Monitoring of Brazilian wheat blast field populations reveals resistance to QoI, DMI, and SDHI fungicides

Abstract

Wheat blast is one of the most important and devastating fungal diseases of wheat in South America, South‐east Asia, and now in southern Africa. The disease can reduce grain yield by up to 70% and is best controlled using integrated disease management strategies. The difficulty in disease management is compounded by the lack of durable host resistance and the ineffectiveness of fungicide sprays. New succinate dehydrogenase inhibitor (SDHI) fungicides were recently introduced for the management of wheat diseases. Brazilian field populations of the wheat blast pathogen Pyricularia oryzae Triticum lineage (PoTl) sampled from different geographical regions in 2012 and 2018 were shown to be resistant to both QoI (strobilurin) and DMI (azole) fungicides. The main objective of the current study was to determine the SDHI baseline sensitivity in these populations. Moderate levels of SDHI resistance were detected in five out of the six field populations sampled in 2012 and in most of the strains isolated in 2018. No association was found between target site mutations in the sdhB, sdhC, and sdhD genes and the levels of SDHI resistance, indicating that a pre‐existing resistance mechanism not associated with target site mutations is probably present in Brazilian wheat blast populations.

Higher fitness and competitive advantage of Pyricularia oryzae Triticum lineage resistant to QoI fungicides

“…Such strategies should be focused on reducing QoI‐R PoTl populations, which include the emergent and preferential adoption of low risk broad spectrum multi‐site fungicides belonging to distinct chemical groups other than QoIs (such as mancozeb, and chlorothalonil, and copper based fungicides) for controlling wheat diseases in general and not only for managing wheat blast 31 . As the PoTl resistance to QoIs fungicides is widespread in wheat fields from Brazil and it is not associated with fitness costs, further spray of QoIs in the crop will lead to the fixation of the resistance across the wheat cropping areas, leading to fungicides becoming inefficient 8,15 . Another important strategy is to adopt integrated management strategies such as biological control spraying labelled biofungicides formulated with Bacillus methylotrophicus , Chaetomium globosum or Trichoderma harzianum , and/or applying other alternative compounds including potassium phosphite or potassium silicate, which have an important role in reducing wheat blast severity 9,32–34 .…”

“…31 As the PoTl resistance to QoIs fungicides is widespread in wheat fields from Brazil and it is not associated with fitness costs, further spray of QoIs in the crop will lead to the fixation of the resistance across the wheat cropping areas, leading to fungicides becoming inefficient. 8,15 Another important strategy is to adopt integrated management strategies such as biological control spraying labelled biofungicides formulated with Bacillus methylotrophicus, Chaetomium globosum or Trichoderma harzianum, and/or applying other alternative compounds including potassium phosphite or potassium silicate, which have an important role in reducing wheat blast severity. 9,[32][33][34] Future studies on the stability of the cytb G143A mutation in pathogen populations may shed light on whether the higher fitness and competitive advantage of QoI-R PoTl isolate group remains after successive cycles without fungicide selection pressure.…”

“…(2021), were selected for testing the relevant hypotheses in this study (Table 1). 6,8 These isolates were obtained from symptomatic heads from wheat fields in Goiás (GO), Mato Grosso do Sul (MS) and Parana states, Brazil, in 2006, 2012, and 2018, respectively (Table 1). These isolates were deposited in the Molecular Plant Pathology fungal collection from São Paulo State University, Ilha Solteira, SP, and the fungal collection from the Department of Plant Pathology at the Federal University of Lavras, Lavras, MG.…”

“…5 Due to the lack of durable blast resistance on wheat cultivars, fungicide sprays targeting wheat heads have been the main strategy for managing wheat blast, despite its low efficacy resulting from fungicide resistance. [6][7][8] Resistance to quinone outside inhibitor (QoI) fungicides in populations of PoTl was reported in Brazil in 2015. 6,9 The QoI resistance described is provided by a nonsynonymous mutation in the cytochrome b (cytb) gene, in which the amino acid glycine is replaced by alanine at codon 143 (G143A).…”

“…21 In fact, contrasting between older (2012) and newer (2018) populations of the PoTl from Brazil indicated that resistance to QoI fungicide is pervasive and persisted in the same geographical locations after 6 years. 8 Therefore, the aim of this study was to determine if there are any differences in fitness parameters and competitive ability between QoI-resistant (R) and sensitive (S) PoTl field isolates. 1).…”