Dr. Cook, Dean and Professor Emeritus of the Washington State University, United States, clears up a long-believed theory with the help of his seven-year-old granddaughter.
Dr. James Cook retired in 2005 as Professor of Plant Pathology and Crop and Soil Sciences and Interim Dean of the College of Agricultural, Human and Natural Resource Sciences at the Washington State University (WSU). His long and productive career in plant pathology had begun in 1965 when he was assigned to work on wheat root diseases for the U.S. Department of Agriculture, Agricultural Research Service (USDA-ARS), stationed at WSU, Pullman, Washington.
Teaching students and post graduates was always an important part of his work. From 1998 – 2005, he held the R. James Cook Endowed Chair in Wheat Research, which was established in 1997 with a $1.5 million gift to the WSU Foundation by the Washington Wheat Commission. Washington State University created this Chair to strengthen research and graduate education in the Plant, Soil, and Microbiological Sciences.
Prof. Cook received a number of honors through his career, especially recognizing that he always worked closely together with growers to ensure that his research was applicable to the farming system practiced.
Dr. Cook’s work aimed at wheat health and the factors that impact plant pathogens, as well as non-pathogenic microbes. His discoveries and conclusions gave insight to the whole picture of factors influencing Root Health, rather than focusing only on single factors. Root Health is a topic dear to his heart, as he states himself.
We asked Dr. Cook about his view on the impact of today’s agronomic practices on Root Health:
One factor clearly impacting Root Health is the lack of rotation in today’s agriculture where continuous crop monoculture is more and more common. Traditional crop rotations are no longer an economical option. In the US Pacific Northwest (PNW), all different classes of wheat can be grown, including spring and winter wheat as well as high grain protein and low grain protein wheat. Still, rotation is based on intensive and continuous cereal growing that includes spring and winter barley.
When I started working at Washington State University, I had to overcome a number of theories that were taken for granted. One of them was that, in monoculture systems, yield is driven down over the years due to depletion of soil nutrients. It took some time to prove that the symptoms of nutrient deficiency that occurred in continuous monoculture were caused by an increase of root diseases in this system. The damaged root system was subsequently unable to take up the fertilizer available in the soil, resulting in nutrient deficiency symptoms.
Direct seeding employed in the late 70s is the other factor that increased pressure from root diseases. Despite all benefits that this system has with respect to less soil erosion, improvement in soil quality and reduction of fuel costs by saving the energy for tillage, it favors the soil borne pathogens. With the change to direct seeding, it was at first believed that substances from straw left on the soil surface were toxic or allelopathic to wheat. Our laboratory found that the same toxic effect could be observed when dry oatmeal was added to the soil instead of straw. The symptoms of stunted wheat and yellow leaves were overcome by either pasteurizing the soil with moist heat at 45 °C /113 °F for 20 minutes or adding mefenoxam to the soil. With this, we were able to document that the stunted yield was not caused by the straw, but due to soil diseases (Pythium in this case) that were favored by additional crop residues on the soil surface. In field studies we were able to document that the yield loss and disappointingly poor growth of wheat when direct seeded into wheat stubble was due to pathogens in the soil and not in the straw.
What diseases constitute a threat for Root Health in wheat from an US view and how has this changed over the past 40 years?
Fusarium root rot became more important with the introduction of dwarf varieties in the early 1960s. These varieties were able to respond to additional nitrogen (N), resulting in increased yield without the danger of lodging. Unfortunately the higher N uptake correlated to an increased need for water uptake. The resulting drought stress of the plants under dry land conditions was the cause of an increase of Fusarium root rot because Fusarium is a root and foot rot pathogen of water-stressed wheat. By adapting the amount of N fertilizer to the estimated water availability this problem could be minimized.
Take-all (caused by Gaeumanomyces graminis) is an important root disease that is wide spread and favored by cool and moist conditions. In monoculture wheat systems with adequate moisture, it is potentially the dominant root disease. We learned as others in Europe had reported that this disease can decline in wheat monoculture over time due to the establishment of a suppressive soil. The suppressive soil is the result of certain antibiotic-producing strains of Pseudomonas fluorescence that build-up following one or more Take-all outbreaks in a field.
Pythium is a pathogen so wide spread that probably every cup of soil on this world has Pythium in it. Therefore we started working with soil fumigation as a research tool. This treatment allowed us to see what wheat with a healthy root system looks like. Plant physiologists suspected that the improved yield after soil fumigation was because of the dead microbial biomass releasing additional N that gave the increased growth response (IGR) of the wheat plant. However, we were able to document that of two fumigants that each gave the flush of N, only one gave the IGR, thereby providing the first evidence separating the two effects. Later we recognized that the fumigant that gave the IGR eliminated Pythium much like pasteurization of the soil. We also showed that no fumigant left N unused in the soil.
Pythium is classified as a root nibbler. It penetrates the roots in the zone of root hair formation and destroys root tips and root hairs. Pythium generally likes juvenile cells – for example, the embryo of a germinating seed. The introduction of mefenoxam in the early 1980s as a seed treatment resulted in similar effects as soil fumigation and supported the awareness on the impact of this disease. Direct seeded soils over time have better water drainage than conventional tilled soils, which can reduce infections with Pythium over time, as well.
Rhizoctonia root rot caused by Rhizoctonia solani AG8 in the PNW of the US, is an increasing problem especially under direct seeding. This disease can “prune off” the root like a beaver chews off a tree. Since Rhizoctonia has a wide host range, crop rotation is not expected to have a large impact on this disease. The practice of spraying volunteer plants with glyphosate two to three days prior to seeding increased the severity of Rhizoctonia root rot. The pathogen present in the field “becomes a millionaire overnight” with the sudden increase of dying roots already occupied by the pathogen through parasitism. By spraying glyphosate four weeks prior to seeding, these volunteer seedlings are small and therefore represent less potential food base when they die, and the decomposers have more time to destroy this food base and starve Rhizoctonia. I focused on these four disease pathogens. They were discovered in this order during my career. Take-all, Pythium and Rhizoctonia like cool soils, whilst Fusarium, especially Fusarium graminearum, is a pathogen of warmer soils and drought stress.
What is the value of a healthy root, and what is its role in crop production?
A healthy root is key for nutrient uptake and particularly so during early stages of development. If three out of five seminal roots get infected or worse, cut off by girdling lesion, only two are left to provide the seedling with water and nutrients. Under phosphorus (P) deficiency, plants become more susceptible to soilborne diseases. At the same time, a diseased root results in reduced P uptake. Zinc and other trace nutrients are likewise important for disease defense mechanisms of the plant.
Furthermore, it has been shown that hormones important to the tops are produced in the root tips of plants. A root system damaged by a disease like Pythium or Rhizoctonia can therefore result in symptoms of hormone imbalances or deficiencies that have an impact on the plant’s growth, development, and finally yield.
An important finding in plant physiology was discovered by Betty Klepper, plant physiologist and research leader at the USDA-ARS in Oregon, US. She found that after a shoot comes out from the soil, the bud of the axil of the first true leaf breaks dormancy at the time of appearance of the fourth leaf on the main stem, initiating the first tiller. Similarly the bud of the axil of the second leaf breaks dormancy with appearance of the fifth leaf on the main stem, initiating the second tiller and so forth. The development of leaves and hence tillering is driven by growing degree days. For the PNW, one leaf develops and hence another tiller every 100 growing degree days. If, at the time the fourth leaf elongates, the plant encounters stress such as nutrient deficiency caused by a diseased root or drought, the first tiller will be skipped and never be initiated. In this case, the first tiller we will see when looking at that plant might actually be a second or third tiller. Because the first two tillers have the highest yield potential, the most important effect of root health is its effect on tillering. The number of tillers initiated, plus having the “right” tillers, has a big effect on final crop yield.
What needs to be done in order to reach most of the plant’s potential in a cropping system with direct seeding and monoculture?
Spray glyphosate four weeks prior to seeding in order to prevent or eliminate the ”green bridge” (green growth by volunteer plants or weeds after harvest before the next seeding). This can help limit damage caused by pathogens such as Rhizoctonia.
Spring sown wheat after winter wheat gives an eight month break that allows reduction of inoculum. The application of a seed treatment protects the embryo and ensures vigorous seedling development and hence stand establishment. At the same time, use fresh (new) high quality seeds. Older seeds can have dead cells; in contact with moist soil, seed content may leak from those cells to the soil and stimulate infections with Pythium.
Phosphorus fertilizer placement under the seed at the time of seeding can help to reduce the negative effect of root diseases; the P placed is where it is needed in direct proximity to the young roots, even if they are diseased. As mentioned earlier, on one hand P has a positive effect on the plant’s defense mechanisms. On the other hand, even if the root is attacked by disease, there is still enough P available to ensure further development of secondary roots. Once the soil begins warming up, the plant can outgrow its vulnerable phase and overcome the cooler conditions with higher disease pressure.
Adopting paired row spacing (two rows sown 17 cm apart alternating with 43 cm spacing, instead of uniform 30 cm row spacing) showed in our studies that the resulting greater warming and drying of the top few centimeters of soil where the pathogens are most active helped reduce root diseases.
A combination of these practices can bring yields up to at least 85% of what can be reached by eliminating root diseases using soil fumigation.
Dr. Cook was interviewed by Dr. Melanie Goll, Technical Innovation Manager, Syngenta. Melanie holds a Ph.D. in plant pathology and focuses on seed treatment development, especially fungicides and nematicides.