Understanding and mitigating the impact of off-target herbicide drift

Dr Gerhard Rossouw, CSIRO Agriculture and Food, Waite Campus

Herbicides are widely used to control weeds in broadacre crop fields, often during springtime. Grapevines, however, are sensitive to the phytotoxic effects of many herbicides, especially during the stage of early shoot growth and around flowering. Climatic conditions, including prevailing winds, low relative humidity and high atmospheric temperatures can all increase the risk for off-target spray vapour drift, potentially moving for several kilometres (Felsot et al. 2010). Phenoxyacetic acid type herbicides, including 2,4-D, are particularly renowned for causing drift incidents, and may impair fruit yield and canopy growth in exposed vineyards (Ogg et al. 1991, Al-Khatib et al. 1993). Other herbicides, including glyphosate, are sometimes used in vineyards to control weeds between vines and may therefore be absorbed through grapevine leaves and roots.

A simulated herbicide drift study was conducted at the National Wine and Grape Industry Centre in Wagga Wagga during the 2017/18 growing season. The study formed part of Wine Australia’s Incubator Initiative with the NSW Wine Industry Association as regional partners. Four commonly used herbicides (2,4-D, Dicamba, MCPA and glyphosate) were applied to five-year-old potted Tempranillo grapevines at the cessation of flowering to simulate drifts of approximately 10% relative to the label rates. The hormonal-type herbicides, 2,4-D, Dicamba and MCPA, mimic the action of the natural plant hormone auxin in plants, and ultimately cause uncontrolled cell division and high ethylene production in affected tissues (Cobb and Raede 2010). Glyphosate, on the other hand, works by inhibiting the biosynthesis of aromatic amino acids (phenylalanine, tryptophan and tyrosine) in the target plant, subsequently affecting many aspects of plant metabolism including the production of phenolic compounds (Cobb and Raede 2010). The main aim of this study was to better understand the repercussions of the four herbicides on vegetative and reproductive grapevine development until fruit maturity. The visual foliar, shoot and fruit injuries caused by the different herbicides were additionally documented. Photographs and a detailed description of these injuries are available in the 2018/19 NSW DPI Grapevine Management Guide.

Figure 1. Bunch and berry necrosis following 2,4-D exposure at the cessation of flowering.
Figure 2. Shoot tip necrosis developing in a basipetal direction following Dicamba exposure.

Exposure to 2,4-D was particularly detrimental to fruit yield by the maturity stage. Necrosis of bunches and berries (Figure 1) corresponded with a 35% reduction in yield following the 2,4-D treatment. The auxin-type herbicides (2,4-D, Dicamba and MCPA) reduced the structural development of roots by 18, 19 and 28%, respectively by fruit maturity as compared to the control. Furthermore, auxin-type herbicides, markedly Dicamba, induced distinct shoot tip necrosis which moved in a basipetal direction over time (Figure 2). Primary bud necrosis (PBN) increased by 17% in the compound buds usually retained with spur pruning, following the 2,4-D treatment. 2,4-D and Dicamba additionally increased PBN by 52 and 50%, respectively in the younger buds important for cane pruned vines. In contrast to the rapid mode of action exhibited by the auxin-type herbicides, glyphosate related injuries only appeared two to three weeks post-treatment and were generally milder in nature.

The late spring period poses a considerable risk for herbicide drift damage in vineyards. Exposure to 2,4-D may especially be hazardous during this period, substantially increasing the potential for yield loss through bunch and berry necrosis. Grape growers should ensure that their neighbours are aware of the sensitivity of grapevines toward the phytotoxic effects of hormonal-type herbicides, notably 2,4-D, which should be avoided near vineyards during spring especially around flowering. The likely inhibition of root development following exposure to 2,4-D, Dicamba or MCPA is of particular importance for younger vineyards where the root system is still being established. Promoting soil conditions which favour root health during the late season and post-harvest period could mitigate the detrimental impact of these herbicides on roots. A flush in root growth usually occurs late in the season up to after harvest (Van Zyl 1984), and irrigation and fertilisation strategies could, for example, be applied to help boost the development of the root system. Furthermore, exposure to hormonal-type herbicides may cause a negative residual effect on fruit yield the season following exposure (Ogg et al. 1991). Results from the current study suggest that 2,4-D and Dicamba exposure in particular promotes PBN, which align with reduced fruitfulness the next season (Collins et al. 2006). Younger compound buds on higher shoot positions are more likely to be damaged by the herbicides. Therefore, cane pruned grapevines are likely to suffer more from the residual effects of herbicide drift in terms of fruit yield for the next season. Spur pruning would, therefore, be a more suitable option for the majority of vineyards which are regularly affected by herbicide drift.

References

Al-Khatib, K., Parker, R. and Fuerst, E.P. (1993) Wine grape (Vitis vinifera L.) response to simulated herbicide drift. Weed Technology 7, 97-102.

Cobb, A.H. and Reade, J.P.H. (2010) Herbicides and Plant Physiology. (Wiley-Blackwell, Hoboken, NJ, USA).

Collins, C., Coles, R., Conran, G. and Rawnsley, B. (2006) The progression of primary bud necrosis in the grapevine cv. Shiraz (Vitis vinifera L.): A histological analysis. Vitis 42, 57-62.

Felsot, A.S., Unsworth, J.B., Linders, J.B., Roberts, G., Rautman, D., Harris, C. and Carazo, E. (2010) Agrochemical spray drift; assessment and mitigation – A review. Journal of Environmental Science and Health Part B 46, 1-23.

Ogg, A.G., Ahmedullah, M.A. and Wright, G.M. (1991) Influence of repeated applications of 2,4-D on yield and juice quality of concord grapes (Vitis labruscana). Weed Science 39, 284-295.

Van Zyl, J.L. (1984) Response of Colombar grapevines to irrigation as regards quality aspects and growth. South African Journal of Enology and Viticulture 5, 19-28.

Links for further information:

Rossouw, G., Holzapfel, B., Rogiers, S. and Schmidtke, L. (2018) Visual symptoms of herbicide drift on grapevine shoots, leaves and fruit. In The 2018-19 DPI NSW Grapevine Management Guide Pp 73-81.

Rossouw, G.C. (2018) Incubator initiative: Can we visually identify different sorts of herbicide injury in grapevines based on foliage and fruit symptoms? Final Report to Wine Australia, Project CSU 1701, July 2018.