New host resistances in Brassica napus and Brassica juncea from Australia, China and India: Key to managing Sclerotinia stem rot (Sclerotinia sclerotiorum) without fungicides

• Most resistant Brassica napus Oscar (Australia), Zhongyou-za No. 8, Fan 168, Ding 110 (China).
• Most resistant Brassica juncea Aravali and Bio-902 (India).
• B. napus Oscar and Ding 110 express high value isolate-independent resistance.
• Host resistance is key to future Sclerotinia management rather than fungicides.

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is a serious problem in oilseed rape and mustard worldwide. Locating effective sources of host resistance to this disease offers the best long term prospects for its improved management. For this reason, 19 Brassica napus genotypes from Australia (6), China (7) and India (6) and 34 Brassica juncea genotypes from Australia (6), China (3) and India (25), were screened for resistance to SSR under field conditions using a stem inoculation test. There were significant differences (P < 0.001) among the B. napus and B. juncea test genotypes in relation to the stem lesion length. The most resistant B. napus genotypes were Oscar from Australia, Zhongyou-za No. 8, Fan 168 and Ding 110 from China, all with stem lesion lengths ≤4.1 cm. The most susceptible B. napus were GSL2 from India, and 03-p74-11 from China, with stem lesion lengths ≥12 cm. The most resistant B. juncea genotypes were Aravali and Bio-902 from India with stem lesion lengths ≤5.7 cm; while the most susceptible were JM018 from Australia, Xinyou 8 and Xinyou 9 from China, Sanjucta Ascsh and Basanti from India, all with stem lesion lengths >10 cm. In particular, this high level resistance in B. napus Oscar from Australia, Zhongyou-za No. 8, Fan 168 and Ding 110 from China provides sources of resistance for oilseed Brassica breeding programs in Australia; particularly resistance to pathotype 76, the dominant prevailing S. sclerotiorum pathotype. It is noteworthy that progenies from B. napus crosses of Oscar with Ding 110 express isolate-independent host resistance, making Oscar and Ding 110, and potentially their progenies, an ideal target to exploit in developing new commercial rapeseed cultivars that not only have more effective resistance to SSR, but resistance that is effective across multiple pathotypes of this pathogen. These findings ensure successful management of SSR based on host resistance rather than fungicides is now possible providing breeders take the opportunities now presented.