New sources of resistance to Sclerotinia sclerotiorum for crucifer crops.
Current management of Sclerotinia rot (Sclerotinia sclerotiorum) relies heavily upon cultural and chemical control options that provide, at best, only partial, sometimes sporadic control and can be cost prohibitive. Effective host resistance to S. sclerotiorum is urgently needed if Sclerotinia rot is to be successfully managed across diverse oilseed, forage and vegetable crucifer crops worldwide. Differences in resistance across 127 diverse cruciferous genotypes to S. sclerotiorum in one experiment, and a set of 55 Brassica napus lines carrying one or more B. carinata B genome introgressions in a second experiment, were assessed by field-inoculation of stems and natural ascospore infection. Reactions on B. fruticulosa, B. oxyrrhina, B. parachinensis, B. tournefortii, Camelina sativa, Carrichtera annua, Diplotaxis tenuifolia, Eruca vesicaria, Hirschfeldia incana, Raphanus raphanistrum, Raphanus sativus, Rapistrum rugosum, Sinapis arvensis and Sisymbrium irio to this pathogen are reported for the first time. Among these, and across other species previously explored, including B. carinata, B. napus, B. juncea, B. rapa, B. cretica, B. incana, B. insularis, B. nigra, and B. montana, responses ranged from highly resistant (stem lesion length 2 mm) to highly susceptible (155 mm). Against a highly virulent and prevailing pathogen pathotype, R. raphanistrum RRA 41, B. napus Mystic, B. carinata BRA 926/78, B. carinata 054113, R. sativus Krasnodar. Market B, B. carinata PI 193459 and R. raphanistrum WARR 20 all showed very high-level resistance, with stem lesion length <5 mm; with a further 29 genotypes across these species and/or B. incana, B. rapa var. rosularis, B. oleracea vars acephala and italica, and S. arvensis, that showed slightly less but still high resistance (lesions 5-10 mm). Natural air-borne ascospore leaf infection occurred in 89% of plots, yet some forage and/or vegetable crucifers, such as B. rapa var. chinensis Ivory and R. sativus Oriental radish, showed high-level resistance to such infections. B. napus with introgression(s) from B. carinata, ranged from highly resistant (lesions <2 mm) to very susceptible (>67 mm). There appeared to be separate genetic control for stem vs. leaf resistance, crucial to the understanding needed in developing resistant cultivars across different cruciferous crop types. While this study highlighted individual genotypes that offer great potential for improving resistance to Sclerotinia rot in commercial cruciferous crops, it also demonstrated that assessment of the overall value of a species is only possible when a significant number of genotypes within the species are tested. Overall, while genotypes with high-level stem or leaf resistance identified are of particularly significant value for developing new disease-resistant cultivars of oilseed, forage and vegetable crucifers, even genotypes with moderate levels of resistance, if deployed commercially, offer prospects of improving current integrated disease management strategies compared with current reliance upon cultural and/or chemical controls utilized in conjunction with cultivars lacking any effective resistance. As resistance was identified for the first time across many of these diverse cruciferous species, it is likely that some resistances identified constitute new sources and/or types of host resistance not previously identified.