A Closer Look At Mid Season Plant Health Decline Syndrome
by David Miller, Vice President of Agronomy Education at AEA
July 25, 2016
Why do plants get sick just as the crop is starting to size and ripen? Why do insects and diseases destroy hundreds of thousands of dollars worth of food each year? Why are crops infested with fungal and bacterial diseases like downy and powdery mildew? Why do cherries get bacterial canker? Why is there citrus greening in oranges? What is the cause of diseases in plants and why do they mostly show up late in the season?
Insects and diseases not the problem, but rather, they are a symptom of the real problem.
It’s important to remember that insects and diseases are never the problem, just a symptom of the real problem. It reminds me of a trip I made to Kansas a while ago. I overslept (an alarm malfunction) rushed to the airport, and got there just in time for my flight. Great! But after boarding, my flight was delayed by 30 minutes so now I was going to be late for my appointment with the distributor and all the farmers that were planning to be at the meeting. We landed, I rushed to the car, meeting my colleague, and headed out on the three-hour drive. About halfway there, my head started throbbing, my eyes hurt, and I wanted to puke. As I pondered the reason I was this sick, it dawned on me that in the rush of my morning I had not consumed enough liquids. I was very dehydrated.
The thought never crossed my mind that my sickness was from an ibuprofen deficiency.
As soon as I realized what the real problem was, we stopped to buy a lot of liquid and some acetaminophen to relieve my headache. By the time we arrived at the farm for our meeting, I had started rehydrating and I didn’t feel the headache.
Let’s think about this. Was my headache the problem? Did the acetaminophen solve the problem? No. The problem clearly was dehydration and the solution was increasing liquids. Had I thought the headache was the problem and acetaminophen the solution, I would have had to take increasing amounts
of it to stay free of the symptoms. After a while, I would have believed I couldn’t live without the painkiller and finally, I would have been hurt or killed by the increased levels.
Can you see the correlation between my experience and agriculture? If insects and diseases were headaches and dehydration was a lack of nutrient availability, what would be the solution to our crop loss from insects and diseases? More pesticides? No, while it might be ok to use it occasionally in an emergency or where we don’t manage well (like me on my trip), it will kill your farm if you try to solve the problem with it.
The real problem is lack of nutrient availability.
So what are the reasons nutrients aren’t available and what can we do to solve them? Let me clarify that there is a big difference between having a good looking soil report that has lots of minerals and having minerals that are in a form the plant can absorb and utilize. Bacteria and fungi (microbes) are the key players in making and keeping the nutrients available to plants throughout the season. What do microbes need to thrive and support our crops with available nutrients all season long?
First, let’s consider oxygen. Microbes are just like humans—they can't live without oxygen. They need food—again, just like us humans. As well, microbes need shelter. While microbes can live for a long time without a shelter, they won’t be as efficient at work without one to provide a good night's rest and protection from the weather. Let's look at each one of those three components: oxygen, food and shelter. Starting with oxygen:
Tight, compacted soil limits the flow of oxygen to soil microbes. Compaction can result from an imbalance of minerals, especially magnesium to calcium. When magnesium is greater than 18 to 20 percent and calcium is less than 65 to 70 percent, chances are you will have a deflocculated, deoxygenated soil in which it is impossible for microbes to live. A calcium-to-magnesium imbalance can be corrected with an application of sulfur or gypsum. Poor tillage management also can contribute to compaction. When soil is tilled when too wet, or tilled too finely, its structure is destroyed. Healthy soil has space among the soil colloids for oxygen. So, check your soil for compaction layers and check your soil samples for mineral imbalances.
Besides oxygen, microbes need a source of food to survive. What do microbes eat? Many of the microbes, especially bacteria, are fed by root exudates (sugars) which the plant provides in exchange for minerals. Because these sugars are produced through photosynthesis, let’s take a quick look at what might limit a plant’s ability to photosynthesize efficiently. Plants take up moisture through their roots then absorb carbon dioxide from the soil through the stomata on the underside of its leaves. This water and carbon dioxide is then converted into glucose via the energy of the sun shining on the chloroplast in the leaves. Glucose contains 24 atoms: 6 carbon atoms, 12 hydrogen atoms, and 6 atoms of oxygen. Up to 70% of this glucose is transported out through the roots to feed the bacteria if the plants are healthy. So a lack of moisture, low soil carbon, poor microbial activity, and limited sunshine are all components that will reduce photosynthesis and thus the food supply to the microbes.
The other source for food is crop residue and organic matter. Microbes, primarily fungi, will break down crop residue and organic matter and release the nutrients that are in that organic matter for the next crop. They will live on the sugars, fats, and all the various plant secondary metabolites and compounds that are within the crop residue. And, as they do that, the crop residue will break down and minerals will be released back into the soil in a plant-available form.
Microbes also need shelter, which comes in the form of crevices or pores within stable humic substances that cannot be degraded any further. In other words, microbes live in organic matter that has gone through the entire microbial digestive process.
The Basic Needs Of Plants:
Shelter
Food
Oxygen
When I overlay these biological needs and processes with the realities of the growing season, the plague of late season plant health decline starts to come into focus. The microbe narrative could go like this:
There are no plants using minerals during the winter month yet some of the microbes will slowly continue digesting minerals, thus there are usually plenty of available nutrients to start off the season.
In the spring farmers hit the fields renewed, eager to put winter’s plans into action and determined that this year will be better than the last. Toward that end, soil is prepared, seeds planted, nutritional applications are on time and consistent, the fields are scouted frequently, the moisture level is monitored and everything that can be controlled is diligently tended to. The microbes are well fed and provided for, and they hold up their end of the equation.
As summer comes on the reserves of minerals start to dwindle and, unless the microbes can keep up with the demand, nutrient availability starts diminishing, photosynthesis becomes less efficient, the food supply is shorted to the microbes, and nutrient availability suffers even more and so the cycle continues.
Time pressures emerge as harvest begins just as plants need the most nutrients to size and ripen fruit, for which so much energy has been expended to set and grow to this point, nutrient availability hits a low point—this is when applications sometimes get missed as farmers are busy. This is also the time when insects attack susceptible plants, destroying your crop.
Plants don’t have a pesticide deficiency but rather, they suffer from not having enough nutrients available for some reason or another.
Take a look at everything that is holding microbes back and limiting nutrient availability and eliminate each one. I know you can and I know you will. This year, prevent mid/late season plant health decline syndrome and harvest your profit. Those first pickings pay for your investments to plant the crop, but the last pickings are the profit in your pocket; don’t lose them to midseason decline!
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