A Technical Guide for Wine Producers

RECENT ARTICLES   |   WYNBOER HOME The threat of herbicide resistant weeds to the wine industry Andy Cairns A.L.P. Cairns and F.H. Eksteen Department of Agronomy University of Stellenbosch Stellenbosch, 7600 Key words: herbicide resistance, glyphosate, paraquat, ACCase-inhibitors, identification of resistance, rotation of herbicides, danger signs. Abstract Western Cape vineyards are home to the most herbicide-resistant ryegrass in the world. Not only are some populations of the weed resistant to glyphosate and paraquat but also to ACCase-inhibiting graminicides such as haloxyfop-R methyl ester (Gallant) and cycloxydim (Focus Ultra). Resistance has largely been induced by the exclusive use of the same non-selective herbicides year after year for a long time. Guidelines as to the identification and management of herbicide-resistant populations of weeds are given. Rotation of herbicides with different modes of action must form an integral part of this strategy. Introduction Recently the following complaints are frequently heard from producers complaining about the lack of efficacy of herbicides used for weed control in vineyards: "The herbicides are not working as well as they used to what has happened to the quality of our herbicides?" "The agent has calibrated my sprayer incorrectly. My dosage rate is to low to control the weeds." "My water quality is bad that is why the herbicide doesn't work"." " I have to spray a higher dosage rate every year to get the same degree of weed control, why?" " I can't control my weeds and more with any herbicide, what must I do? " " My weed control is costing me a fortune what can I do to bring my herbicide costs back to what they were"? These complaints stem from the fact that a high degree of resistance to the most popular non-selective herbicides has developed in several weed species in vineyards of the Western Cape. If not attended to, this problem can present a threat to profitability of the wine industry in some areas. Glyphosate and paraquat Herbicide resistance has developed to both glyphosate and paraquat in vineyards of the Western Cape. Paraquat has been on the market for about 50 years and glyphosate for 30. These two products form the basis of non-selective weed control in perennial crops in the Western Cape and worldwide. When glyphosate came on the market in the mid 1970's it largely displaced paraquat due to its following superior characteristics: i) lower toxicity, ii) far more systemic, iii) could be applied with a lower spray volume per unit surface area, iv) could control a much wider spectrum of weeds both annual and perennial. Initially when glyphosate came on the market the product was more expensive than paraquat but later glyphosate became much cheaper. All these factors led to the situation where glyphosate became the product of choice for up to 90% of producers and it was applied up to 3 times a year non-stop for more than 30 years. The stage was thus set for the development of widespread resistance to this herbicide. Glyphosate-tolerant ryegrass was first discovered in South Africa in a vineyard at Tulbagh in 2001. The number and extent of glyphosate-resistant ryegrass populations have increased dramatically since that time. Many of these weed populations are also resistant to paraquat (and diquat) and also to selective grass killers such as the "dims" and "fops" which are ACCase-inhibiting herbicides. Nowhere else in the world has any weed biotype developed resistance to these three herbicide groups simultaneously. We do not yet know why the Western Cape is home to the most herbicide-resistant ryegrass in the world. Overuse of glyphosate and earlier paraquat certainly is a contributory cause but similar situations exist all over the world and other factors must be sought to explain our unique resistance situation. It is tempting to speculate that the environmental conditions pertaining in the Western Cape is somehow contributing to the diversity of weed biotypes. The higher the diversity the more likely those biotypes with herbicide resistant characteristics will be found in the population. Some of these will survive herbicide applications and their progeny, which will be even more resistant, will be found at a disproportionately high level in the next generation. In most cases the development of glyphosate and paraquat resistance starts as a gradual annual decrease in the efficiency of the herbicide concerned. This may take place so slowly that it is almost imperceptible but over the years the producer has to gradually increase the dosage rate of the herbicide to achieve the same degree of control. Most people have forgotten that in days gone by ryegrass could be controlled very effectively at dosage rates less than 1L of 360g a.i. glyphosate ha-1. Very few producers now use less than 3-4L ha-1 and many now spray up to 6L ha-1. In extreme cases individual ryegrass plants can survive up to 40 L glyphosate 360 ha-1. This means that the vast majority of ryegrass populations in vines are resistant to glyphosate to a greater or lesser extent. Resistance to paraquat would appear to be less widespread than resistance to glyphosate but this is almost certainly a function of the much lower incidence of paraquat use in the wine industry today. What is interesting is that paraquat resistance in ryegrass has survived in places where the herbicide has not been used for many years. The problem of herbicide resistance in vineyards is not confined to ryegrass. Our unique weed flora boasts the only case of glyphosate resistant tongblaar (Plantago lanceolata) in the world. We were also the first in the world to discover glyphosate-resistant kleinskraalhans (Conyza bonariensis). (Spain has recently also found glyphosate resistant populations of this weed). Widespread resistance of kleinskraalhans to paraquat is also rampant. We are of the opinion that resistance to glyphosate is emerging in populations of several other weed species. Producers are having to use increasing dosage rates of glyphosate to control populations of perennial weeds such as uintjies, (Cyperus rotundus) and kweek (Cynodon dactylon). In addition increasing numbers of winter and summer annuals are escaping applications of glyphosate applied at standard registered rates. Populations of ryegrass that are resistant to both glyphosate and paraquat have also been found to be resistant to ACCase-inhibiting graminicides such as Haloxyfop-R methyl ester (Gallant) or cycloxydim (Focus Ultra). This further limits the producer's options as regards herbicides. Management of weeds to overcome resistance to herbicides What must the producer do to if he suspects that he has herbicide resistant weeds in his vines? The first thing to do is to confirm that the weeds are indeed resistant and that the poor control achieved is not due to other factors such as herbicide dosage rate, application stage and water quality. Either seed or plants of the suspected resistant biotype can be sent to the Department of Agronomy, University of Stellenbosch, Stellenbosch 7600 for testing free of charge. These tests determine the resistant status of any population of weeds. Providing seed of suspected resistant species is the better than sending in plants as a more representative sample of the weed population can be taken in the form of seed. Help with the gathering of seed and/or plant samples can be obtained by contacting Prof A.L.P. Cairns on 021-8082317 or 0845879928. These tests involve either laboratory or glasshouse tests. If the test confirms resistance to a particular herbicide or herbicide group, further trials are carried out to see if there are any other herbicides or combination of herbicides that can be used to control the biotype concerned. Resistance profiles differ from vineyard to vineyard and from estate to estate. A product that works well in one vineyard may be completely unsuitable for use in another. Every vineyard block on an estate should be subjected to a test for resistance and the herbicide recommendation made accordingly. In most cases there is a solution. Sometimes however the is no herbicidal solution and the only solution is hand weeding. This is of course hugely expensive and only high-value vineyards with many years of production ahead of them can enjoy the luxury of being hand weeded. For older less valuable blocks the most cost-effective solution is to plough out the vines and sow the land to a crop such as oats for three years or more. This is a drastic measure but sometimes the only solution. Producers tend to be very loyal to the herbicides they use and it is sometimes difficult to wean someone from a product that they and their father have used for 30 years or more. However if the same product is applied year after year it will be a matter of time before herbicide resistance presents itself. A very good rule of thumb is never to use the same product two years in a row. A heavy onus lies on the agrochemical representative from whom the producer buys his herbicides. He must be in a position to be able to recognise full-blown herbicide resistance in his client's vineyards or the telltale danger symptoms that point to the immanent development of resistance. If the chemical representative is in any way unsure as to the advice he should give his client he should contact us at the address given above. Danger signs It is important to recognise the first stages of the development of resistance to herbicides such as glyphosate and paraquat. As mentioned earlier the development of resistance is insidious and can go unnoticed until it is too late. If glyphosate, paraquat or a selective grass killer such as haloxyfop-R methyl ester (Gallant) or any other herbicide applied at the registered rate gives less than satisfactory control, it is imperative to investigate. Advice of where and how to look for resistance and danger signs that warn of the impending of herbicide resistance are the following: Control of weeds is good except for a few individual plants which appear to be relatively unaffected by the herbicide application despite the fact the plant is surrounded by totally dead plants. Weed control is poor in oval or round patches. Rectangular strips of poor weed control between or within the vine rows are usually related to the swath width and indicative of application problems not herbicide resistance. Weed plants are all burned down to the ground with scorch-like products such as paraquat but in a week or so regenerate sphinx-like by sprouting again from the crown of the plant at the soil surface. Resistance can originate spontaneously in the middle of a block of vines but more often starts at the ends of the rows or adjacent to the edge of the block. Also, if the vineyard is not cultivated resistance usually spreads very slowly. Resistance nearly always starts in the older blocks of vines. This is because the development of resistance is usually a function of the number of times a population of weeds has been subjected to applications of the same herbicide. Also, for reasons that we don't understand, the incidence and degree of herbicide resistance tends to occur far more in some areas than others. If your neighbour has glyphosate-resistant ryegrass the chances are that you weed population is or on its way to resistance. Glyphosate resistant ryegrass also occurs in Australia. There the farming community views the matter so seriously that the selling price of a property can drop up to 50% as a result of the presence of these weeds. They at least can use paraquat - an option not open to many Western Cape farmers.

Wynboer is incorporated in WineLand, magazine of the SA wine producers. Subscribe to WineLand Visit our sister sites: South African wine farmers' representative organisation Facts, figures, contact details and much more in the 2009/10 Directory

UP	COPYRIGHT (C) 2000 WineLand