The Mystery Of The Yellow Leaves
 | Author:
Allison Kosto, MSU Broadwater County Extension Agent
MSU Broadwater County Extension Agent |
This time of year, I get lots of calls and emails about trees or plants that look unhealthy. I hear “my tree is dying”, “my plants are turning yellow”, “the leaves are curling” or other statements describing the unhealthy plant. With easy access to information on the internet, many people have already self-diagnosed the issue. Unfortunately, solving the problem isn’t as easy as looking at the yellow leaves.
I think it’s the same problem a doctor faces. If you have a fever, is the fever a symptom or the cause of you not feeling well? The fever is a sign or symptom that there is a bigger issue, likely a viral or bacterial infection. The same goes for trees. If leaves are yellowing, it’s a symptom of a larger cause. Unfortunately, determining the cause is not always straightforward and it means becoming a detective to figure it out.
First, let’s take a quick dive into plant biology to understand why a plant is green. To put it simply, it is green because it is producing chlorophyll. When a plant stops producing chlorophyll or produces at a reduced rate, the leaves change color, usually yellow. A fancy way to say the plant is turning yellow is to call it chlorosis, which is defined as the lack of chlorophyll. This sometimes affects the entire plant, sections of the plant, or even a speckled appearance. Sometimes, the leaves will be yellow between the veins which is called interveinal chlorosis.
What causes chlorosis? This is where it gets complicated. One possible cause is an environmental factor such as soil condition, temperature, nutrient imbalances or weather conditions. The soil is the lifeblood of the plant. When soil conditions are not ideal, plant and root growth can be impacted, which in turn affects the plant’s ability to absorb water and nutrients.
One example of a common soil condition in Broadwater County that causes interveinal chlorosis is soil pH. Broadwater County has alkaline soils (high pH). When the pH gets high, certain minerals become difficult for plants to absorb. Specifically, the main issue is iron. Iron deficiencies show up as interveinal chlorosis.
Another possible cause of yellowing leaves is herbicide injury. The pattern depends on the herbicide type. Typically, you see yellowing along with other symptoms such as leaf distortion or curling, browning and dieback.
Yellowing could also be caused by diseases, nematodes, insects or mites. Diseases can interfere with nutrients and plant functions. This can lead to a reduction in chlorophyll which causes the leaves to turn yellow. Nematodes are microscopic worms that are important to the soil food web by feeding on bacteria, fungi and plants. Plant parasitic nematodes can be an issue though when they feed on plant roots, stems or leaves. One of the first symptoms of nematodes is yellow plants.
There are many insects, mites and other arthropods that can cause yellow leaves. As insects feed on leaves, they cause injury which reduces the plant’s ability to produce chlorophyll, turning the plant yellow. Some insects feed on roots which reduce the plant’s ability to absorb water and nutrients. Because the plant’s ability to thrive is compromised, it can turn yellow.
Many things cause a plant to turn yellow, so how do you figure it out? Just like a doctor can figure out a cause of a fever using the process of elimination or analyzing multiple symptoms, you can do the same for a plant. Here are a few things to look for:
• What other symptoms do you see? Leaf curling? Stunted growth? What else? If you can’t see any obvious symptoms above ground, then dig up some of the plant to look at the roots.
• Could weather be a factor? Review the recent and seasonal weather patterns. For example, this year we had a really wet June followed by hot dry weather in July and August which made perfect conditions for fire blight.
• What are the soil conditions? What is the pH? Is the soil compacted? Have you applied any fertilizer or herbicides?
• Do you see any patterns? Is this issue only affecting one plant or multiple plants?
• Have you done anything differently this year that could have caused the issue?
With some investigative work, the cause can hopefully be determined. Sometimes fixes to chlorosis are easy. Other times, such as issues caused by drought or herbicide injury, it might be too late to fix the problem.
Keep in mind that your local MSU Extension Office provides free services to help identify issues. Don’t hesitate to contact us at 406-266-9242 or allison.kosto@ montana.edu if we can help in any way.
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