We can manage plant disease by managing plant nutrition
Editor’s note: This is an edited transcript of part of Dr. Don Huber’s talk from the 2013 Acres U.S.A. Eco-Ag Conference. Dr. Huber will be presenting a full-day Eco-Ag U session at this year’s conference — December 4-7 in Covington, Kentucky. Learn more and sign up at events.acresusa.com.
Agriculture is the basic infrastructure of our society. Everything you as a farmer do determines what the rest of society does — you provide that whole basic infrastructure that determines the number of doctors, the number of school teachers, and everything else.
So, it’s important that we maximize our agricultural efficiency and production, and that we produce high-quality food. And to do that, we have to control diseases and pests. So, I’d like to address some of the interactions of nutrient deficiency and how we recognize them — and how they tie in with disease.
Remember that in farming, your purpose is to run a beautiful factory — an amazing factory — that captures the sun’s energy and stores it as sugar. To do this, you have to have some nutrients. If it wasn’t for manganese, none of us would be here, because manganese is required to split the water molecule that’s required for photosynthesis, which takes the hydrogen, and then — through chlorophyll and the energy of the sun, with magnesium as that central element in the chlorophyll molecule — combines hydrogen with carbon dioxide and produces sugar.
The oxygen is what we get — and what sustains us. It’s the most important element that we have — but producing it requires all of the other nutrients.
We’re only harvesting 25 to 30 percent of the genetic potential of our crops. There’s a lot more room out there for us to improve our yields. And that’s what we do in our breeding programs — getting better expression of that wealth of genetic potential that’s in those crops. We’re told we can’t feed another two billion people unless we have genetic engineering. But there isn’t a single genetically engineered plant that has increased the intrinsic yield of any crop. All of the yield increases come about through the traditional breeding programs. Yield and quality are factors that you can’t produce with a silver bullet — and farming isn’t a silver-bullet approach to start with.
Genetic potential is lost through stresses — either biotic or abiotic — and we only harvest what’s left over. So, as we reduce those stresses — as we reduce disease and the pressure from pests — we can harvest more of that genetic potential.
All of these stressors are linked to nutrition. A lot of them will reduce nutrient efficiency. But we can also take those nutrients and use them to reduce or eliminate the stressors. Whether it’s drought stress with manganese, whether it’s cold stress with copper, whether it’s Fusarium with zinc, or whether it’s Verticillium or Streptomyces or other pathogens with manganese — nutrients are linked to resistance and to the vigor and ability of the plant to offset damage, or reduce damage, from those stressors.
So, what we’re doing when we’re farming is managing an ecology. It’s an ecology that involves the plant and also the chemical and physical environments, and especially the dynamic, biological environment of the soil, which then impacts both the plant and the pathogens and pests. As we manage this ecology, we’re trying to make the conditions as favorable for the plant as possible, and we’re trying to reduce the opportunity for those pests and diseases to have an impact and to diminish the plant’s genetic potential.
You can’t do one thing without affecting all of the components of the ecology. Because when you do one thing — whether it’s tillage or adding fertility or crop rotation or planting cover crops — you change the interaction of all of those components, which are going to determine whether you have a healthy crop or a diseased crop — a nutrient-dense crop or a bunch of empty carbon.
Each of these interactions impacts a specific nutrient, or the overall nutrient relationship, that we have in the plant. There are many factors that change the host environment. Nutrient needs change with different growth stages. Temperature changes both the plant physiology as well as the soil physiology and biology. We can look at all of those interactions and try to manage more effectively, or more efficiently, to provide the nutrients that are optimum for the plant and for its resistance to the stressor.
An example of one of these nutrient interactions that comes into play in the biological component is the relationship between take-all — a root-and-crown rot on wheat — and manganese. One time I was studying a uniform plot, and I went back in the spring, at about heading time, and there were spots in the field, randomly scattered around the plot, with severe take-all. And as I looked at those spots, I found a signature from the neighbor’s cows that had gotten out in the wintertime and had left their signature — that pile of manure that upset the nutrient balance. Now, I recommend nitrogen sufficiency to reduce this disease. But too much of a good thing can also throw a lot of the system out of bounds — and that’s what happens when you have that much manure on one spot. You interfere with the manganese uptake because it favors the oxidizing organisms — it reduces manganese availability.
Between a pH of 5.2 and 7.8, manganese availability for a plant is determined entirely by the soil biology. The effect of pH and the redox potential in that large range is on the organisms that reduce or oxidize manganese to influence its availability. In sterile soil, there is no change in manganese between 5.2 and 7.8. But in a field soil, there’s a dramatic change, because of that soil biology — that 10 to 20 tons of living matter for every acre-foot of soil.
We need to remember that a balance is important. All of these nutrients are interrelated. They interact in an interdependent system with the plant’s genetics and the environment. That’s why quite often the root of the problem is underground — in the primary absorptive area of the plant. I’ve done work in Guatemala, where you see typically black roots on the sugarcane and on banana. Water management becomes extremely important because it shifts the balance — the biological equilibrium — that we need for certain nutrients. The black roots are actually showing oxidized manganese. It’s oxidized, because if it wasn’t — if it wasn’t for those organisms — with those heavy, wet soils, you’d have manganese toxicity. It’s a protective situation for them. In the dry season we have to add more water, and in the wet season we add calcium to reduce the availability, so that we don’t have some of those root pathogens that would respond in those conditions.
Quite often we don’t recognize that it’s the micronutrients that are the keys — that turn on and off the physiology of the enzymes of the plant. There are thousands of enzymes in a plant that’s functioning properly. All are under genetic control, but the genetics are also regulated by the nutrients. If you want to turn a function off, you merely pull that mineral away from the enzyme by chelation. Now, we use chelators like EDTA and citric acid and malic acid, and some of the amino acids, to increase the solubility and availability of some of these nutrients and move them across cell walls. But we also use chelators to immobilize them.
All of our herbicides, for instance, are chelators. They pull the key out of the ignition, and they shut down a process. We say that they’re an ALS inhibitor, or that they’re an EPSP synthase inhibitor. But we don’t recognize that the way they’re inhibiting those enzymes is by pulling the key out of the ignition, because those particular nutrients are the cofactors for that particular physiological function. That’s why you see that a symptom of diquat, for instance, is a yellowing and an iron deficiency — because you pull the iron out of the system and immobilize it, so that all of those energy relationships that are involved can no longer take place. With EPSPS, we shut down the shikimate pathway. All of our herbicides, and many of our other pesticides, are chelators. They work based on nutrition.
If we don’t recognize that, and we’re using these materials, then we induce a nutrient deficiency. For our farmers up in Canada and the Pacific Northwest — where we use fenoxaprop herbicides — those are copper chelators. If farmers there who are on low-copper soils don’t put on five to 10 pounds of copper nitrate or copper sulfate, they’re going to induce a nutrient deficiency, and in many of those soils, they’re going to end up with a severe case of ergot, and yield reduction. But we can compensate for it if we recognize how some of these chemicals work. We need to remember how these nutrients interact with the physiology.
Dr. Don Huber is Professor Emeritus of Plant Pathology at Purdue University. He is the co-editor of Mineral Nutrition and Plant Disease, available at the Acres U.S.A. bookstore, which examines in depth how individual minerals improve or reduce the health of plants.
