Field-evolved pyrethroid resistance in Tychius picirostris (Coleoptera: Curculionidae) populations in Oregon white clover seed crops.

The clover seed weevil, Tychius picirostris Fabricius, a serious pest of white clover, Trifolium repens L., grown for seed in western Oregon, causing feeding damage to flowers and developing seeds. Since 2017, white clover seed producers have anecdotally reported T. picirostris control failures using foliar pyrethroid insecticide applications. This mode of action (MoA) is an important chemical control option for T. picirostris management. To evaluate insecticide resistance selection to pyrethroids (bifenthrin) and other MoAs labeled for T. picirostris management (malathion and chlorantraniliprole), adult populations were collected from 8 commercial white clover grown for seed fields in the Willamette Valley, OR, in 2022 and 2023. Among collected Oregon populations, very high resistance ratios (RR50 = 178.00-725.67) were observed to technical grade bifenthrin and low to high resistance ratios (RR50 = 7.80-32.80) to malathion in surface contact assays compared to a susceptible Canadian field population. Moreover, >2.73 times the labeled rate of formulated product containing bifenthrin as the sole MoA was required to kill >50% of T. picirostris in topical assays. Synergistic assays with a mixed-function oxidase inhibitor, an esterase inhibitor, and a glutathione-S-transferase inhibitor revealed phase I and II detoxification enzymes are present in Oregon T. picirostris populations and confer metabolic resistance to bifenthrin. This is the first report of T. picirostris insecticide resistance selection to pyrethroid and organophosphate insecticides. Results will inform continued monitoring and insecticide resistance management strategies to slow the evolution of T. picirostris insecticide resistance selection in Oregon's white clover seed production.

Methods for Screening Wheat Genotypes for Resistance to Sharp Eyespot in the Field and Greenhouse.

Screening methodology of wheat genotypes for resistance to sharp eyespot (caused by Rhizoctonia cerealis) was developed. Disease severity differed among cultivars and between field and greenhouse trials. However, the cultivars Bobtail and Rosalyn had consistently lower severity in field experiments with high sharp eyespot disease pressure. Artificial inoculation was crucial to achieving adequate disease levels for effective screening but planting date had very little effect. Greenhouse inoculation of adult wheat plants was much less successful in categorizing resistance to sharp eyespot. Seedling inoculations in the greenhouse were highly inadequate as a screening method. Selection for resistance to sharp eyespot by artificial inoculation in field trials is feasible in wheat breeding programs.

Temporal Dynamics and Spatial Variation of Azoxystrobin and Propiconazole Resistance in Zymoseptoria tritici: A Hierarchical Survey of Commercial Winter Wheat Fields in the Willamette Valley, Oregon.

Fungicide resistance can cause disease control failure in agricultural systems, and is particularly concerning with Zymoseptoria tritici, the causal agent of Septoria tritici blotch of wheat. In North America, the first quinone outside inhibitor resistance in Z. tritici was discovered in the Willamette Valley of Oregon in 2012, which prompted this hierarchical survey of commercial winter wheat fields to monitor azoxystrobin- and propiconazole-resistant Z. tritici. Surveys were conducted in June 2014, January 2015, May 2015, and January 2016. The survey was organized in a hierarchical scheme: regions within the Willamette Valley, fields within the region, transects within the field, and samples within the transect. Overall, frequency of azoxystrobin-resistant isolates increased from 63 to 93% from June 2014 to January 2016. Resistance to azoxystrobin increased over time even within fields receiving no strobilurin applications. Propiconazole sensitivity varied over the course of the study but, overall, did not significantly change. Sensitivity to both fungicides showed no regional aggregation within the Willamette Valley. Greater than 80% of spatial variation in fungicide sensitivity was at the smallest hierarchical scale (within the transect) of the survey for both fungicides, and the resistance phenotypes were randomly distributed within sampled fields. Results suggest a need for a better understanding of the dynamics of fungicide resistance at the landscape level.

Evidence of Selection for Fungicide Resistance in Zymoseptoria tritici Populations on Wheat in Western Oregon.

Plant pathogens pose a major challenge to maintaining food security in many parts of the world. Where major plant pathogens are fungal, fungicide resistance can often thwart regional control efforts. Zymoseptoria tritici, causal agent of Septoria tritici blotch, is a major fungal pathogen of wheat that has evolved resistance to chemical control products in four fungicide classes in Europe. Compared with Europe, however, fungicide use has been less and studies of fungicide resistance have been infrequent in North American Z. tritici populations. Here, we confirm first reports of Z. tritici fungicide resistance evolution in western Oregon through analysis of the effects of spray applications of propiconazole and an azoxystrobin + propiconazole mixture during a single growing season. Frequencies of strobilurin-resistant isolates, quantified as proportions of G143A mutants, were significantly higher in azoxystrobin-sprayed plots compared with plots with no azoxystrobin treatment at two different locations and were significantly higher in plots of a moderately resistant cultivar than in plots of a susceptible cultivar. Thus, it appears that western Oregon Z. tritici populations have the potential to evolve levels of strobilurin resistance similar to those observed in Europe. Although the concentration of propiconazole required to reduce pathogen growth by 50% values were numerically greater for isolates collected from plots receiving propiconazole than in control plots, this effect was not significant (P > 0.05).