Reducing Selection Pressure for PPO Resistance in No-till Cotton

Currently, glyphosate resistant (GR) Palmer amaranth exists in ten states throughout the mid-south and southeastern United States and infests a large portion of the cotton acreage. Furthermore, acetolactate synthase (ALS) resistant is prevalent and multiple resistance to both modes of action (MOA) is also present. With widespread resistant Palmer amaranth, academic recommendations suggest utilizing residual herbicides in weed management systems. In addition, previous work embraces preemergence (PRE) herbicides as vital to weed control in reduced tillage cotton. Residual herbicides reported to be most effective PRE or preplant (PPL) include fomesafen, flumioxazin, and pyrithiobac, for Palmer amaranth control in cotton. In no-till cotton one strategy for managing glyphosate-resistant Palmer amaranth includes an early preplant (PPL) application of flumioxazin followed by a preemergence application of fomesafen. Although fomesafen and flumioxazin provide adequate control of Palmer amaranth, utilizing both sequentially is more effective than either herbicide alone and is attractive to cotton producers fighting glyphosate resistance. Whitaker (2009) reported 95% late-season Palmer amaranth control by flumioxazin PPL followed by fomesafen PRE. However, fomesafen and flumioxazin are both protoporphyrinogen oxidase (PPO) inhibiting herbicides and sequential use of both herbicides in no-till cotton is reason for concern over PPO resistant Palmer amaranth. Additionally, PPO-resistance has been reported in waterhemp. This is particularly alarming, considering it shares the same genus with Palmer amaranth and has many similar morphological characteristics. One available option for replacement of PPO inhibitors in no-till cotton includes diuron. Diuron can be applied both PRE and 15 to 45 days prior to planting cotton. Therefore, diuron can serve as a suitable replacement for either flumioxazin PPL or fomesafen PRE. The objective of this research was to determine if diuron can serve as a suitable alternative for one PPO-inhibiting herbicide in reduced tillage cotton, limiting selection pressure on this valuable MOA.