Delineation of Sclerotinia sclerotiorum pathotypes using differential resistance responses on Brassica napus and B. juncea genotypes enables identification of resistance to prevailing pathotypes

Abstract

Sclerotinia stem rot caused by the fungus Sclerotinia sclerotiorum is one of the most damaging and difficultto-manage diseases of oilseed rape (Brassica napus) and mustard (B. juncea). Identifying oilseed Brassicagenotypes with effective resistance offers the best long-term prospect for improved management of thedisease. Despite some significant interactions between oilseed Brassica genotypes and S. sclerotiorumisolates reported in earlier studies, mostly a single pathogen isolate has been used to identify resistantgenotypes. This paper reports the results of studies involving 53 isolates of S. sclerotiorum from the northernand southern agricultural regions of Western Australia where Sclerotinia stem rot is a serious diseaseof oilseed rape. Colony characteristics of isolates on potato dextrose agar were determined, and two fieldvirulence (i.e., levels of disease severity) studies conducted. The first field study included 14 Brassicagenotypes against three S. sclerotinia isolates and the second had eight Brassica genotypes against 50 S.sclerotinia isolates. Only colony diameter of isolates growing on potato dextrose agar was correlated withstem lesion length in the field. In both field experiments, there were significant effects of isolates and hostgenotypes, as well as a significant interaction between isolates and genotypes in relation to stem lesionlength. In the first field experiment, mean stem lesion length ranged from 1 cm in the highly resistantB. napus ZY006 to at least 9 cm for susceptible genotypes Brassica juncea #2 and B. juncea Montara. Thelatter genotype was the most susceptible with a mean stem lesion length of 11.1. Expression of high-levelresistance in B. napus ZY006 was largely independent of S. sclerotiorum isolate. In contrast, responses inB. napus genotypes, Zhongyou 821, 06-6-3792, RT108 and Charlton were much more isolate-dependent.In the first field experiment, with a greater number of host genotypes than S. sclerotiorum isolates, ahigher variance ratio occurred for isolates (VR = 158.4) than for host genotypes (VR = 10.7). In the secondfield experiment, with a larger number of S. sclerotinia isolates than hosts, host genotypes had a highervariance ratio (VR = 458.9) compared to the isolates (VR = 71.2). Increasing the number of isolates greatlyimproved the capacity to differentiate levels of resistance among test genotypes. In this experiment,some genotypes showed more consistent resistant reactions (e.g., B. napus Mystic and B. juncea Xinyou9) across different isolates and these are ideal targets for commercial exploitation of this resistance inoilseed Brassica breeding programs. From this experiment, a standardized set of three B. napus and threeB. juncea genotypes were shown to be suitable for use as universal differentials to characterize pathotypesof S. sclerotiorum using octal nomenclature. Eight distinct pathotypes of S. sclerotiorum were delineatedand characterized and these six host differential genotypes can now be utilized to identify and monitorthe incidence and distribution of current and future pathotypes of S. sclerotiorum. Further, by providinga reliable means to characterize pathotypes of S. sclerotiorum, for the first time not only allows identificationof resistance(s) against the predominant pathotype(s) of S. sclerotiorum prevailing in a particularregion, but also now allows oilseed rape breeding programs to combine host resistances against severalspecific pathotypes of S. sclerotiorum into future cultivars.