Mefenoxam sensitivity and fitness analysis of Phytophthora nicotianae isolates from nurseries in Virginia, USA

Abstract

Mefenoxam is one of the most commonly used fungicides for managing diseases caused by Phytophthora spp. on ornamentals. The objectives of this study were to determine whether Phytophthora nicotianae , a destructive pathogen of numerous herbaceous annual and perennial plant species in nurseries, has developed resistance to mefenoxam, and to evaluate the fitness of mefenoxam-resistant isolates. Ninety-five isolates of P. nicotianae were screened for sensitivity to mefenoxam on 20% clarified V8 agar at 100 a.i. μ g mL -1. Twenty-five isolates were highly resistant to this compound with EC 50 values ranging from 235·2 to 466·3 μ g mL -1 and four were intermediately resistant with EC 50 values ranging from 1·6 to 2·9 μ g mL -1. Sixty-six isolates were sensitive with EC 50 values less than 0·04 μ g mL -1. Nine resistant and seven sensitive isolates were tested for mefenoxam sensitivity on Pelargonium × hortorum cv. White Orbit. Mefenoxam provided good protection of pelargonium seedlings from colonization by sensitive isolates, but not by any resistant isolates. Four resistant and four sensitive isolates were compared for fitness components and their relative competitive ability on Lupinus Russell Hybrids in the absence of mefenoxam. Resistant isolates outcompeted sensitive ones within 3 to 6 sporulation cycles on lupin seedlings, regardless of their initial proportions in mixed zoospore inoculum. Resistant isolates exhibited greater infection rate and higher sporulation ability than sensitive ones when they were applied separately onto lupins. These results suggest that fungicide resistance may pose a serious challenge to the continued effectiveness of mefenoxam as a control option for nursery growers.

Diversity and fitness of Plasmopara viticola isolates resistant to QoI fungicides

“…As with other studies on the fitness of oomycetes, such as Phytophthora nicotianae or P. infestans resistant to metalaxyl or mefenoxam (Hu et al 2008;Kadish and Cohen 1988), QoI resistance did not show reduced fitness. However, it is possible that resistance cost might only become apparent under a specific set of environmental conditions which were not encountered in the experimental set-up, or that the associated antagonistic pleiotropy might not be detectable (Rosenberg 2001); or, yet again, that a fitness deficit linked to a resistance cost can be compensate for after numerous generations by different compensatory mechanisms (Anderson 2005;Cowen et al 2001).…”

Race distribution and distribution of sensitivities to mefenoxam among isolates of Phytophthora parasitica var. nicotianae in Guizhou province of China

“…Due to high activity of the chemical, isolates of P. nicotianae had rarely occurred in the Table 1 Distribution and ranking of EC 50 values of Phytophthora parasitica isolates collected from various cultivars of tobacco in 19 tobacco commercial districts in Guizhou province of China and exposed to mefenoxam. fungicide-treated areas until 2008 (Hu et al, 2008). However, the disease was frequently found in flue-cured tobacco fields in the last five years.…”

Competitive Parasitic Fitness of Mefenoxam-Sensitive and -Resistant Isolates of Phytophthora erythroseptica under Fungicide Selection Pressure

“…Lesion size on potato leaves increased significantly in the absence of selection pressure when the leaves were inoculated with a metalaxyl-resistant isolate compared with a metalaxyl-sensitive isolate of P. infestans; however, there was no significant difference in sporulation capacity (22,23). Metalaxyl-resistant isolates were better competitors than metalaxyl-sensitive isolates in studies conducted with P. infestans from potato and P. nicotianae from citrus as well as lupin (20,22,40). Additionally, no significant difference was observed between the aggressiveness (as measured by disease incidence and severity) of mefenoxam-resistant and -sensitive P. erythroseptica isolates in the absence or presence of mefenoxam selection pressure (37).…”