New resistances offer opportunity for effective management of the downy mildew (Hyaloperonospora parasitica) threat to canola

Abstract

Downy mildew (Hyaloperonospora parasitica) is a problem for canola production worldwide, including in Australia where it has remained a persistent threat since 1998. Testing of 131 Brassicaceae varieties, including 109 Australian canola varieties (Brassica napus and B. juncea) and 22 diverse Brassicaceae (including B. napus, B. carinata, B. juncea, B. nigra, B. rapa, Crambe abyssinica and Raphanus sativus) highlighted excellent resistance to downy mildew. Using a mixture of 10 H. parasitica isolates, R. sativus Colonel and Boss showed highest resistance to H. parasitica, with per cent disease index (%DI) values of 3.7% and 10.2%, respectively. These were followed by (%DI values): B. carinata ATC 94011 (11.1%), B. napus CB™ Tanami (14.1%) and Komet-741 A (14.3%), B. juncea 397.23.2.3.3 (14.8%), B. napus ATR-Banjo (16.9%), Hyola 575 CL (16.9%), Komet-744 A (18.1%), Cresor-770 B (18.5%), Wamus (18.5%), Surpass 400 (19.2%), Hyola 432 (19.4%) and Hyola 76 (19.4%), and C. abyssinica (19.9%). These varieties were also considered highly resistant. Another five B. juncea genotypes and B. nigra P.23845 were considered highly resistant with %DI of 22.2%. Those considered resistant (but not highly resistant) included hybrid B. napus Hyola 444 TT, Hyola 500 RR, Hyola 504 RR, Pioneer 46Y78, Pioneer 45Y77 and Hyola 650 TT, and the non-hybrid variety ATR-Eyre, all with %DI values 23.1–28.2%. By contrast, B. napus Thunder TT, Hyola 450 TT and ATR-Grace were highly susceptible with %DI values of 90.3%, 88.2% and 81.7%, respectively. Cluster analysis revealed six distinct clusters (highly resistant, resistant, moderately resistant, moderately susceptible, susceptible, very susceptible) for the tested Brassicaceae genotypes that, on average, showed similar responses within each cluster against H. parasitica based on their %DI values. From 2000 onwards (with the exception of Surpass 400), 10 B. napus varieties and one B. juncea released were classified as highly resistant; however, there was no overall correlation between year of variety release and level of resistance expressed against H. parasitica. This is the first study to demonstrate the existence of very high levels of pathotype-independent resistance in Australian canola varieties to H. parasitica. The most resistant varieties identified can be used in canola breeding programs and also directly deployed into regions where downy mildew is prevalent, providing the canola industry with an immediate and effective option for management of this important disease.

Molecular characterization of disease resistance in Brassica juncea – The current status and the way forward

“…In Australia, blackleg and sclerotinia have been identified as the most destructive diseases for oilseed-and vegetable-type mustards (Murray and Brennan, 2012;Uloth et al, 2013Uloth et al, , 2014Uloth et al, , 2015aUloth et al, , 2015bVan de Wouw et al, 2016). However, there are several other diseases also identified as important in Australia, including TYV, downy mildew, white leaf spot, alternaria blight, bacterial leaf blight, and powdery mildew (Li et al, 2006a(Li et al, , 2007(Li et al, , 2008a(Li et al, , 2009bEshraghi et al, 2007;Murray and Brennan, 2012;Gunasinghe et al, 2014;Van de Wouw et al, 2016;Mohammed et al, 2017Mohammed et al, , 2018aMohammed et al, , 2018bMohammed et al, , 2019Al-lami et al, 2019aAl-lami et al, , 2019bAl-lami et al, , 2019cGuerret et al, 2017;Uloth et al, 2016Uloth et al, , 2018Murtza et al, 2019).…”

“…Although H. brassicae has been present on Australian B. napus and B. juncea crops for decades (Barbetti and Khangura, 2000), it shows increasing severity and threat to commercial brassica crops in some regions of Australia, particularly since 1998 (Barbetti and Khangura, 2000). This may be in part associated with periods of less than ideal rainfall in autumnearly winter and to the continued deployment of one or more highly susceptible varieties, as both seem to foster development of downy mildew (Mohammed et al, 2017 (Nashaat and Awasthi, 1995). Similarly, in Australia, where several varieties of B. napus were highly susceptible to H. brassicae isolates, several genotypes of B. juncea were highly resistant (Mohammed et al, 2017 .…”

“…This may be in part associated with periods of less than ideal rainfall in autumnearly winter and to the continued deployment of one or more highly susceptible varieties, as both seem to foster development of downy mildew (Mohammed et al, 2017 (Nashaat and Awasthi, 1995). Similarly, in Australia, where several varieties of B. napus were highly susceptible to H. brassicae isolates, several genotypes of B. juncea were highly resistant (Mohammed et al, 2017 . sativus, B. carinata, B. oleracea, and C. abyssinica.…”

“…juncea crops for decades (Barbetti and Khangura, 2000), it shows increasing severity and threat to commercial brassica crops in some regions of Australia, particularly since 1998 (Barbetti and Khangura, 2000). This may be in part associated with periods of less than ideal rainfall in autumn–early winter and to the continued deployment of one or more highly susceptible varieties, as both seem to foster development of downy mildew (Mohammed et al, 2017). In addition, the diverse pathotypes and phylogenetic variation recently reported in Australian H .…”

Current Status and Challenges in Identifying Disease Resistance Genes in Brassica napus

“…However, the study failed to delineate pathotypes of H. parasitica , which might explain variability in the resistance outcome. More recently, Mohammed et al ( 2017 ) tested 131 Brassicaceae lines which highlighted excellent resistance to Downy Mildew, and was the first study to demonstrate the existence of very high levels of pathotype-independent resistance in Australian B. napus rapeseed varieties. However, no molecular analysis of these lines has been performed in order to identify resistance genes.…”