Detection of the Phoma pathogens Plenodomus biglobosus subclades ‘brassicae’ and ‘canadensis’ on wasabi, and ‘canadensis’ in Europe

Abstract

Phoma stem canker / blackleg is an internationally important disease of Brassicas including B. napus (oilseed rape, OSR), caused by multiple genetic subclades of the fungi Plenodomus lingam (formerly Leptosphaeria maculans) and P. biglobosus (L. biglobosa). In Spring 2021, Phoma-like disease symptoms were observed on leaves and stems of Eutrema japonicum (wasabi) crops at three UK sites (Northern Ireland, Southern England and the West Midlands). Fungal isolation from wasabi leaf spots yielded colonies with two distinct phenotypes on potato dextrose agar (PDA). Isolates from the Northern Ireland and Southern England sites had white colonies with abundant pink cirri that were confirmed (based on ITS rDNA, beta tubulin and actin sequences) as P. biglobosus subclade ‘canadensis’ (Pbc). Those from the West Midlands site, however, had yellow pigmented colonies and were confirmed by sequencing as P. biglobosus subclade ‘brassicae’ (Pbb). Greenhouse pathogenicity testing showed that Pbb and Pbc wasabi isolates were pathogenic not only to this host but also OSR, B. oleracea (cabbage), and B. rapa (pak choi). Re-isolation of the fungi was attempted and confirmed from lesions that developed on inoculated OSR and wasabi, thus completing Koch’s postulates. These findings represent new discoveries for both Pbb and Pbc on wasabi, plus for Pbc in Europe. The crop health implications of these results are briefly considered.

Metabolic Capacity Differentiates Plenodomus lingam from P. biglobosus Subclade ‘brassicae’, the Causal Agents of Phoma Leaf Spotting and Stem Canker of Oilseed Rape (Brassica napus) in Agricultural Ecosystems

“…The species and sub-clade occurrence and frequency greatly depend on the region and host plant [23][24][25][26]. For example, wasabi plants are mainly infected by P. wasabiae [27], currently called P. biglobosus, with two sub-clades, 'brassicae' and 'canadensis', both found recently in Europe [28].…”

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“…Although the P. biglobosus pure culture on PSA differed from P. lingam in production of diffusible dark yelloworange pigment, this feature alone would not be sufficient for reliable species identification, because it is known that only some P. biglobosus isolates produce this pigment (King, West, 2022). There is little difference between these species in micromorphological features of spore-bearing structures.…”

“…Koidz., Raphanus (radish and daikon), and Sinapis (black and white mustard). Several wild Brassicaceae (Descurainia, Sisymbrium, and Thlaspi) can also be infected with these fungi (West et al, 2001;Fitt et al, 2006;Zou et al, 2019;King, West, 2022). Plenodomus biglobosus and P. lingam differ from each other by micromorphological, cultural, physiological, biochemical and molecular phylogenetic features (Mendes-Pereira et al, 2003;Liu et al, 2014;Frac et al, 2022).…”

“…Seven subclades have been identified for P. biglobosus within the subclade 'brassicae' occurring mostly in Brassicaceae crops (Mendes-Pereira et al, 2003;Zou et al, 2019). The representatives of this subclade was also found in wasabi in the UK (King, West, 2022). In Oregon, USA, the subclade P. biglobosus 'americensis' was recorded in oilseed rape (Zou et al, 2019).…”

“…In Oregon, USA, the subclade P. biglobosus 'americensis' was recorded in oilseed rape (Zou et al, 2019). In Australia, Canada, China, Mexico, the UK and the USA, P. biglobosus 'canadensis' was recorded in the Brassicaceae species, such as E. japonicum and Thlaspi arvense L. (Dilmaghani et al, 2009;Mendes-Pereira et al, 2003;Van de Wouw et al, 2008;Luo et al, 2021;King, West, 2022). In Australia and the USA, P. biglobosus 'australensis' was recorded in B. napus and B. juncea (L.) Czern.…”