Weed Problems in Uruguayan Agriculture: Evolution and Current Situation

Uruguayan agriculture has undergone dramatic changes in the last 50 years driven by the adoption of new agricultural production systems that incorporate zero tillage and herbicide resistant crops. This has resulted in a shift in weed species frequencies and the dispersion of introduced herbicide resistant weed populations. Finally, integrated weed management tools are being developed by research and extension services to manage herbicide-resistant (HR) weeds better and to reduce environmental impact of herbicides.

How Many Ways Can Nature Kill the Goose That Laid the Golden Egg? – The Many Mechanisms of Evolved Glyphosate Resistance

Weeding has been the bane of humanity since the dawn of agriculture. For about 70 years, synthetic herbicides have removed much of the drudgery of this onerous task. Glyphosate was introduced as a non-selective herbicide in 1974. Its ideal properties made it a very popular herbicide, and the introduction of glyphosate-resistant (GR) crops allowed its use as a selective herbicide, greatly expanding its use to become the most used herbicide on earth. For farmers who used glyphosate in GR crops, it was the golden age of weed management, as this technology significantly improved the efficacy and reduced the cost of weed management. Weed management was also simplified, an asset that was particularly valuable to part-time farmers. Furthermore, this technology provided the environmental benefits (reduced soil loss and fossil fuel use) of significantly reducing tillage. Farmers saved billions of US$, and weed management became more effective and simple. Indeed, to many farmers, glyphosate with GR crops became the goose that laid the golden egg. After more than 20 years of use, the first cases of GR weeds were reported in the latter 1990s. After a lag period of less than 10 years after the first GR weed was reported, the number of species reported to have evolved glyphosate resistance began to increase in a linear fashion, reaching 53 species in 2021, third only to atrazine (66 species), a much older herbicide and to ALS inhibitors (168 species), which include several different herbicides used in numerous crops since the 1980s. The long lag phase before any resistance was detected led some to believe by the mid-1990s that evolution of resistance was improbable. By this time, glyphosate use was greatly increasing, especially in GR crops, an ideal situation for the evolution of resistance. After this, the number of glyphosate-resistance cases exploded, and the mechanisms of resistance to many of these cases of resistance were determined. A recent, short commentary detailed these mechanisms after a new mechanism of resistance was reported. The number of mechanisms for resistance to no other herbicide comes close to those of glyphosate. In the present paper, we briefly describe the many evolved mechanisms by which weeds have evolved resistance to glyphosate.

Prospects for Bioherbicides

New solutions to weed management are needed now more than ever. Ag retailers, university extension specialists and farmers consider the evolution of herbicide-resistant weeds "one of the most significant developments in agriculture today." Indeed, weed resistance is not just a US problem, but a global one. There are currently 505 unique cases of herbicide resistant weeds globally, with 264 species. Weeds have evolved resistance to 21 of the 31 known herbicide sites of action and to 164 different herbicides. Herbicide resistant weeds have been reported in 95 crops in 71 countries. According to Duke, no new herbicide modes of action have been introduced since the 1980s. Since this paper, only one herbicide with a new mode of action (cyclopyrimorate) has been commercialized. Some major agrichemical companies have announced new discoveries and new modes of action coming in the future, but these are some years off. Given this state of weed management, one would think that biological or natural product herbicides with new modes of action would already be making headway in the market. Unfortunately, few new biological products have been commercialized. The technical difficulty in finding bioherbicides that can compete with the spectrum and price of chemical herbicides has left agriculture with a paucity of new herbicides based on natural products. Most products are targeted at organic agriculture because of their higher manufacturing cost. These products are typically short residual, burndown products that require high volumes and multiple applications. Examples are clove and orange oils. Other natural active ingredients include acetic acid (vinegar), FeHEDTA, NaCl, pelargonic acid, and caprylic and capric acids. This paper reviews the reasons for the lack of new bioherbicides, current and pipeline products and how new technologies may accelerate their progress.

Citrus Huanglongbing (HLB) and the Brazilian Efforts to Overcome the Disease

Recorded in Brazil since 2004, Huanglongbing (HLB), or simply Greening, is the most impacting disease in citrus production worldwide. It induces qualitative and quantitative losses in production, but also can be lethal to the infected citrus plant. The disease is caused by phloem-limited bacteria with three identified species, Candidatus Liberibacter asiaticus (CLas), Ca. L. africanus (CLaf), and Ca. L. americanus (CLam). CLas and CLam are reported in Brazil, being vectored by the Asian citrus psyllid Diaphorina citri. HLB-infected citrus exhibit blotchy mottling with green islands on leaves, and small and lopsided fruits. As the infection progresses, excessive fruit drop and dead branches are frequently observed, shortening the trees' lifespan and reducing fruit yield and quality. HLB occurrence is restricted to the States of São Paulo, Minas Gerais, Mato Grosso do Sul and Paraná, which corresponds to more than 85% of citrus production in Brazil. Although citrus production is concentrated in these states, commercial citrus groves are present in 85% of the microregions of Brazil, reinforcing the economic and social importance of citrus for the country. Thus, in addition to the economic damage already caused to the national citrus industry, the spread of HLB to other regions of Brazil represents a great risk to the economic and food security of these non-affected regions. For this reason, the management of HLB is supported by legislative force, both at the federal and state levels. In 2008, based on scientific knowledge at the time, the Brazilian Ministry of Agriculture published a Normative Instruction 53 (IN53) which established criteria related to HLB surveillance and control, imposing the obligation to carry out periodic phytosanitary inspections, eradication of symptomatic hosts and planting HLB-free nursery trees. The implementation of IN53 has already resulted in the eradication of 55.5 million HLB-infected citrus plants in the State of São Paulo alone in the period from 2007 to 2019. In May 2021, IN53 was revoked and Portaria 317 was published. This new policy establishes the National Programme for HLB Prevention and Control. It addresses the problem according to the HLB status in each State, with or without the disease, and sets the general guidelines for local control actions.<br/> In view of the economic importance and complexity of this pathosystem, the disease has required concerted mobilization of the research community and inter-institutional cooperation, which has been working intensively to understand all the components directly and indirectly involved with the pathosystem. Also, a major cooperation effort is needed to gather all the information to develop and/or improve technologies and cultural practices to fight the disease better and reduce its impacts as much as possible. Here we present some of these efforts which are being carried out to develop short, medium and long term solutions for HLB control and management in areas of occurrence, as well as for surveillance and risk mitigation in non-affected areas.

Challenges for Pesticide Regulation and Use

Many farmers still apply a wide range of pesticides, but are exposed to some which are highly hazardous to use. Is there sufficient legislation and control of pesticide use to remove these pesticides? Even applying the less hazardous pesticides, farmers need to be better trained to improve their application and minimise exposure to the chemicals. The use of chemical pesticides has increased immensely over the last 80 years, since the development of DDT, with a range of many other insecticides, fungicides and herbicides being marketed. Although legislation to control the use of insecticides had been introduced in the USA back in 1910, it was only after World War II that other countries began to start registration of pesticides and WHO created a Classification based on the toxicity of the chemicals. More recently there has been greater concern about the extent to which certain pesticides, especially those rated as Highly Hazardous Pesticides (HHPs) have been used. WHO estimates that there are 1.6 million deaths per year from poisoning by chemicals. There are other problems, such as the herbicide glyphosate, use of which has expanded with farmers growing maize, soybean and cotton crops genetically modified to provide tolerance to this weedkiller. The use of this herbicide has resulted in many court cases claiming it use results in ill health.