Role of foliage component and host age on severity of Alternaria leaf spot (caused by Alternaria japonica and A. brassicae) in canola (Brassica napus) and mustard (B. juncea) and yield loss in canola

Abstract

Studies were undertaken under controlled conditions into the effects of different foliage components (cotyledon, first, second and third leaf) at three plant ages (3, 5 and 7 weeks old) on development of Alternaria leaf spot disease, caused by Alternaria japonica or A. brassicae, in canola (Brassica napus cv. Thunder TT) and mustard (B. juncea cv. Dune). Alternaria japonica generally showed percentage disease index (%DI) values similar to A. brassicae across the two Brassica species, different foliage components and plant ages. %DI from either pathogen was greater in older plants than younger plants for the same foliage components in both cultivars. Field studies were then undertaken with canola to compare disease development from A. japonica and A. brassicae across different plant components (leaf, pod and stem) and the consequent adverse impact on seed yield. Alternaria japonica was more severe in terms of leaf area diseased (%LAD 62.6) and stem area diseased (%SAD 69.8) than pod area diseased (%PAD 25.5), whereas A. brassicae was more severe on leaves (%LAD 61.9) than on pods (%PAD 47.4) or stems (%SAD 41.0). Stem disease incidence was greater for A. japonica (%SDI 94.0) than for A. brassicae (%SDI 56.5), but pod disease incidence was greater for A. brassicae (%PDI 93.5) than for A. japonica (%PDI 86.1). For A. japonica, AUDPC values of leaf disease incidence (LDI, 283.5), leaf area diseased (LAD, 253.3) and leaf collapse (LCI, 149.5) resulted in a yield loss of 58.1%, similar to A. brassicae, where AUDPC values of LDI (277.8), LAD (247.2) and LCI (111.0) caused a yield loss of 59.4%. These findings explain observed acceleration of Alternaria leaf spot severity from A. japonica, as from A. brassicae, through the growing season as plants become more susceptible with increasing age, and as more susceptible, later developing leaves become abundant. For the first time, we demonstrate that under conducive field conditions for disease development, A. japonica can cause serious seed-yield losses of a magnitude similar to those occurring with A. brassicae.

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

“…In Victoria, Australia, a B. juncea advanced breeding line had shown resistance against A. brassicae(Van de Wouw et al, 2016), yet recent studies have shown that Australian B. juncea and B. napus varieties tested were all susceptible to A. brassicae, although some of these same varieties did show good resistance to other Alternaria spp. such as A.japonica(Al-lami et al, 2019b). Other recent studies in Australia (Allami et al, 2019a) confirmed 10 species(A. alternata, A. arborescens, A. brassicae, A. ethzedia, A. hordeicola, A. infectoria, A. japonica, A. malvae, A. metachromatica, and A. tenuissima) as present on commercial rapeseed and pathogenic on both B. juncea and B. napus.…”

“…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).…”

Virulence variability across the Alternaria spp. population determines incidence and severity of alternaria leaf spot on rapeseed

“…, ), while A. japonica causes yield loss up to 58% on rapeseed in Australia (Al‐lami et al . , ). In addition, these species are reported to have varying levels of virulence on Brassicaceae.…”

“…, ) and, on rapeseed ( B. napus ), losses of 59% in Australia (Al‐lami et al . , ), 42% in Canada (Degenhardt et al . , ), 17% in India (Kumar, ), and up to 60% in Europe (Smith et al .…”

Understanding Host–Pathogen Interactions in Brassica napus in the Omics Era

“…), and White Rust (obligate biotrophic oomycete Albugo candida ) [ 22 , 23 ]. These diseases have widely caused a yield loss of 24–50% and economic loss of up to USD 200 million in the B. napus industry, with the potential to wipe out the entire crop where not effectively controlled [ 24 , 25 , 26 , 27 , 28 , 29 ]. The most promising approach to controlling diseases of Brassica is through breeding disease-resistant varieties [ 30 ].…”