Developing Genetic Markers for Metribuzin Tolerance in Wheat to Aid in the Management of Italian Ryegrass Through Herbicide Synergism

Project Summary/Abstract Italian ryegrass has been an ever-increasing threat for small-grain production in the commonwealth of Kentucky. Small-grain producers in Kentucky have been reporting multi- herbicide resistant Italian ryegrass in the commonwealth of Kentucky, increasingly frequently. Some Italian ryegrass samples obtained from producers were reported to have resistance to glyphosate (HRAC 9), glufosinate (HRAC 10), ALS-inhibitors (HRAC 2) and/or ACCase-inhibitors (HRAC 1), sometimes all in the same population. If more herbicides aren’t added to wheat production systems or used in new ways quickly, Italian ryegrass will become resistant to all the available herbicides, making chemical management of Italian ryegrass no longer viable. Recently, small-grain producers in Kentucky have been adding post-emergent metribuzin to their herbicide regiments to help manage Italian ryegrass. However, if metribuzin is applied post-emergence to the wrong variety of wheat in optimal conditions for metribuzin efficacy, severe damage to the crop will occur. Bill Bruening and Dr. Revolinski documented that in soft-red winter wheat varieties frequently planted in Kentucky, metribuzin tolerance is predominantly controlled genetically with repeatability levels up to 0.90. The best dose to differentiate tolerant and susceptible wheat varieties was 400g of metribuzin per hectare. Unfortunately, those previous trials were performed using genotypes of wheat that were entered into variety testing at the University of Kentucky which we cannot genotype and use to develop rapid assays to determine metribuzin tolerance in wheat. After running metribuzin trials with wheat that is genotyped, genetic markers with a short turn- around time will be developed for local wheat breeders to use so that they can ensure varieties that they develop are metribuzin tolerant. In addition to metribuzin safety in wheat, this work is also useful because photosystem II inhibiting herbicides, like metribuzin, have synergistic effects with HPPD-inhibitors, like tembotrione. Although tembotrione has been found to only have moderate performance in the management of Italian ryegrass, mixing tembotrione with metribuzin may be highly effective for managing multi-herbicide resistant Italian ryegrass. Due to the herbicide synergy, it may be safer to add tembotrione to wheat that is already metribuzin tolerant. To develop a rapid genetic assay for metribuzin tolerance in wheat, a panel of genotyped soft-red winter-wheat varieties will be assessed for metribuzin tolerance, allowing a genome-wide association study to be performed. Using the results from genome-wide association study, KASP™ markers will be developed to rapidly assess the metribuzin tolerance of new wheat lines. The most metribuzin tolerant wheat lines and multi-herbicide resistant Italian ryegrass (that we have already collected seed from producers) will be sprayed with varying levels of metribuzin and tembotrione, allowing the synergism to be assessed regarding the management of Italian ryegrass and safety in metribuzin tolerant wheat.