By Christine Engelbrecht

Plant Pathologist

Iowa State University Extension

I love watching TV crime shows. Those detectives can solve a complicated murder and apprehend the criminal within an hour. But what can we do when the crime happens in our garden, and the victim is our favorite oak tree or petunia?  Fungi, viruses, insects, improper cultural practices and nasty weather are common garden criminals that can stalk, terrorize or outright kill our favorite plants. We need to know “whodunit” so we can manage the problem now and in the future. When garden criminals have left our plants wilting, dying and begging for justice, how can we solve the crime?

1.  Identify the victim

First, it’s important to know what kind of plant has been attacked. What characteristics are normal for that particular cultivar? Is a yellow plant actually sick, or is that its usual appearance? What problems are common on that type of plant? For example, pin oaks and birches in parts of Iowa frequently turn yellow because they can’t take up iron from alkaline soils. Scotch pines are especially susceptible to a disease called pine wilt, but white pines are more resistant. Knowing what kind of plant you have is the first step to solving the crime.

2.  Examine the entire body

Wilted, spotted or discolored leaves may be the first symptom you notice on the victim, but carefully examine the rest of the plant, too. Do the stem or trunk and branches appear normal? If it’s a tree, examine the base of the trunk—is it planted too deep, or are there any injuries to the bark such as from a lawn mower or animal? Are any roots growing around the base of the plant, girdling it? If feasible, cut open the plant and look for evidence of internal decay. On smaller plants, dig up the roots—are they white and healthy, or brown and rotted?

3.  Investigate the crime scene

The location in which a plant lives is crucial to its health. What can you see in the surrounding environment that may be a clue to the plant’s problems? Is there plenty of room for the roots to grow, or is part of the rootzone covered with concrete? What is the soil like? Is the area wet or dry, sunny or shady? Are nearby plants healthy or are we dealing with an indiscriminant mass murderer? 

4.  Run a background check

To solve a plant crime you must know some history. Is the victim a recent transplant or is it well-established? What’s happened near the plant recently? Has there been a traumatic site change, such as nearby construction, changing the grade of the lawn or installing a pond in recent years? Did something happen shortly before symptoms appeared, such as a dry spell, heavy rainstorm or a springtime frost?

5.  Do your bookwork

Once you’ve gathered as much forensic evidence as possible, it’s time to hit the books (or the Internet). What possible diseases, insects or environmental conditions could cause the symptoms you observe on the plant?  A quick web search, such as “spruce diseases,” should yield many suspects. University Extension websites are good sources of information on plant problems.

6.  Use the crime lab

You won’t have the equipment or expertise to solve every plant crime.  Sometimes, you need help from an expert with a microscope, lab tests, and extensive knowledge of plant problems. Some serious plant diseases, such as oak wilt, cannot be accurately diagnosed without culturing the causal fungus in a lab. Every state’s land grant university has a plant clinic to help the public solve plant problems.  In Iowa, you can go to your county extension office, or contact the Iowa State University Plant and Insect Diagnostic Clinic directly at (515) 294-0581, or sickplant@iastate.edu.  

7.  Expect some unsolved mysteries

Even with background information, extensive research and the best help from experts, some plant crimes go unsolved. Scientists continually discover new plant diseases and techniques to diagnose plant problems, just as the field of human forensics is rapidly evolving. For now, some insidious plant criminals stump even the best detectives.

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