Glyphosate effects on the occurance and development of soybean diseases

Glyphosate inhibits a key enzyme which is involved in the synthesis of key amino acids in the plant, and many fungi and bacteria also have this same pathway. In plants, aromatic amino acids are the building blocks for many of the defense compounds such as glyceollin in soybean as well as suberin and lignin. When round-up ready soybeans were first planted this was one concern that was soon alleviated. Few studies have evaluated the development of specific diseases in response to glyphosate applications, but in those that have, the results did not support this claim.

One of these studies was done by a group at Southern Illinois University, which compared round-up ready soybean cultivars with and without glyphosate for the development of SDS. There were no significant differences in the level root infection, SDS symptom development, nor colonization of roots between the sprayed and unsprayed plots of the same variety. Their primary conclusion was that the development of SDS in their region on round-up ready soybeans was due to the lack of resistance to this pathogen and NOT due to glyphosate applications. We have witnessed this in Ohio as well. Specifically during 2009, Ohio had widespread occurrence of SDS. One field in particular still stands out in my mind, where the producer ran out of soybeans of one variety and filled the planter with another variety-and to the row, the SDS was in all of the plants of one variety and none of the other. Same planting date, same herbicide program, same environment-only the variety was different.

Since 2003, this lab has sampled a great number of fields for soybean and corn seedling blight pathogens, including Pythium spp., Phytophthora sojae, and Fusarium graminearum. These have been sampled across conventional corn, soybean, as well as fields that receive predominately a round-up ready program. There are many very reasonable explanations that we have been able to attribute to the development of these pathogens: changes in resistance levels in varieties, increase in inoculum due to soil conservation practices and changes in seed treatment chemistry. We have plots on our farms that have not had Round-up as a routine part of the management, we can achieve the same level of disease today (no greater-no less) as previous researchers working on the same pathogen in these very same fields. One of the strengths of the land grant university system is the ability to maintain these long-term study plots which can monitor effects such as these.

In the scouting we have done, during the last 10 years, the outbreaks of Phytophthora sojae, Frogeye leafspot and Sclerotinia stem rot have directly led to a reduction/or lack of resistance to these pathogens in the varieties. These were not due to the application of round-up. A group at the University of Illinois/USDA-ARS screened glyphosate tolerant cultivars for their response to bacterial pustule. They identified that approximately 30% of the cultivars were susceptible while the rest were resistant in a greenhouse screen. Again, it was the inherent resistance level in the variety and not glyphosate tolerance that resulted in the development of this disease. We have seen the same trends when we evaluate varieties for their response to Phytophthora sojae each year for the Performance Trials. When the glyphosate tolerant lines first entered the trials, there levels of partial resistance were quite low but as time increased, more and more lines had higher levels of resistance. Much of the resistance that we use in field crops is governed by multiple genes. It takes a lot of crosses to get that resistance combined with the high yielding genes and many of these resistant traits do not have the best tightly linked markers.

In addition, glyphosate applications have been shown to reduce fungal growth in plates and as a result of applications in the field. No, we are not going to start recommending glyphosate as a fungicide. Don't even think about it. But in two cases, studies of soybean rust in Florida and wheat stem rust at Washington State University, researchers were able to show in experimental systems that there was indeed a reduction. And this makes sense since this enzyme is present in plant and fungi.

Lastly, every year, soybean pathologists from around the world complete a survey that examines the yield losses in soybean due to plant pathogens. Alan Wrather at Univ. of Missouri has coordinated this effort for the past 20 years. These surveys are based on actual field stops, diagnostic samples, and research plots. These summaries have also not shown an increase in disease. The following table is a summary of these surveys and the different diseases. The yield loss recorded for each disease group fluctuates based on the incidence and severity of specific diseases due to the widespread planting of susceptible varieties and/or environmental factors that favor infections.

Leaf diseases = bacterial diseases, brown spot, downy mildew, frogeye leaf spot.
Stem diseases = anthracnose, brown stem rot, pod and stem blight, Sclerotinia stem rot, stem canker.
Root diseases = charcoal rot, Fusarium root rot, Phytophthora root rot, sudden death syndrome.

There is also a statement concerning the occurrence of a "new" pathogen that can be found in soybean and wheat meal that has negative impacts on animal health. I can't comment on this new pathogen as there is very little data and a lot of speculation, including that wheat was treated with glyphosate, which at this point wheat is not treated. For this part, we will have to wait to see what the evidence truly is and what methods were used to identify this "new" pathogen.

One final thought or maybe this is a reality check. Ohio producers, as well as those worldwide, need to double the world food supply within the next 20 years due to predicted changes in the world population. Our 2 to 3 bushel per year is not going to cut it. We will need every single tool, approach and tactic to make this happen and in my opinion includes genetically modified strategies. We are reaching the limits of doing this via breeding and for some plant diseases, no resistance to the pathogen has been identified; thus novel genetically modified means are going to be the way that we can produce the food for the future. I would like to say that it is in some plant somewhere but to get the resistance from one plant to another-a gene transformation procedure can make this happen in 2 years vs 20. If there are claims on safety - let's get the data out there so we can adapt, put corrections in place and keep moving forward. We have a lot of food to produce for a hungry world. It must be safe and healthy to eat, sustainable to produce and affordable for the consumer.

SOURCE: Ohio State University Extension