Virulence assessment of Australian Pyrenophora tritici‐repentis isolates

Pao Theen See,Nikki Schultz,Manisha Shankar,Blake Wood,Kalai A Marathamuthu,Kefei Chen,Caroline S Moffat

doi:10.1111/ppa.13484

Abstract

AbstractThe virulence of 57 Australian isolates of Pyrenophora tritici‐repentis (Ptr), a necrotrophic fungal pathogen responsible for the major wheat disease tan spot, was assessed through plant infection assays. Isolates collected from the northern, southern, and western wheat‐cropping regions of Australia were evaluated against 16 Australian bread wheat cultivars under controlled growth conditions. Following infection, the wheat panel displayed varying disease symptoms ranging from tiny necrotic specks to spreading chlorotic and necrotic lesions. Analysis of variance indicated that the wheat cultivar exhibited a greater effect on the disease response, explaining 62.7% of the variation, in comparison to the isolate (10.4%). The interaction between the cultivar and the isolate was statistically significant and was attributed to 9.8% of the total variation. All Ptr isolates examined were able to cause disease, but did not display a clear distinction in virulence on the wheat panel investigated, instead showing subtle differences in aggressiveness. Based on the disease responses, there was no obvious pattern between isolate aggressiveness and cropping region. Some cultivars, such as Hydra, exhibited an effective level of resistance in relation to the panel of isolates tested. All 57 Ptr isolates were found to possess the ToxA effector gene and lack the ToxB effector gene. The gene expression level of ToxA was up‐regulated at 3 days postinfection in both ToxA‐sensitive and ‐insensitive cultivars, independent of ToxA–Tsn1 recognition.

Highlights

Tan spot disease of wheat is caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis (Ptr)
This study aims to (a) determine the level of Ptr virulence in Australia across the three cropping regions, and (b) assess the effect of Ptr isolates and wheat cultivars on the tan spot disease response
Plant infection assays were performed in a controlled growth environment on a panel of 16 Australian bread wheat cultivars with tan spot disease ratings that ranged from moderately resistant (MR) to susceptible/very susceptible (SVS) (Table S2)

Summary

Introduction

Tan spot disease of wheat ( known as yellow spot) is caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis (Ptr). The extent of symptom development varies depending on the host genotype, with lesion types ranging from small black spots to extensive chlorosis or tan necrosis, which differentiate a resistant or susceptible reaction to the pathogen, respectively (Lamari & Bernier, 1989). Like many parts of the world, in Australia the predominant Ptr effector is ToxA, which causes extensive necrosis on wheat lines possessing the sensitivity gene Tsn and is generally correlated with tan spot disease susceptibility (Faris et al, 2010; Friesen et al, 2006; Kamel et al, 2019; See et al, 2018, 2019). The disease was initially reported in the northern cropping region (New South Wales and Queensland; Valder & Shaw, 1952), the occurrence of tan spot is widespread across the country

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