In vivo sensitivity of Phakopsora pachyrhizi to fungicides

Amanda Chechi,Walter Boller,Carolina Cardoso Deuner,Valéria Cecília Ghissi-Mazetti,Elias Zuchelli,Carlos Alberto Forcelini

doi:10.1590/0103-8478cr20190593

Abstract

ABSTRACT: Asian soybean rust is one of the most destructive diseases that can be found in this crop. It can be largely controlled by fungicide application. The objective was to assess the sensitivity of P. pachyrhizi isolates to fungicides. The tests were performed in a completely randomized design, with six replicates. The sensitivity of twelve isolates to site-specific and multisite fungicides at concentrations of 0.1; 1.0; 10.0, and 100.0 mg L-1, plus a control with absence of fungicide (0.0 mg L-1) was assessed. Soybean leaflets were immersed in the appropriate fungicide solutions, disposed in wet chambers in plastic boxes, and inoculated using each uredinia suspension of P. pachyhrizi (5.0 x 104 uredospores mL-1), separately. Boxes were incubated for 20 days at a temperature of 23°C and a 12-hour photoperiod. Next, the number of uredinia per cm2 on the abaxial face of each leaflet was evaluated. The active ingredients prothioconazole, trifloxystrobin, fluxapiroxade, trifloxystrobin + prothioconazole, trifloxystrobin + bixafen + prothioconazole, azoxystrobin + benzovindiflupyr, and azoxystrobin + benzovindiflupyr + diphenoconazole were highly fungitoxic for the majority of the isolates, with EC50 lower than 1.0 mg L-1. Diphenoconazole, azoxystrobin, and fenpropimorph were considered moderately fungitoxic for nine of the twelve isolates, with EC50 between 1 and 10 mg L-1. The multisites mancozeb and copper oxychloride presented EC50 responses classified as low toxic for the twelve isolates and eight for chlorothalonil (EC50 between 10 mg L-1 and 50 mg L-1). Site-specific fungicides showed high-to-moderate fungitoxicity to P. pachyrhizi isolates, even as the multisites presented moderate-to-less toxic activity.

Highlights

The P. pachyrhizi fungus is the causal agent for Asian soybean rust (ASR) and it had been first reported in Brazil, at the end of the 2000/2001 season, as being a threat to the American continent (GODOY et al, 2006)
Different sensitivity values of P. pachyrhizi isolates were observed for the tested fungicides
Among the sterol demethylation inhibitors (Table 3), the fungicide prothioconazole showed differences up to 8.2 times in the effective concentration that controlled 50% of the uredinia caused by P. pachyrhizi isolates

Summary

Introduction

The P. pachyrhizi fungus is the causal agent for Asian soybean rust (ASR) and it had been first reported in Brazil, at the end of the 2000/2001 season, as being a threat to the American continent (GODOY et al, 2006). In Brazil, the application of fungicides for ASR control was Approved 10.25.19 Returned CR-2019-0593.R1 by the author. V.50, n.1, initiated in the 2002–2003 soybean season, using products from the triazoles group (demethylation inhibitors - DMIs), followed by mixtures of triazoles and strobilurins (quinone outside inhibitors - QoIs) (REIS et al, 2014); and mixtures containing carboxamides (succinate dehydrogenase inhibitors SDHIs), with the use of multisites and morpholines as disease control reinforcement. EDGINGTON et al (1971) defined the criteria for associating a fungicidal substance with the level of fungitoxicity. In this regard, the authors elaborated a scale, based on the concentrations of active ingredients that inhibited 50% of spore germination (50% inhibitory concentration - IC50) or on the ones that controlled 50% of the disease severity (effective concentration of 50% - EC50). Concentrations less than 1 mg L-1 are considered highly toxic to the fungus, values between 1 and 10 mg L-1 are moderate, between 10 and 50 mg L-1 are slightly fungitoxic, and above 50 mg L-1, have no toxic effect on the fungus

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