2019

Herbicides that inhibit HPPD have become very important. In North America, corn-selective HPPD herbicides have long been used and cases of HPPD herbicide-resistant weeds reported. 

This paper reviews the literature to understand the effects of glyphosate resistance on plant fitness at the molecular, biochemical and physiological levels. A number of correlations between enzyme characteristics and glyphosate resistance imply the existence of a plant fitness cost associated with resistance-conferring mutations in the glyphosate target enzyme, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). These biochemical changes result in a tradeoff between the glyphosate resistance of the EPSPS enzyme and its catalytic activity.

In this Plant Physiology paper, it is clearly established that glyphosate resistance in this Echinochloa population is due to an enhanced capacity to metabolise glyphosate, catalysed by an Aldo-keto reductase gene.  The first author on the paper is Dr Pan Lang, now at the Hunan Agricultural University, Changsha, China.

Pre-emergent herbicides play an important role in conservation agriculture, however, crop residues on the soil surface in these systems can intercept a considerable amount of herbicide during application. Cutting crops relatively high at harvest has some advantages, such as allowing faster harvest, and this also means that there is less horizontal residue on the soil surface. This field study tested the impact of standing wheat residue height and amount of horizontal residue on the interception, leaching and weed-control efficacy of the pre-emergent herbicide pyroxasulfone in the 2015 and 2016 growing seasons.

Resistance to the auxinic herbicides 2,4-D and dicamba is widespread in Western Australian R. raphanistrum populations, with the resistance mechanism appearing to involve alterations in the physiological response to synthetic auxins and in plant defense. This study aimed to determine whether these alterations cause inhibition in plant growth or reproduction that could potentially be exploited to manage 2,4-D–resistant populations in cropping areas.

Weed resistance to foliar herbicides has dramatically increased worldwide in the last two decades. As a consequence, current practices of weed management have changed, with increased adoption of soil-applied herbicides to restore control of herbicide-resistant weeds.

Dr Qiong Peng came to AHRI for one year to study Chinese Echinochloa populations resistant to the rice-selective auxinic herbicide quinclorac.  This paper documents two Echinochloa populations as being very highly resistant to quinclorac. She found that quinclorac resistance in these Echinochloa populations is not due to differential quinclorac uptake, translocation or metabolism.  

The research by AHRI PhD student Jinyi Chen (now Dr Jinyi Chen!) documents both target site and non-target site resistance to dinitroaniline herbicides in Lolium individuals. We are long accustomed to documenting multiple mechanisms of resistance existing within Lolium individuals/populations because of the high genetic diversity and obligate cross-pollination between Lolium which enables multiple resistance mechanisms to be a frequent occurrence.  In this particular Lolium population, resistance to dinitroaniline herbicides is due to a mutation in the alpha tubulin gene (Val-202-Phe) and non-target site enhanced rates of dinitroaniline herbicide trifluralin metabolism.   Unequivocal evidence for the target site Val-202-Phe mutation endowing resistance was obtained by expressing in transgenic rice, where it endowed resistance.

This paper outlines three case studies of weed species that have demonstrated a great propensity for HR gene flow: B. scoparia in western North America, Amaranthus palmeri S. Watson (Palmer amaranth) in the United States (U.S.), and Lolium rigidum Gaud. (annual or rigid ryegrass) in Australia. These three species share three common features: (1) a top troublesome and economically damaging weed in their respective jurisdictions; (2) high incidence of multiple resistance in populations; and (3) rapid expansion of resistance incidence across jurisdictions in a short period of time.

This review covers recent developments and trends in herbicide-resistant (HR) weed management in agronomic field crops. In countries where input-intensive agriculture is practised, these developments and trends over the past decade include renewed efforts by the agrichemical industry in herbicide discovery, cultivation of crops with combined (stacked) HR traits, increasing reliance on preemergence vs. postemergence herbicides, breeding for weed-competitive crop cultivars, expansion of harvest weed seed control practices, and advances in site-specific or precision weed management.