Expert chat with Prof. Gregory L. Tylka, Professor and Extension Plant Pathologist, Iowa State University, United States – April 2017
Greg Tylka, Professor and Extension Plant Pathologist at the Iowa State University, United States, focuses on the biology and management of plant-parasitic nematodes – specifically the soybean cyst nematode (SCN), Heterodera glycines. His current research includes studies of SCN-resistant soybean varieties, effects of nematode-protectant seed treatments, and effects of cover crops on SCN population densities.
Greg, can you briefly summarize your professional history and passion?
My passion is to get the information about nematodes into the hands of farmers and field representatives – people who need to know how to manage them in the field. This means translating the basic biology of nematodes in a way that the end user can understand and use it, in addition to conducting research. This passion holds my attention and thoughts at all times and drives me. It was a bit unexpected that I ended up studying nematodes because after high school I wanted to become a marine biologist to train animals for sea life shows. I was even accepted at an out-of-state university to major in marine biology; however, my financial situation at that time did not allow for it. I eventually studied biology at the California University of Pennsylvania, which was located less than 20 miles (32 km) from home. With my Master’s thesis, I entered the field of plant pathology, studying root rot of pine trees. It was my Doctorate study at the University of Georgia (with Richard Hussey) that introduced me to the world of nematodes – namely SCN, which I’ve now worked on for more than 30 years. After completing my Ph.D., I joined the Iowa State University in 1990, where I have had the luxury of specializing on this one pathogen. My faculty duties split evenly between research and extension. I also serve as the founding director of the Iowa Soybean Research Center, through a 25% appointment. This university center is supported by the Iowa Soybean Association and by private agricultural companies. It plays a coordinative function among these three entities and helps to align efforts. This extra 25% effort is possible because I really love what I am doing and feel that my job is also my hobby!
Can you give us a bit of background on the relevance of soybean cyst nematodes (SCN) in the United States and your predictions for the future?
Since being introduced in the United States from China during the 1950’s, SCN has spread across the entire Midwest. I observed over the past decades that SCN seemed to operate in cycles. When I started working on this pest in Iowa in 1990, it began to occur and cause severe damage in limited regions of the state and the Midwest. Over the years, it spread out. SCN impact would decline in the locations where it initially caused severe damage, while the damage was most severe in the newly infested areas. The reason for this might have been that farmers used resistant varieties after SCN was discovered in their fields, but also perhaps because of the development of antagonists in the soil (i.e. development of suppressive soils). Although by now high numbers of SCN are present everywhere in the Midwest, the general awareness of this pest declined over the last 10 years due to the simple fact that SCN thrives in hot and dry conditions. In the 1980’s and 1990’s, we had very hot years, but since 2000 we have seen cooler years and more rain, resulting in less SCN crop damage. This outcome seems to have lessened the stature of SCN as a very destructive pest in the minds of soybean farmers. The decreased concern about SCN worries me greatly because we only need some hotter years and the damage will be devastating. Even though it may seem comforting that the number of SCN-resistant varieties for Iowa farmers has increased from 29 in 1991 to 956 in 2016, 97% of the varieties available use the same source of resistance (PI 88788). Of the remaining 29 varieties, 26 possess the “Peking” source of resistance. To emphasize the danger of such a narrow base of SCN resistance, I often ask farmers in meetings, “Can you imagine what would happen if you used the same herbicide for 20 years?” They start giggling because that is what they did, and now they are battling to control herbicide-resistant weeds. The same thing is happening with PI 88788 SCN resistance, except that with weed resistance, there are many other herbicides available for farmers to use, but there are very few SCN-resistant varieties on the market with anything other than PI 88788 resistance. Farmers can’t diversify their SCN resistance if they recognized the problem and wanted to.
To put data behind this matter, we recently reviewed the results of experiments in which we tested hundreds of SCN-resistant varieties at nine (9) locations in farmers’ fields throughout Iowa, over 20 years. From 1991 to 2009, the work was funded by fees paid by seed companies to have their varieties evaluated in the experiments. But since 2010, the Iowa Soybean Association has funded the work. The experiments were conducted in different fields every year. Most of the SCN-resistant varieties had the PI 88788 source of resistance. Our data showed a clear increase of virulence in the SCN populations in those farmers’ fields over the last decade, resulting in increasing SCN population densities in the soil and a decline in yields of the resistant varieties. The decrease in yield is sometimes not that big when the weather is not in favor of SCN, but as it gets hot and dry, the damage will be severe. The results of this study will be published in the scientific journal Plant Health Progress.
What do you think is key to managing the SCN problem?
In my view, it is key to keep farmers aware of the SCN presence in their fields and the need to manage the pest despite not seeing dramatic yield declines over the recent years. SCN represents a chronic soybean health risk that likely will result in devastating losses under certain growing conditions. I liken the threat of SCN to the threat of high blood pressure in humans. If you don`t monitor your blood pressure, you might not know that it is high, and you could eventually die from its consequences. Many farmers may not know that they have high population densities of SCN – they may suffer devastating losses during the next hot and dry growing season. That is why I keep telling farmers to sample for SCN in autumn, after harvest, so that they know what they have in their fields and to at least monitor their sites to be able to make educated decisions.
To manage this disease, it is important to rotate the soybean crop with other crops that are non-hosts. We also need more tools such as soybean varieties with different sources of resistance, more seed treatments, and information on the effects of cover crops. Cover crops are planted in autumn to stop erosion over winter when the field lies fallow. They either die during winter or are killed with herbicide in the spring prior to planting soybean. (For a list of possible cover crops see table below.) Some companies claim a nematode population declining effect is associated with the cover crops. We decided to analyze this concept for SCN and to determine if any or all of these possible mechanisms are true with cover crops and SCN:
- The cover crop induces hatching of the nematodes before soybean, as a host crop, is present. The hatched nematodes starve.
- The cover crop acts as a trap crop. The nematode enters the root of the cover crop and starts initiating feeding sites. The nutrition status is not as optimal in this cover crop as in soybean; therefore, females cannot develop in the roots – resulting in a population decline.
- T3. The impact of allelopathy, which is a form of plant defense, usually against other plant species. The root exudates secreted through this plant defense mechanism can have a nematicidal effect – e.g. methylisothiocyanate, which is released by some plants when glucosinolate in the green leaf tissue degrades.
We still don’t have a lot of information about the cover crop concept related to SCN, but the first greenhouse studies we did determined that none of the non-legume cover crops that we tested were host plants for SCN. In the legumes that were tested, a maximum average of three (3) females were found per plant as compared to 100-300 females per plant on soybeans. No SCN females formed on the non-legume plants. Below are the cover crop plants that we tested to see if they were hosts for SCN:
What is the biggest challenge to using the key SCN management measures?
One challenge is to convince farmers to use a seed treatment. In 2012, farmers received nearly USD 18 per bushel of soybean. If a seed treatment costs USD 15 per acre, he needs to gain only one bushel of yield per acre to make this investment worthwhile. But in 2016, soybeans were worth less than USD 9 per bushel; so, the amount of yield increase needed to recover the cost of the seed treatment was double. If the required yield increase is not likely to happen, the farmer will not invest in seed treatment. Because there are additional pests and pathogens in the fields, for which seed treatments are also available, in the end the farmer must make a bet: for which pest to buy the “extra insurance” of seed treatment.
Are SCN the number one nematode species in soybean everywhere in the United States?
In the Midwest, we believe that SCN is the most widely prevalent and damaging nematode to soybean, but in the southern United States, SCN is not that much of a problem. Down south, root-knot and reniform nematodes seem to cause much more damage to soybean than SCN. And root-lesion nematodes appear to be an increasing soybean problem in the United States as well.
Learn more about Greg Tylka’s passion for sharing the information and results he has captured in practical, easy-to-understand materials: https://www.plantpath.iastate.edu/scn/ and http://www.plantpath.iastate.edu/tylkalab/