Fungicide resistance in Pyricularia oryzae populations from southern and northern Brazil and evidence of fitness costs for QoI-resistant isolates

Rice blast management is heavily reliant on chemical control, and the rise of fungicide resistance is contingent on fitness costs. This study contrasts the responses of 322 blast isolates from southern (n = 224) and northern Brazil (n = 98) to fungicides in the Quinone outside inhibitor (QoI), Sterol demethylation inhibitor (DMI), and Melanin biosynthesis inhibitor (MBI) groups, and associated epidemiological traits. Resistance was more prevalent in northern Brazil, where conditions are most conducive to blast. Northern populations displayed higher EC50 for spore germination, faster mycelial growth rates, and higher rates of appressorium formation for fungicides in all chemical groups. Results were most striking for the QoI fungicide azoxystrobin. We examined possible fitness penalties and related in planta competitive abilities associated with the gain of azoxystrobin resistance. QoIresistant (QoI-R) genotypes, carrying the G143A cytochrome b gene mutation, were found in 90% of the northern isolates. The EC50 values of these QoI-R isolates exceeded 10 mu g L-1. However, QoI-R isolates had a reduced number of conidia, longer latent period, reduced infection efficiency and aggressiveness, compared to the sensitive isolates. These impairments were reflected in a decrease of competitive ability in planta in the absence of azoxystrobin. Nevertheless, under continuous fungicide applications, the proportion of resistant and sensitive isolates in mixed cultures in planta remained constant after four generations. The results are valuable for managing strategies as the increase of resistance may hamper the competitive abilities of QoI-R mutants in the absence of the respective active ingredient.