (n°321) étaient identifiées à l'aide d'un test spécifique de la pourriture au cours duquel les plants en fleur étaient inoculés avec des chevilles de mycélium. Cette étude examine la résistance partielle à la pourriture des quatre accessions suivantes, PAK54, PAK93, DC21 et K22, après inoculation avec des isolats australiens. Les groupes de compatibilité mycélienne et les haplotypes des régions des espaceurs intergéniques ont été identifiés chez 71 isolats provenant de champs de colza oléagineux et de lupin australiens. Onze isolats génétiquement différents ont affiché des variations quant à leur agressivité lorsqu'ils ont été utilisés pour inoculer neuf variétés de colza oléagineux et une accession chinoise. Les isolats CU8.24, CU10.17 et CU11.19 ont été choisis en fonction de leur diversité génétique, de leur taux de croissance in vitro et de leur énorme agressivité lors du criblage initial, puis ont servi à inoculer les quatre accessions de B. napus. Ces accessions ont développé des lésions particulièrement petites comparativement aux variétés témoins réceptives ('AV Garnet' et 'Westar'), avec une fréquence moyenne de moins de 20% de lésions molles...

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The fungus Sclerotinia sclerotiorum infects hundreds of plant species including many crops. Resistance to this pathogen in canola (Brassica napus L. subsp. napus) is controlled by numerous quantitative trait loci (QTL). For such polygenic traits, genomic prediction may be useful for breeding as it can capture many QTL at once while also considering nonadditive genetic effects. Here, we test application of common regression models to genomic prediction of S. sclerotiorum resistance in canola in a diverse panel of 218 plants genotyped at 24,634 loci. Disease resistance was scored by infection with an aggressive isolate and monitoring over 3 wk. We found that including first-order additive × additive epistasis in linear mixed models (LMMs) improved accuracy of breeding value estimation between 3 and 40%, depending on method of assessment, and correlation between phenotypes and predicted total genetic values by 14%. Bayesian models performed similarly to or worse than genomic relationship matrix-based models for estimating breeding values or overall phenotypes from genetic values. Bayesian ridge regression, which is most similar to the genomic relationship matrix-based approach in the amount of shrinkage it applies to marker effects, was the most accurate of this family of models. This confirms several studies Abbreviations: AGG, Australian Grains Genebank; ASSYST, Associative expression and systems analysis of complex traits in oilseed rape/canola; AUDPC, area under the disease progress curve; BLUP, best linear unbiased predictor; DPI, days post inoculation; G-BLUP, genomic best linear unbiased predictor; GEBV, genomic estimated breeding value; h, square root of narrow-sense heritability; H 2 , broad-sense heritability; h 2 , narrow-sense heritability; LMM, linear mixed model; MCMC, Markov chain Monte Carlo; PSCL, proportion of soft or collapsed lesions; QTL, quantitative trait loci; RR-BLUP, ridge regression best linear unbiased predictor; SNP, single nucleotide polymorphism. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Sclerotinia sclerotiorum is an important fungal pathogen of chickpea (Cicer arietinum L.) and it can cause yield losses up to 100%. The wild progenitors are much more diverse than domesticated chickpea and this study describes how this relates to S. sclerotiorum resistance. Initially, the pathogenicity of nine Australian S. sclerotiorum isolates was examined on three Cicer lines to develop a robust phenotyping assay and significant differences in isolate aggressiveness were identified with 6 isolates being classed as highly aggressive and 3 as moderately aggressive. We identified two S. sclerotiorum isolates, CU8.20 and CU10.12, to be highly aggressive and moderately aggressive, respectively. A subsequent phenotyping assay was conducted using the two isolates to evaluate 86 wild Cicer accessions (Cicer reticulatum and Cicer echinospermum) and two C. arietinum varieties for resistance to S. sclerotiorum. A subset of 12 genotypes was further evaluated, and subsequently, two wild Cicer accessions with consistently high levels of resistance to S. sclerotiorum were examined using the initially characterised nine isolates. Wild Cicer accessions Karab_084 and Deste_063 demonstrated consistent partial resistance to S. sclerotiorum. There were significant differences in responses to S. sclerotiorum across wild Cicer collection sites. The Cermik, Karabahce and Destek sites’ responses to the aggressive isolate CU8.20 ranged from resistant to susceptible, highlighting an interaction between isolate genotype and chickpea collection site for sclerotinia stem rot resistance. This is the first evidence of partial stem resistance identified in wild Cicer germplasm, which can be adopted in chickpea breeding programs to enhance S. sclerotiorum resistance in future chickpea varieties.