# Intrapopulation changes in Puccinia hordei induced by two-component fungicides from different chemical classes

## TL;DR

This study shows how fungicides affect the virulence and aggressiveness of a barley rust pathogen in Russia.

## Contribution

The paper provides the first analysis of fungicide effects on Puccinia hordei population dynamics in Russia.

## Key findings

- Higher fungicide application rates reduced virulence to specific Rph genes and increased virulence to Rph14.
- Fungicide-treated populations showed lower average virulence, longer latent periods, and reduced spore production.
- Increased fungicide use led to decreased viability of P. hordei isolates.

## Abstract

Fungicide resistance is a global problem that reduces the effectiveness and duration of action of these compounds due to changes in the racial composition and virulence of phytopathogen populations. Currently, resistance to 100 active substances has been registered in more than 230 fungal plant pathogens. Leaf rust of barley (Puccinia hordei Otth.) is one of the most widespread and harmful pathogens in the barley pathocomplex; it is recorded in southern Russia every year. There are very few studies on the effect of fungicides on the characteristics of rust fungi populations, and none have been carried out on P. hordei in Russia. This research aimed to analyze the effect of fungicides belonging to the chemical classes of triazoles and strobilurins on intrapopulation changes in P. hordei in terms of pathogenicity (virulence and aggressiveness) under the conditions of the North Caucasus region of Russia. Two-component fungicides approved for use in the Russian Federation were selected for the study: Delaro, SC; Amistar Extra, SC; Amistar Gold, SC. Plants were treated using several application rates: 50, 100, 150 and 200 % (the recommended application rate was determined to be 100 %). Treatment of winter barley plants with fungicides with different application rates revealed intrapopulation changes in the virulence structure of P. hordei. In all treatment variants, the frequency of isolates virulent to the Rph4, Rph5, Rph6+2, Rph12 genes decreased with increasing fungicide application rate and the frequency of isolates virulent to Rph14 increased. No isolates virulent to Rph7 were found in either the original population or the experimental variants. The average virulence of the fungal populations treated with the fungicides in all experimental variants was lower compared to the original population (no treatment (48.5 %)) and depending on the application rate varied from 33.8 % (Amistar Gold, 50 %) to 28.5 % (Amistar Gold, 200 %). Under the influence of the increased application rates of the fungicides, an increase in the duration of the latent period was observed: from 168 h (original population) to 216 h (Delaro, Amistar Gold, 200 %). A decrease in sporulation ability (spore mass per pustule ranged from 0.013 mg (original population) to 0.002 mg (Delaro, Amistar Gold, 200 %)) and in the viability of P. hordei (from 100 % for the original population to 22.5 % in Amistar Gold, 200 % treatment) was found under the action of the fungicides. Thus, a fungicide-treated P. hordei population is characterized by intrapopulation changes in aggressiveness and virulence, which can significantly increase barley yield losses due to a decrease in the effectiveness of chemical protection, as well as an increase in the harmfulness of the pathogen.