Purdue Extension comments on recent glyphosate stories

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Glyphosate’s Impact on Field Crop Production and Disease Development

The U.S. Department of Agriculture’s recent decision to approve Roundup Ready alfalfa renewed a debate about the safety of genetically modified crops and the use of glyphosate in the environment.
This is not a new controversy, but many statements released in recent weeks by groups opposed to the use of genetically modified (GM) crops have claimed that glyphosate use and Roundup Ready® technology will be disastrous and that glyphosate has damaged crop production by decreasing nutrient availability to plants, reducing nutrient content of food and livestock feed, and increasing plant susceptibility to disease (Zerbe, 2011). There also are claims that glyphosate is contributing to an increase in more than 40 plant diseases that may also affect human and animal health (Smith, 2011; Zerbe, 2011). However, evidence to support these claims has neither been presented to nor evaluated by the scientific community

As scientists, we are equally concerned about the health of the environment and the sustainability of agricultural production. We have previously addressed questions on the impact of glyphosate and manganese
(Mn) interactions on soybean (see http://www.btny.purdue.edu/weedscience/2010/GlyphosateMn.pdf).

 In this article, we discussed the limited research available on the impact of glyphosate and glyphosate resistant crops on Mn nutrition of soybeans, and encouraged producers to avoid “insurance” applications of Mn for the sole purpose of counteracting perceived plant health damage due to glyphosate use. However, the most recent press releases around this issue are focused on the impact of glyphosate on plant and human disease development. This article is intended to clarify the relationship between glyphosate and plant disease development.

The claim that herbicides, such as glyphosate, can make plants more susceptible to disease is not entirely without merit. Research has indicated that plants sprayed with glyphosate or other herbicides are more susceptible to many biological and physiological disorders (Babiker et al., 2011; Descalzo et al., 1996; Johal and Rahe, 1984; Larson et al., 2006; Means and Kremer, 2007; Sanogo et al., 2000; Smiley et al., 1992). Our research with glyphosatesusceptible weeds has shown that some weeds die more rapidly after they have been sprayed with glyphosate when grown in soil that contains certain soil-borne fungi. This suggests that some soil fungi are more effective in infecting Harikrishnan and Yang, 2001; Sanogo et al., 2000). Based on observations from our research, we speculate that this happens when weeds are exposed to ACCase inhibitors as well.

Despite the potential for herbicides to increase disease levels in certain plants, plant pathologists have NOT observed a widespread increase in susceptibility to plant diseases in glyphosate-resistant corn and soybean….

…Although some research indicates there is an increase in disease severity on plants in the presence of glyphosate, it does NOT necessarily mean that there is an impact on yield. The most important point to make about the majority of research available on glyphosate-disease interactions is that the research does not always quantify the effect of glyphosate-influenced disease development on yield. Despite claims linking glyphosate use to increases in yield-limiting diseases such as Goss’s wilt of corn, or sudden death syndrome (SDS) of soybean, we are not aware of published research that fully examines the impact of glyphosate on disease development and yield under disease pressure. Previous research examining the effect of herbicides, including glyphosate, on disease development in soybean has been conducted in greenhouse or limited field trials, and has not examined the effect of these interactions on yield (Bradley et al., 2002; Sanogo et al., 2000). All plant diseases do not have an equal impact on yield. Plants have natural defense systems that are able to limit infection and prevent yield loss in some cases. Disease-causing organisms exist naturally in the environment, but only cause infection when a susceptible host and a favorable environment are present. Even when infection occurs, the disease must reach a level in the host where the plant is weakened enough to cause yield loss.

The claim that plant disease has “skyrocketed” due to glyphosate usage is also unfounded…


…The articles and websites state that fungi in the genus Fusarium cause not only plant diseases but also disease outbreaks in humans and animals. In fact, very few pathogens infect both plants and animals. Some fungi can produce toxic compounds called mycotoxins that can be harmful to animals and humans (Desjardins and Proctor, 2007). However, only certain species within the genus Fusarium have been shown to produce mycotoxins. The majority of Fusarium fungi that produce mycotoxins are pathogens of corn and wheat. Wheat and food-grade corn are non-GMO crops, meaning that mycotoxin development in these crops would not be directly linked to glyphosate usage or interactions. Plants and grain affected by the fungus that causes SDS, Fusarium virguliforme, have not been shown to be toxic to humans or livestock. Additionally, the United States Food and Drug Administration has set levels for the amount of mycotoxins that can be in animal feed, and in food for human consumption, and these markets are closely regulated to prevent introduction of mycotoxin-contaminated grain into the market.

Overall, the claims that glyphosate is having a widespread effect on plant health are largely unsubstantiated. To date, there is limited scientific research data that suggest that plant diseases have increased in GM crops due to the use of glyphosate. Most importantly, the impact of these interactions on yield has not been demonstrated. Therefore, we maintain our recommendations of judicious glyphosate use for weed control. We encourage crop producers, agribusiness personnel, and the general public to speak with University Extension personnel before making changes in crop production practices that are based on sensationalist claims instead of facts.

References:
1. Anderson, J.A., and Kolmer, J.A. 2005. Rust control in glyphosate tolerant wheat following application of the herbicide glyphosate. Plant Dis. 89:1136-1142.
2. Babiker, E.M., Hulbert, S.H., Schroeder, K.L., and Paulitz, T.C. 2011. Optimum timing of preplant applications of glyphosate to manage Rhizoctonia root rot in barley. Plant Disease 95:304-310
3. Baley, G.J., Campbell, KG., Yenish, J., Kidwell, K.K., and Paulitz, T.C. 2009. Influence of glyphosate, crop volunteer and root pathogens on glyphosate4. Cotton, T.K., and Munkvold, G.P., 1998. Survival of Fusarium moniliforme, F. proliferatum, and F. subglutinans in maize stalk residue. Phytopathology 88:550555.
5. Descalzo, R.C., Punja, Z.K., Levesque, C.A., and Rahe, J.E. 1996. Identifiaction and role of Pythium species as glyphosate synergists on bean (Phaseolus vulgaris) grown in different soils. Mycological Research 100:971-978.
6. Desjardines, A.E., and Proctor, R.H. 2007. Molecular biology of Fusarium mycotoxins. International Journal of Food Microbiology 119:47-5o.
7. Feng, P.C.C., Baley, G.J., Clinton, W.P., Bunkers, G.J., Alibhai, M.F., Paulitz, T.C., and Kidwell, K.K. 2005. Glyphosate inhibits rust disease sin glyphosate-resistant wheat and soybean. Proceedings of the National Academy of Sciences. 48:1729017295. http://www.pnas.org/cgi/doi/10.1073/pnas.0508873102.
8. Flett, B.C., McLaren, N.W., and Wehner, F.C. 1998. Incidence of ear rot pathogens under alternating corn tillage practices. Plant Disease 82:781-784.
9. Harikrishnan, R., and Yang, X.B. 2001. Influence of herbici
des on growth and sclerotia production in Rhizoctonia solani. Weed Science 49:241-247.
10. Johal, G.S., and Rhae, J.E. 1984. Effect of soilborne plant-pathogenic fungi on the herbicidal action of glyphosate on bean seedlings. Phytopathology 74:950-955.
ii. Kishore, G.M., and Shah, D.M. 1998 Amino acid biosynthesis inhibitors as herbicides. Annual Review of Biochemistry 57:627-663.
12. Larson, R.L., Hill, H.L., Fenwick, A., Kniss, A.R., Hanson, L.E., and Miller, S.D. 2006. Influence of glyphosate on Rhizoctonia and Fusarium root rot in sugar beet. Pest Management Science 62:1182-1192.
13. Means, N.E., and Kremer, R.J., 2007. Influence of soil moisture on root colonization of glyphosate-treated soybean by Fusarium species. Communications in Soil Science and Plant Analysis 38:1713-1720.
14. Njiti, V.N., Myers, 0., Schroeder, D., and Lightfoot, D.A. 2003. Roundup ready soybean: glyphosate effects on Fusarium solani root colonization and sudden death syndrome. Agronomy Journal 95:1140-1145.
15. Smiley, R.W., Ogg., A.G. Jr, and Cook, R.J. 1992. Influence of glyphosate on Rhizoctonia root rot, growth, and yield of barley. Plant Disease 76:937-942.
16. Smith, J. 2011. Monsanto’s Roundup triggers over 4o plant diseases and endangers human and animal health. Foodconsumer.org. http://www.foodconsumer.org/ newsite/Non-food/Environment/roundup_0118110818.html. Posted 1/19/2011, Accessed 2/8/11.
17. Workneh, F. Yang, X.B., and Tylka, G.L. 1998. Effect of tillage practices on vertical distribution of Phytophthora sojae. Plant Disease 82:1258-1263.
18. Zerbe, L. 2011. Roundup: What you need to know about the pesticide poised to “push us all off of the cliff.” Rodale Press. http:/www.rodale.com/roundup. Posted 2/3/2011, Accessed 2/8/11.

Purdue Authors listed in alphabetical order
Jim Camberato, Extension Soil Fertility
Specialist, Purdue University
Shaun Casteel, Extension Soybean Agronomist, Purdue University
Peter Goldsbrough, Department Head, Botany and Plant Pathology Department,
Purdue University
Bill Johnson, Extension Weed Scientist, Purdue University
Kiersten Wise, Extension Field Crop Pathologist, Purdue University
Charles Woloshuk, Extension Corn/ Mycotoxin Pathologist,
Purdue University