Use of mefenoxam to control pink rot [Phytophthora erythroseptica] of potato in Prince Edward Island

Abstract

Pink rot of potato, caused by Phytophthora erythroseptica, causes yield losses in the field and in storage. Surveys were conducted in Prince Edward Island (P.E.I.) during 1999-2001 to determine the sensitivity of local isolates of the pathogen to mefenoxam (Ridomil  ). In addition, field trials were established to test the efficacy of foliar applications of Ridomil to control pink rot in stored progeny tubers. Of 47, 24, and 4 samples of infected tubers collected from harvested fields or warehouses across P. E.I. in 1999, 2000, and 2001, respectively, 15 (32%), 2 (8%), and 2 (50%) contained tubers showing symptoms of pink rot. Tissues excised from infected tubers of samples showing symptoms of pink rot yielded 62, 7, and 8 field isolates of P. erythroseptica in 1999in , respectively. In 1999 and 17 single-zoospore isolates were also generated, respectively, for testing of sensitivity to mefenoxam. An assay on agar amended with mefenoxam revealed that all field and single-zoospore isolates collected from P.E.I. were highly sensitive to mefenoxam (EC 50 < 0.5 µg mefenoxam/mL). By contrast, tester isolates of P. erythroseptica from Maine were either sensitive, moderately resistant, or highly resistant to mefenoxam. Tubers ('Russet Burbank' and 'Kennebec') harvested from field plots managed with foliar applications of mefenoxam were inoculated with selected isolates of P. erythroseptica. Isolates that were inhibited on agar medium amended with mefenoxam were also suppressed in tubers from plants treated with foliar applications of the chemical. One foliar application of mefenoxam at tuber initiation was often sufficient to provide a significant (P = 0.05) reduction in pink rot incidence and severity. In some instances, two foliar applications (at tuber initiation and 2 weeks thereafter) reduced disease incidence and severity significantly (P = 0.05) more than a single application. By contrast, tester isolates of P. erythroseptica from Maine designated as either moderately or highly resistant following the amended-agar assay, also infected tubers from plants treated with mefenoxam. Fungicide efficacy was found to vary between cultivars and among years of study; suppression of pink rot was greater in 'Kennebec' than 'Russet Burbank', and fungicide efficacy was reduced in drier years. Management of resistance to mefenoxam and application scheduling are discussed.

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

“…Although growers can manage pink rot successfully by implementing such techniques, field applications of mefenoxam, the R-enantiomer of the parent isomer metalaxyl (Ridomil Gold EC and Ultra Flourish EC), continue to provide the most effective means of controlling the disease (29,31,41,46). Originally introduced as metalaxyl in 1977, this phenylamide fungicide has been used on a variety of crops to control diseases caused by the Peronosporales.…”

Prevalence of Mefenoxam Resistance Among Phytophthora erythroseptica Pethybridge Isolates in Minnesota and North Dakota

“…Each isolate collected across years and location was evaluated twice for mefenoxam sensitivity using these methods. Further analysis was performed by categorizing P. erythroseptica isolates into sensitive (EC 50 <1 μg/ml), intermediately resistant (EC 50 ≥1≤99.9 μg/ml) and resistant (EC 50 ≥100 μg/ml) populations Peters et al , 2003aTaylor et al 2002aTaylor et al , b, 2004. Similarly, fields were categorized into sensitive (only mefenoxam-sensitive isolates recovered), resistant (only mefenoxam-resistant isolates recovered), and mixed (mefenoxam-sensitive, -intermediately resistant and -resistant isolates recovered), and the frequency of each isolate and field category was calculated by year and location within year.…”

“…Mefenoxam has increased activity against oomycetes as compared to metalaxyl (Nuninger et al 1996) and is currently used commercially to control pink rot, with applications made at planting, hilling, and on foliage when the tubers are approximately 10 mm in diameter (Secor and Gudmestad 1999;Taylor et al 2004;Wicks et al 2000). Since the release of these compounds, variations in sensitivity to metalaxyl/mefenoxam have been reported among and within numerous species of Phytophthora Csinos and Bertrand 1994;Ferrin and Kabashima 1991;Goodwin and McGrath 1995;Goodwin et al 1996;Hunger et al 1982;Peters et al 2003a;Sujikowski et al 1995;Taylor et al 2002aTaylor et al , 2004Taylor et al , 2006. The widespread development of metalxyl-resistance among P. infestans isolates was attributed to the fitness of metalaxyl-resistant isolates (Cohen and Coffey 1986).…”

Effect of Application Method and Rate on Residual Efficacy of Mefenoxam and Phosphorous Acid Fungicides in the Control of Pink Rot of Potato

“…Initially released as metalaxyl in 1977, the fungicide provided excellent control of most oomycete pathogens, but resistance soon developed and now is widespread throughout tbe P. erythroseptica population in North America (52). Although mefenoxam still can be used effectively to control P. erythroseptica populations predominantly composed of sensitive strains (31,32,36,51,53,56), the continual spread of resistance reafftrms the need to investigate altemative fungicides capable of managing populations resistant to mefenoxam. Although the first phosphonate fungicide, fosetyl-Al (aluminum tris-O-ethyl pbosphonate), was released commercially at approximately the same time as metalaxyl, phosphonates only recently have been invesfigated as potential control agents of oomycete fungi.…”