Plant Disease (2011) 95, 691-696

Venkataramana Chapara, Raymond J. Taylor, Julie S. Pasche and Neil C. Gudmestad (2011)
Competitive parasitic fitness of mefenoxam-sensitive and -resistant isolates of Phytophthora erythroseptica under fungicide selection pressure
Plant Disease 95 (6), 691-696
Abstract: A 2-year field and laboratory experiment was initiated to study the competitive parasitic fitness of mefenoxam-resistant (50% effective concentration [EC₅₀] > 100 μg ml^-1) and mefenoxam-sensitive (EC₅₀ = 0.07 μg ml^-1) isolates of Phytophthora erythroseptica with equal aggressiveness. The competitive ability of the mefenoxam-resistant and -sensitive isolates was tested under no selection pressure (nonfungicide treated) as well as under the influence of mefenoxam and non-mefenoxam (phosphorous acid) fungicides. P. erythroseptica isolates were combined in four ratios of mefenoxam-resistant (R) to mefenoxam-susceptible (S) (0R:0S, 1R:1S, 3R:1S, and 1R:3S) and subsequently infested into the soil at the time of planting. In-furrow mefenoxam applications were applied to the soil immediately following infestation with P. erythroseptica. Phosphorous acid was applied at tuber initiation and 14 days after tuber initiation. Noninfested, nonfungicide-treated plots served as controls. P. erythroseptica isolates recovered from field-infected pink rot tubers at harvest and 3 to 4 weeks after harvest were tested for mefenoxam sensitivity in vitro. In vivo studies were performed by challenge inoculating a zoospore suspension in the four ratios described above onto potato tubers harvested from nontreated, phosphorous acid-treated, or mefenoxam-treated field plots. These field plots were not infested with P. erythroseptica at planting. Results from both field and in vivo studies demonstrate that mefenoxam-resistant isolates of P. erythroseptica are as fit as sensitive isolates in the absence of selection pressure or in the presence of a phosphorous acid fungicide treatment. Under mefenoxam selection pressure, mefenoxam-resistant P. erythroseptica isolates were more parasitically fit than -sensitive isolates. These studies suggest the lack of an apparent fitness penalty in mefenoxam-resistant P. erythroseptica populations under field conditions and that these isolates could be stable in most agroecological systems. Based on these results, mefenoxam-based fungicides are no longer recommended for the management of pink rot once mefenoxam-resistant P. erythroseptica populations are detected in a specific field.
(The abstract is excluded from the Creative Commons licence and has been copied with permission by the publisher.)
Link to article at publishers website
Database assignments for author(s): Neil C. Gudmestad

Research topic(s) for pests/diseases/weeds:
pesticide resistance of pest

---

Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Phytophthora erythroseptica	Potato (Solanum tuberosum)