Novel Resistances Provide New Avenues to Manage Alternaria Leaf Spot (Alternaria brassicae) in Canola (Brassica napus), Mustard (B. juncea), and Other Brassicaceae Crops

Abstract

Alternaria leaf spot (Alternaria brassicae) can be a devastating disease in canola (Brassica napus) and mustard (B. juncea), but there are no highly effective host resistances available. Screening of 150 diverse Brassicaceae varieties under glasshouse conditions highlighted important novel resistances. In particular, Camelina sativa '4076' and Diplotaxis erucoides 'Wasabi Rocket' had complete resistance across disease assessment parameters (leaf incidence [%LDI]; severity [%LAD]; consequent defoliation [%LCI]). The next most resistant varieties were C. sativa 'CSA' (%LDI 0.6; %LAD 0.4), '4144' (%LDI 1.2; %LAD 0.5), '405' (%LDI 1.7; %LAD 0.7), C. sativa '3274' (%LDI 2.5; %LAD 0.8), Carrichtera annua 'CAN3' (%LDI 7.7; %LAD 4.0), and Sisymbrium irio 'London Rocket' (%LDI 2.1; %LAD 0.8), all with %LCI values of 0. Other genotypes showing high-level resistance included S. erysimoides 'SER 4' (%LDI 11.8; %LAD 5.6; %LCI 0) and D. cardaminoides 'Wild Rocket' (%LDI 15.5; %LAD 7.2; %LCI 0), and those showing moderate resistance were Brassica carinata 'ML-EM-1' (Rungwe), B. insularis 'Moris', B. napus 'ZY006', B. oxyrrhina 'BOX1', B. oleracea var. capitata 'Sugarloaf', B. tournefortii 'CN01-104-2', and Sinapis alba 'Concerta' with %LDI 21.6 to 29.8, %LAD 12.8 to 21.0, and %LCI 0 to 5.7. In particular, B. napus 'ZY006' for canola and B. oleracea var. capitata 'Sugarloaf' can now be directly utilized (i.e., without crossing impairment) for Brassica species and vegetable breeding programs, respectively. While all B. juncea genotypes were susceptible, there were some less susceptible varieties from India in comparison with genotypes from Australia or China. The most susceptible test genotype was Rapistrum sativus (%LDI 89.4; %LAD 83.9; %LCI 71.0), highlighting the value of the resistances identified. These findings not only highlight a range of novel resistances against A. brassicae for canola, mustard, and other diverse Brassicaceae breeding programs to develop resistant commercial varieties, but also emphasize highly susceptible varieties to avoid in both breeding programs and commercial situations conducive to Alternaria leaf spot.

Keywords: Alternaria brassicae; Alternaria leaf spot; Brassica juncea; Brassica napus; Brassicaceae; canola; mustard.