Biological Significance of Mefenoxam Resistance in Phytophthora erythroseptica and Its Implications for the Management of Pink Rot of Potato

Abstract

Tubers from plants treated with in-furrow and foliar applications of mefenoxam were inoculated with eight isolates of Phytophthora erythroseptica having varying levels of sensitivity to the fungicide. Two isolates with effective concentration causing 50% reduction of mycelial growth (EC50) values of 0.02 and 0.04 μg ml-1 were categorized as being mefenoxam sensitive. Isolates with EC50 values >1.0 μg ml-1 were designated as insensitive to mefenoxam and were grouped two each into low intermediate (EC50 = 1.1 and 5.3 μg ml-1), high intermediate (EC50 = 26 and 74 μg ml-1), and resistant (EC50 ≥ 100 μg ml-1). The biological significance of these isolates was examined by quantifying disease control. P. erythroseptica isolates classified in the resistant group infected a significantly greater proportion of untreated tubers than isolates in any other group. Mefenoxam reduced infection frequency of sensitive isolates by as much as 37%. Mefenoxam did not provide disease control of any isolate possessing insensitivity to the fungicide, with the greatest decrease in control observed with the low intermediate group. Aggressiveness indices, representing tuber infection frequency and depth of penetration, were calculated for untreated and mefenoxam-treated tubers. According to these indices, both isolates classified in the resistant group and high intermediate isolate 252–4 were more aggressive than sensitive isolates in the absence of mefenoxam pressure, and significantly so in the presence of mefenoxam. These results suggest that pink rot may become more severe in fields known to contain P. erythroseptica populations with mefenoxam EC50 values >1.0 μg ml-1 if the fungicide is applied. These factors should be considered when developing strategies to manage pink rot and mefenoxam-resistant populations of P. erythroseptica.

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

“…One problem with fungicide resistance assays of any type is the degree to which biological significance can be attributed to the results (Taylor et al 2006). In an effort to improve interpretation, the results have been compared with concentrations applied in the field under Andean conditions (Table 3).…”

Phytophthora erythroseptica (Pink Rot) Development in Russet Norkotah Potato Grown in Buffered Hydroponic Solutions II. pH Effects

“…This study has helped define a possible cultural method as an additional option to fungicides in managing outbreaks of pink rot. This could be increasingly important as P. erythroseptica becomes resistant to the few effective fungicides currently available for pink rot control (Taylor et al 2006). …”

Biological Control Agents for Suppression of Post-Harvest Diseases of Potatoes: Strategies on Discovery and Development

“…Most commercially grown potato cultivars in Canada and the United States are susceptible to pink rot and breeding efforts against this disease have been minimal (Peters et al, 2004). Mefenoxam, a phenylamide fungicide that formerly was effective in reducing the disease in storage, has lost much of its effectiveness (Taylor et al, 2006) due to widespread genetic resistance (Taylor et al, 2002) and the stability of the resistance (Abu-El Samen et al, 2005). The use of various salts (Mills et al, 2005), foliar applications of phosphorous acid (Johnson et al, 2004) and the oomycete fungicides "zoxamide" and phosphite (Miller et al, 2006) have reduced symptoms of P. erythroseptica on tubers.…”