Trade-Off Between Triadimefon Sensitivity and Pathogenicity in a Selfed Sexual Population of Puccinia striiformis f. sp. Tritici

Sensitivity of Puccinia triticina f. sp. tritici from China to Triadimefon and Resistance Risk Assessment

Wheat leaf rust, caused by Puccinia triticina f. sp. tritici (Pt), is distributed widely in wheat-producing areas and results in serious yield losses worldwide. In China, leaf rust has been largely controlled with a demethylation inhibitor (DMI) fungicide, triadimefon. Although high levels of fungicide resistance in pathogens have been reported, no field failure of wheat leaf rust to DMI fungicides has been reported in China. A resistance risk assessment of triadimefon to Pt was investigated in the present study. The sensitivity of 197 Pt isolates across the country to triadimefon was determined, and the density distribution of EC50 values (concentration at which mycelial growth is inhibited by 50%) showed a continuous multimodal curve because of the extensive use of this fungicide in wheat production, with a mean value of 0.46 μg/ml. The majority of the tested Pt isolates were sensitive to triadimefon, whereas 10.2% developed varying degrees of resistance. Characterization of parasitic fitness revealed that the triadimefon-resistant isolates exhibited strong adaptive traits in urediniospore germination rate, latent period, sporulation intensity, and lesion expansion rate. No correlation was observed between triadimefon and tebuconazole and hexaconazole, which have the similar mode of action, or pyraclostrobin and flubeneteram, which have different modes of action. Overexpression of the target gene Cyp51 led to the triadimefon resistance of Pt. The risk of resistance to triadimefon in Pt may be low to moderate. This study provided important data for fungicide resistance risk management against wheat leaf rust.

Modeling plant diseases under climate change: evolutionary perspectives

Infectious plant diseases are a major threat to global agricultural productivity, economic development, and ecological integrity. There is widespread concern that these social and natural disasters caused by infectious plant diseases may escalate with climate change and computer modeling offers a unique opportunity to address this concern. Here, we analyze the intrinsic problems associated with current modeling strategies and highlight the need to integrate evolutionary principles into polytrophic, eco-evolutionary frameworks to improve predictions. We particularly discuss how evolutionary shifts in functional trade-offs, relative adaptability between plants and pathogens, ecosystems, and climate preferences induced by climate change may feedback to future plant disease epidemics and how technological advances can facilitate the generation and integration of this relevant knowledge for better modeling predictions.

Effects of Flubeneteram on Inhibiting the Development of Puccinia striiformis f. sp. tritici in Wheat Leaves

Stripe rust caused by Puccinia striiformis f. sp. tritici (Pst) is a serious threat to wheat production, and the application of fungicides is one of the most important means for controlling the disease. The purpose of this study is to determine the effects of a new succinate dehydrogenase inhibitor (SDHI) fungicide, flubeneteram, on reducing stripe rust. The baseline sensitivity of 173 Pst isolates from 13 provinces of China to flubeneteram was determined. Flubeneteram displayed significant effects on inhibiting SDH enzymes of Pst. Histological observations showed that after flubeneteram application, the formation and development of Pst hyphae and haustoria were significantly inhibited, and the structures were destroyed. Flubeneteram primed wheat for salicylic acid-induced defenses via upregulating pathogenesis-related genes (PR1 and PR2). Altogether, our study is the first to provide evidence that flubeneteram induces wheat defense against Pst infection. The findings indicate that flubeneteram could be an effective fungicide for managing stripe rust.

Rapid and Sensitive Detection of Fungicide-Resistant Crop Fungal Pathogens Using an Isothermal Amplification Refractory Mutation System

Fungicide abuse leads to the emergence of fungicide-resistant fungal pathogens, thus posing a threat to agriculture and food safety. Here, we developed an isothermal amplification refractory mutation system (termed iARMS) allowing us to resolve genetic mutations, enabling rapid, sensitive, and potentially field-applicable detection of fungicide-resistant crop fungal pathogens. iARMS yielded a limit of detection of 25 aM via a cascade signal amplification strategy of recombinase polymerase amplification (RPA) and Cas12a-mediated collateral cleavage at 37 °C within 40 min. Specificity for fungicide-resistant Puccinia striiformis (P. striiformis) detection was guaranteed by RPA primers and the flexible sequence of gRNA. The iARMS assay allowed us to detect as low as 0.1% cyp51-mutated P. striiformis that showed resistance to the demethylase inhibitor (DMI), which was 50 times more sensitive than the sequencing techniques. Thus, it is promising for the discovery of rare fungicide-resistant isolates. We applied iARMS to investigate the emergence of fungicide-resistant P. striiformis in western China and found that its proportion was over 50% in Qinghai, Sichuan, and Xinjiang Province. iARMS can serve as a molecular diagnostic tool for crop diseases and facilitate precision plant disease management.

Sensitivity and Resistance Risk Assessment of Puccinia striiformis f. sp. tritici to Triadimefon in China

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a destructive disease of wheat that seriously threatens production safety in wheat-producing areas worldwide. In China, the disease has been largely controlled with the fungicide triadimefon. Although high levels of fungicide resistance in other fungal pathogens have been reported, failure to control Pst with any fungicides has seldomly been reported, and fungicide sensitivity of Pst has not been evaluated in China. The distribution of triadimefon-resistant Pst isolates was investigated in the present study. The baseline sensitivity of 446 Pst isolates across the country to triadimefon was determined, and the concentration for 50% of maximal effect showed a unimodal distribution curve, with a mean value of 0.19 μg ml^-1. The results indicated a wide range of sensitivity to triadimefon, with more insensitive isolates collected from Pst winter-increasing areas and northwest oversummering areas, whereas more sensitive isolates were collected from southwest oversummering areas and epidemic areas of Xinjiang and Tibet. The majority of the tested Pst isolates were sensitive to triadimefon; only 6.79% had developed varying degrees of resistance. Characterization of parasitic fitness revealed that the triadimefon-resistant isolates exhibited strong adaptive traits in the urediniospore germination rate, latent period, sporulation intensity, and lesion expansion rate. Positive cross-resistance was observed between triadimefon and tebuconazole or hexaconazole, but not between pyraclostrobin or flubeneteram. The point mutation Y134F in the 14α-demethylase enzyme (CYP51) was detected in triadimefon-resistant isolates. A molecular method (kompetitive allele-specific PCR) was established for the rapid detection of Y134F mutants in the Pst population. Two genotypes with one point mutation Y134F conferred resistance to triadimefon in Pst. The risk of resistance to triadimefon in Pst may be low to moderate. This study provided important data for establishment of high throughput molecular detection methods, fungicide resistance risk management, and the development of new target fungicides.

High frequency of fungicide resistance-associated mutations in the wheat yellow rust pathogen Puccinia striiformis f. sp. tritici

BACKGROUND

Reliance on fungicides to manage disease creates selection pressure for the evolution of resistance in fungal and oomycete pathogens. Rust fungi (Pucciniales) are major pathogens of cereals and other crops and have been classified as low-risk for developing resistance to fungicides; no case of field failure of fungicides in a cereal rust disease has yet been recorded. Recently, the Asian soybean rust pathogen, Phakopsora pachyrhizi evolved resistance to several fungicide classes, prompting us to screen a large sample of the globally widespread wheat yellow rust pathogen, Puccinia striiformis f. sp. tritici (Pst), for mutations associated with fungicide resistance.

RESULTS

We evaluated 363 Pst isolates from Europe, the USA, Ethiopia, Chile, China and New Zealand for mutations in the target genes of demethylase inhibitor (DMI; Cyp51) and succinate dehydrogenase inhibitor (SDHI; SdhB, SdhC and SdhD) fungicides. A high proportion of Pst isolates carrying a Y134F DMI resistance-associated substitution in the Cyp51 gene was found among those from China and New Zealand. A set of geographically diverse Pst isolates was also found to display a substitution in SdhC (I85V) that is homologous to that reported recently in P. pachyrhizi and linked to SDHI resistance.

CONCLUSION

The identification of resistance-associated alleles confirms that cereal rusts are not immune to fungicide resistance and that selection for resistance evolution is operating at high levels in certain locations. It highlights the need to adopt fungicide resistance management practices and to monitor cereal rust species for development of resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Role of Fungicide Applications on the Integrated Management of Wheat Stripe Rust

First described in Europe in 1777, stripe rust (SR) caused by Puccinia striiformis Westend. f. sp. tritici Erikss (Pst) is one of the most important and destructive diseases of wheat worldwide. Until 2000, SR was mainly endemic to cooler regions, but since then, new aggressive strains have emerged, spread intercontinentally, and caused severe epidemics in warmer regions across the world. This has put SR as a disease that poses a threat to the world food security. At present, the preferred strategy for control of SR is the access to wheat cultivars with adequate levels of SR resistance. However, wheat breeding programs are not sufficiently advanced to cope with the recently emerged Pst strains. Under this scenario, foliar fungicide applications have become an important component of SR management, but information on the effects of fungicide applications on SR control and wheat cultivar yield response is scarce. This review seeks to provide an overview of the impact and role of fungicides on SR management. With focus on wheat management in the major wheat-growing regions of the world, the review addresses: (a) the efficacy of different fungicide active ingredients, optimal fungicide timing and number of applications in controlling SR, and (b) the impact of fungicide on wheat grain yield response. Inclusion of fungicides in an integrated crop management approach is discussed.