Sudheesh Manalil

Rapid Evolution of Herbicide Resistance by Low Herbicide Dosages

Herbicide rate cutting is an example of poor use of agrochemicals that can have potential adverse implications due to rapid herbicide resistance evolution. Recent laboratory-level studies have revealed that herbicides at lower-than-recommended rates can result in rapid herbicide resistance evolution in rigid ryegrass populations. However, crop-field-level studies have until now been lacking. In this study, we examined the impact of low rates of diclofop on the evolution of herbicide resistance in a herbicide-susceptible rigid ryegrass population grown either in a field wheat crop or in potted plants maintained in the field. Subsequent dose–response profiles indicated rapid evolution of diclofop resistance in the selected rigid ryegrass lines from both the crop-field and field pot studies. In addition, there was moderate level of resistance in the selected lines against other tested herbicides to which the population has never been exposed. This resistance evolution was possible because low rates of diclofop allowed substantial rigid ryegrass survivors due to the potential in this cross-pollinated species to accumulate all minor herbicide resistance traits present in the population. The practical lesson from this research is that herbicides should be used at the recommended rates that ensure high weed mortality to minimize the likelihood of minor herbicide resistance traits leading to rapid herbicide resistance evolution. Nomenclature: Diclofop; rigid ryegrass, Lolium rigidum Gaud. LOLRI; wheat, Triticum aestivum L

Does cutting herbicide rates threaten the sustainability of weed management in cropping systems

Evolution of herbicide resistance in weeds is a growing problem across the world, and it has been suggested that low herbicide rates may be contributing to this problem. An individual-based simulation model that represents weed population dynamics and the evolution of polygenic herbicide resistance was constructed and used to investigate whether using lower herbicide rates or standard rates at reduced efficacy could reduce the sustainability of cropping systems by causing faster increases in weed population density as herbicide resistance develops. A number of different possible genetic bases for resistance were considered, including monogenic resistance and polygenic resistance conferred by several genes. The results show that cutting herbicide rates does not affect the rate at which weed densities reach critical levels when resistance is conferred exclusively by a single dominant gene. In some polygenic situations, cutting herbicide rates substantially reduces sustainability, due to a combination of faster increase in resistance gene frequency and reduced kill rates in all genotypes, while in other polygenic situations the effect is small. Differences in sustainability depend on combined strength of the resistance genes, variability in phenotypic susceptibility and rate delivered, level of control due to alternative measures, and degree of genetic dominance and epistasis. In the situation where resistance can be conferred by both a single dominant major gene or a number of co-dominant minor genes in combination, the difference made by low rates depends on the relative initial frequency of the major and minor genes. These results show that careful consideration of herbicide rate and understanding the genetic basis of resistance are important aspects of weed management.

Glyphosate resistance of C3 and C4 weeds under rising atmospheric CO2

The present paper reviews current knowledge on how changes of plant metabolism under elevated CO2 concentrations (e[CO2]) can affect the development of the glyphosate resistance of C3 and C4 weeds. Among the chemical herbicides, glyphosate, which is a non-selective and post-emergence herbicide, is currently the most widely used herbicide in global agriculture. As a consequence, glyphosate resistant weeds, particularly in major field crops, are a widespread problem and are becoming a significant challenge to future global food production. Of particular interest here it is known that the biochemical processes involved in photosynthetic pathways of C3 and C4 plants are different, which may have relevance to their competitive development under changing environmental conditions. It has already been shown that plant anatomical, morphological, and physiological changes under e[CO2] can be different, based on (i) the plant’s functional group, (ii) the available soil nutrients, and (iii) the governing water status. In this respect, C3 species are likely to have a major developmental advantage under a CO2 rich atmosphere, by being able to capitalize on the overall stimulatory effect of e[CO2]. For example, many tropical weed grass species fix CO2 from the atmosphere via the C4 photosynthetic pathway, which is a complex anatomical and biochemical variant of the C3 pathway. Thus, based on our current knowledge of CO2 fixing, it would appear obvious that the development of a glyphosate-resistant mechanism would be easier under an e[CO2] in C3 weeds which have a simpler photosynthetic pathway, than for C4 weeds. However, notwithstanding this logical argument, a better understanding of the biochemical, genetic, and molecular measures by which plants develop glyphosate resistance and how e[CO2] affects these measures will be important before attempting to innovate sustainable technology to manage the glyphosate-resistant evolution of weeds under e[CO2]. Such information will be of essential in managing weed control by herbicide use, and to thus ensure an increase in global food production in the event of increased atmospheric [CO2] levels.

An Herbicide-Susceptible Rigid Ryegrass (Lolium rigidum) Population Made Even More Susceptible

Abstract A wild population of a plant species, especially a cross-pollinated species, can display considerable genetic variation. Genetic variability is evident in differential susceptibility to an herbicide because the population can show continuous phenotypic variation. Recent, recurrent selection studies have revealed that phenotypic variation in response to low herbicide rates is heritable and can result in rapid evolution of herbicide resistance in genetically variable cross-pollinated rigid ryegrass. In this study, the heritable genetic variation in an herbicide-susceptible rigid ryegrass population was exploited to shift the population toward greater herbicide susceptibility by recurrent selection. To enhance herbicide susceptibility, herbicide-susceptible rigid ryegrass plants were divided into two identical clones, and one series of cloned plants was treated with a low rate of herbicide (diclofop). The nontreated clones of individuals that did not survive the herbicide treatment were selected and bulk-crossed to obtain the susceptible progeny. After two cycles of selection, the overall susceptibility to diclofop was doubled. The results indicate that minor genes for resistance are present in an herbicide-susceptible rigid ryegrass population, and their exclusion can increase susceptibility to diclofop. Nomenclature: Diclofop; rigid ryegrass, Lolium rigidum Gaudin. LOLRI.

An assessment of weed flora 14 years after the introduction of glyphosate-tolerant cotton in Australia

Abstract. Glyphosate-tolerant (GT) cotton offers a multitude of benefits such as broad-spectrum and cost-effective weed control, simple weed management, and reduced impact on the environment. However, high adoption rates of GT cotton have led to overreliance on glyphosate in weed management and have decreased the use of other herbicide options and non-chemical weed-management strategies, possibly leading to the emergence of many resistant weeds. Previous surveys in 2006 and 2011 in the cotton-growing regions of New South Wales (NSW) and Queensland, Australia, indicated changes in weed populations over the period and increased prevalence of several weeds. These two surveys indicated increased dominance of Conyza bonariensis, Echinochloa colona, and Chloris virgata in these regions. Periodic weed surveys are necessary to assess weed population dynamics and shifts due to overreliance on glyphosate for weed management. A survey was carried out in the cotton-growing regions of NSW and Queensland in 2014–15, covering 135 fields. Survey results indicated the emergence of volunteer GT cotton as the most common weed present across all of the cotton-growing regions, occurring in 85% of fields, followed by E. colona (67% of fields surveyed), and C. bonariensis and Sonchus oleraceus, which were present in 51% of fields. The most prevalent grass weed after E. colona was C. virgata (37%). Broadleaf weeds Ipomoea lonchophylla and Amaranthus mitchellii were present in 40% and 37% of fields, respectively. Regional-level analysis indicated greater prevalence of Sesbania cannabina and Parthenium hysterophorus in Emerald region of Queensland. Lolium rigidum was present in the Griffith and Warren area of NSW during summer, even though it is a winter weed. The results of this study indicate integration of diversified weed-management options and inclusion of both non-chemical and chemical options because many major weeds observed in this study are tolerant to glyphosate and have already evolved resistance to glyphosate.

Eco-biology, impact, and management of Sorghum halepense (L.) Pers.

Sorghum halepense (L.) Pers. is ranked among the worst and extensively disseminated weed species. It is emerging as a potential menace for agroecosystems in 53 different countries across the world. This weed is adapted to warmer regions and is native to Mediterranean areas of Africa, Asia, and Europe. In the mid-1900s, cultivation of this weed species as a potential forage crop resulted in its escape from crop fields and invasion of agricultural and natural areas, but in some European countries, it has been introduced deliberately (e.g., as contamination of seeds and soil). S. halepense interferes with economically important agronomic and horticultural crops and cause 57–88% yield losses. Herbicide tolerance, diverse propagation mechanisms, rapid development, and strong competitiveness are key attributes in its invasion. Conventional management approaches are limited in their scope to control this weed due to its rapid vegetative growth and increasing herbicidal tolerance. Integration of chemical methods with cultural or mechanical approaches is important for restricting its future spread to non-infested areas. This review provides insights into the invasion mechanisms of S. halepense, which will help in its management. A better understanding of ecobiological aspects, survival mechanisms, and genetic variabilities of S. halepense, within a wide range of environmental conditions, will assist in designing more effective management strategies for this serious invasive weed. Collaborative research between the various countries impacted by this weed will assist in developing efficient, sustainable, and economical approaches to restrict its invasion in new areas.

Eco-Biology and Management of Alligator Weed [ Alternanthera philoxeroides ) (Mart.) Griseb.]: a Review

Exotic plant invasion, a global issue, has a tremendous impact on ecology, economy, human, and animal health. Alligator weed (the world’s first aquatic weed) is a serious invasive weed in 32 different countries of South America, Australia, Asia, and North America. Recently, it has been recorded as a threat weed of rice, maize, soybean, vegetables, fruit trees, and pastures, causing 19–45% yield losses in these crops in addition to its infestation in canals, lakes, and ditches. Alligator weed has the potential to ruin agricultural and natural ecosystems and recreational areas. Ability to propagate via vegetative fragmentation, water-borne dispersal of vegetative propagules, and allelopathic potential contribute towards its success as an invasive weed species of terrestrial, semi-aquatic, and aquatic environments. Application of glyphosate, metsulfuron-methyl, dichlobenil, fluridone, hexazinone, triclopyr amine, dimethylamine, imazapyr, diuron, and amitrole herbicides have been found most effective in controlling this weed in different habitats. Agasicles hygrophila, Vogtia malloi pastana, Amynothrips andersoni, and Nimbya alternanthera have been reported as bio-agents for the control of alligator weed. We present a comprehensive review of the biology, interference, and management options of an extremely dangerous invasive weed species. Although management of alligator weed through chemical, biological, and mechanical means are often effective, there is need for well-planned, long-term field experiments to evaluate the role of different factors that are stated to be responsible for its increasing infestation and distribution (e.g., regeneration after damage caused by herbicides, high soil fertility levels, soil disturbances, shallow vs. deep ploughing and grazing management). It is recommended that future research should focus more on the integration of different management approaches in both aquatic and terrestrial ecosystems, and in various ecological regions.

Germination ecology of turnip weed (Rapistrum rugosum (L.) All.) in the northern regions of Australia

In Australia, turnip weed has been rapidly emerging as one of the major weeds in conservation agricultural systems. Germination and emergence of turnip weed were examined for two populations collected from Gatton and St George regions of Australia; two locations with high and low rainfall, respectively. The seeds of turnip weed germinated at all the tested temperatures, but germination was the lowest at 15/5°C, intermediate at 20/10°C and highest at 25/15°C and 30/20°C. The results indicated a high adaptability of turnip weed to warm environmental conditions, although it is a major problem in the winter season. Germination was higher in dark than light/dark regimes except at 30/20°C. Three was a concomitant reduction in germination as the osmotic potential values decreased from 0 to -1.0 MPa. There was 2 and 4% germination at -0.8 MPa for Gatton and St George populations, respectively, and no germination occurred at an osmotic potential of -1.0 MPa. There was a reduction in germination when the sodium chloride (NaCl) concentration was increased from 0 to 150 mM, and no germination was observed at 200 and 250 mM of NaCl. Turnip weed germinated over a broad range of pH (4 to 10). Seedling emergence was higher at 1 cm depth compared to 0.5 cm or at the soil surface. There was 28 and 33% emergence at the surface for the Gatton and St George populations, respectively, compared to 48 and 56% emergence from 1 cm depth for the Gatton and St George populations, respectively and no emergence was observed from 6 cm depth. The results indicated that tillage leading to shallow burial would promote the emergence of turnip weed; on the contrary, tillage that could bury seeds deep into the soil profile might minimise the emergence. Under ideal conditions and lack of integrated weed management programmes, this weed will emerge, set seeds and enrich the soil seed bank and thereby continue to be a problem in the northern grain region of Australia.

Germination ecology of Chloris truncata and its implication for weed management

Chloris truncata is a significant weed in summer crops in the subtropical region of Australia. A study was conducted to evaluate the effect of environmental factors on germination and emergence of two populations of C. truncata. Overall, germination was not affected by the populations. Seeds germinated at a wide range of alternating day/night temperatures, suggesting that seeds can germinate throughout the spring, winter and autumn seasons. Seed germination was stimulated by the presence of light; however, 51 to 71% of these seeds still germinated in the dark. The sodium chloride concentration and osmotic potential required to inhibit germination of 50% of the population were 179 mM and -0.52 MPa, respectively. A high proportion of seeds germinated over a wide pH range (4 to 10). Seeds placed on the soil surface had greatest germination (67%) and a burial depth of 3 cm resulted in complete inhibition of emergence. The sorghum residue amount required to reduce emergence by 50% was 1.8 t ha-1. The results suggest that, although this weed will be favored in no-till systems, residue retention on the soil surface will help in reducing its infestation. Seed bank buildup can be managed by burying seeds below the depth of emergence.

Biology and management of Avena fatua and Avena ludoviciana : two noxious weed species of agro-ecosystems

Avena fatua and Avena ludoviciana are closely related grass weed species infesting a large number of crops around the world. These species are widely distributed in diverse agro-ecosystems from temperate to sub-tropical regions due to their unique seed traits, successful germination ecology, high competitive ability, and allelopathic potential. A. fatua is more widespread, adaptable, and problematic than A. ludoviciana. Both these species infest major winter and spring crops, including wheat, oat, barley, canola, maize, alfalfa, and sunflower, causing up to 70% yield losses depending on crop species and weed density. Chemical control has been challenged by large-scale herbicide resistance evolution in these weed species. A. fatua is the most widespread herbicide-resistant weed in the world, infesting about 5 million hectares in 13 countries. The use of alternative herbicides with different modes of action has proved effective. Several cultural practices, including diverse crop rotations, cover crops, improved crop competition (using competitive cultivars, high seed rates, narrow row spacing, altered crop geometry), and allelopathic suppression, have shown promise for controlling A. fatua and A. ludoviciana. The integrated use of these cultural methods can reduce the herbicide dose required, and lower dependency on herbicides to control these grasses. Moreover, integrated management may successfully control herbicide-resistant populations of these weed species. The use of integrated approaches based on the knowledge of biology and ecology of A. fatua and A. ludoviciana may help to manage them sustainably in the future.