Resistance to metalaxyl-M and cymoxanil in a dominant clonal lineage of Phytophthora infestans in Huánuco, Peru, an area of continuous potato production

Eur J Plant Pathol (2009) 125:8795 DOI 10.1007/s10658-009-9461-z

Resistance to metalaxyl-M and cymoxanil in a dominant clonal lineage of Phytophthora infestans in Hunuco, Peru, an area of continuous potato production

Wilmer Prez & Johanna Lara & Gregory A. Forbes

Received: 3 July 2008 /Accepted: 12 March 2009 /Published online: 25 March 2009 # KNPV 2009

Abstract Fifty-eight isolates of Phytophthora infestans were evaluated in vitro and on detached leaves of potato for their sensitivities to metalaxyl-M and cymoxanil. The isolates belonged to the clonal lineage, EC-1, which is dominant on potato in Peru and Ecuador. All isolates were collected in Hunuco, Peru, an area of year-round potato production, where the potential for development of fungicide resistance is high. All isolates were resistant to metalaxyl-M, with in vitro EC50 values ranging from 468.30813.57 mg l1. In contrast, we found no evidence for resistance to cymoxanil for which in vitro EC50 values ranged from0.031.11 mg l1. Resistance to each fungicide was also evaluated for five isolates in a detached leaf assay in which the fungicide was sprayed on the leaf surface prior to inoculation. With metalaxyl-M, the range of EC50 values was 158.85828.29 mg l1, similar to that for the in vitro assay. For cymoxanil, EC50 values ranged from 1.41 to 2.31 mg l1, which was higher than in the in vitro assay but still two orders of magnitude lower than the concentration applied by farmers in the field.

Keywords Fungicide resistance . Potato late blight . Disease management . EC50

Introduction

The continuing changes in populations of Phytophthora infestans worldwide have made the management of late blight increasingly difficult. Due to the introduction and subsequent global migration of new genotypes of P. infestans, pathogen populations are now generally characterised by greater aggressiveness, earlier outbreaks of disease and increased resistance to fungicides (Hannukkala et al. 2007; Fry 2008).

The initial high efficacy of the phenylamide fungicide, metalaxyl, against oomycete pathogens led to its widespread use for many diseases, including potato late blight. However, resistance to metalaxyl was reported shortly after its introduction (Davidse et al. 1981) and has since been reported in most potato growing regions of the world (Fry et al. 2009).

The population of P. infestans in the Andes north of Bolivia is dominated by...