New specific quantitative real‐time PCR assays shed light on the epidemiology of two species of the Leptosphaeria maculans–Leptosphaeria biglobosa species complex

• Published in: Plant pathology 2021-04, Vol.70 (3), p.643-654
• Main Authors: Jacques, Noémie, Balesdent, Marie‐Hélène, Rouxel, Thierry, Laval, Valérie
• Format: Article
• Language: English
• Subjects: Assaying; Biology; Brassica; Colonization; Environmental Sciences; Epidemiology; Fungi; Genotypes; Inoculum; Leaves; Leptosphaeria; Leptosphaeria biglobosa ‘brassicae’; Leptosphaeria maculans; Leptosphaeria maculans ‘brassicae’; Oilseeds; Plant tissues; Questions; Rape plants; Rapeseed; real‐ time PCR assays; Species; Stem canker; Stems
• ISSN: 0032-0862; 1365-3059
• DOI: 10.1111/ppa.13323

Phoma stem canker, a highly damaging disease of oilseed rape (Brassica napus), is caused by fungi of the Leptosphaeria maculans/Leptosphaeria biglobosa species complex. On B. napus, at least in Europe, L. maculans ‘brassicae’ (LMB) and L. biglobosa ‘brassicae’ (LBB) are often found together, in the same fields. The biology and epidemiology of LMB have been intensively studied, but few studies have investigated the infection and colonization processes of LBB, and unanswered questions remain concerning the relative contributions of these two species to phoma stem canker disease. We addressed this question by developing two species‐specific robust quantitative PCR (qPCR) assays, one for each species, for detecting and quantifying these species in field samples of oilseed rape. The assays were highly specific, reproducible, and sensitive. We used them to obtain an overview of stem canker epidemiology in a two‐year field study of a B. napus genotype susceptible to LMB, and a near‐isogenic line harbouring Rlm11, a gene conferring effective resistance to LMB. Inoculum pressure differed considerably between the two years of the experiment, and we found that LMB leaf spotting intensity at limited observation time points was insufficient for predicting final disease severity. We also found that LBB was more prevalent than previously thought, with epidemiological and colonization biology characteristics very similar to those of LMB, but a necrotrophic lifestyle. The general presence of these two species together on leaves and in the stem tissues of the same individual plant raises questions about their relative incidence and biological interactions within plant tissues. Species‐specific qPCR assays were developed and used to quantify Leptosphaeria maculans and L. biglobosa 'brassicae' on Brassica napus genotypes for two years, revealing the impact of the resistance gene Rlm11 on the two fungal species.