Overview of Plant Diagnostics

Getting to the bottom of a plant problem can be a challenge.
Overview of Plant Diagnostics - Articles


Impatiens, Downy Mildew - sporulation closeup, photo: Gary Moorman

This is definitely the case with ornamental plants because there are so many different species. What do you need to be a good diagnostician?

You need to have some familiarity with horticulture (plant identification); also basic, yet important, landscape cultural practices; and finally what diagnostic references can lead you to a correct answer. A plant with symptoms of damage or stress may have one key issue, or it may have a combination of problems. Typically, problems fall within one of these categories:

  1. Plant's environment (soil, light, temperature parameters)
  2. Plant care (how was it planted, what has been done to it since)
  3. Insects, mites, and other animal pests
  4. Infectious diseases (fungi, viruses, bacteria)

Abiotic or Biotic?

Before we dive into the diagnostic procedure, we need to define two terms. The first is abiotic. Abiotic means a plant problem caused by a non-living thing. Examples would be poor horticultural practices, environmental stress, and storm damage. The other type of plant problems is biotic. These include insects, diseases, and weeds that compete with the plants of interest.

Management of these problems can be done:

  • culturally--putting plants in the right growing conditions so that they are not stressed
  • mechanically--prune out the problem
  • biologically--predators and parasites can play a major role in managing some insect pests
  • chemically--pesticides

There are some definite steps to follow for an accurate diagnosis. The first step is to identify the plant. Knowing the plant allows you to determine whether it is planted in an appropriate site and if it is being cared for correctly. This assessment is critical as a majority of the plant problems I diagnose are abiotic. This is often a result of "wrong plant in the wrong place" or just a poor planting job.

It's also important to know what is normal for a particular plant. You don't want to spend valuable time looking for a problem that doesn't exist. Knowing the plant also helps with this next step--identifying key pests. Every plant is host to a unique group of insects and diseases. Some are threatening to the life of the plant; others are simply part of the food web. If you aren't familiar with the plant and have no idea what its cultural requirements are or what pests can affect it, you'll need to do a little research.

Things you need to know about the site

  • Is the site sloped? If so, how steep?
  • Does water puddle on the site, or does it drain? Remember that some plants, like yews, can't handle "wet feet." Others, like winterberry holly, can grow in saturated soil.
  • Find out the soil texture and pH. Some plants do well in heavy clay; others thrive in sandy or loamy soils. It's important to know the soil type for the plant you are diagnosing. The same is true for pH. Most plants do best in slightly acidic soil with a range 6.0 to 7.0. Some plants, however, such as azaleas, need a lower pH to thrive.
  • Check for soil compaction. Compaction from foot traffic or equipment can keep vital air and water from reaching a plant's root system.
  • Is there irrigation on site? If so, where is it located? Is it operating properly?
  • Does the available light meet the plant's needs? Is the site windy?
  • Lastly, is the plant near a building, sidewalk or road near salt or excessive heat?

Next, find out how the plant was transplanted and cared for. This requires talking with the client to get as much information as possible. Be a good listener and use open-ended questions such as "How do you usually water your plants?" Find out when the plant was transplanted. If it is a recent planting, spend some time on this step. Explore how it was planted. Was it a container, or balled and burlap? How deep and wide was the planting hole?

Now get a little history about how this plant has been managed. Has any fertilizer or pesticide been applied? Many homeowners assume chemicals are needed to grow plants and misapply. Also, take note of excessive mulch or bad pruning cuts. Has any construction gone on near the plant? Trenching for gas lines, changing the grade by adding top soil, running heavy equipment over the root system can all have a negative impact.

Observations for a correct diagnosis

Armed with knowledge about the site and the plant's care, you are ready to inspect the plant. You'll want to look at the entire plant, not just a specific branch. Where is the damage? Is it on just one part of the plant or is it spread throughout? Does it start at the outer tips and work inward? Or does it start inside and work outward? If the entire plant or a major part of it is affected, the cause is likely further down on the main trunk or in the root system. This could be insect or disease related, but often is caused by a planting or plant care problem.

Planting too deep, a girdling root, too much mulch - all these affect plant health. If only one or two branches on your client's plant are affected, trace the damage to the point where the branch is healthy. If there is a disease or insect involved, the intersection between healthy and infected tissue is where you would most likely find it. Damage patterns can be a clue to whether the problem has a biotic or abiotic cause. If more than one species is affected, it is most likely an abiotic cause such as weather.

Next, take a close look at the plant for signs and symptoms of a problem. A sign is the physical presence of an organism. This might be the offending insect, or frass or other parts it left behind. Or in the case of disease, it could be fungal fruiting structures. A symptom is the response of the plant to the damage; examples include dieback, wilting, or discoloration. Keep in mind that just because an insect is present doesn't mean that it is a pest. In fact, less than 1% of all insect species are considered pests. The rest perform vital ecosystem services as predators, pollinators or decomposers. Injury symptoms from insects, diseases and abiotic causes are sometimes very similar and difficult to distinguish.

Make sure you have a good sample:

  1. When possible and practical, the entire plant with root system is most helpful.
  2. When entire plant is not possible, select a sample showing the symptoms.
  3. The sample should be placed in plastic bag with a dry paper towel.

If you can't reach a diagnosis, contact the Penn State Extension office in your county. If the problem can't be diagnosed locally, the sample can be sent to the Penn State Plant Disease Clinic for further analysis.


Many plant problems encountered are common, but each situation is unique. Such as the location, or your client and the expectations each has for plant aesthetics. So evaluate the damage--is the problem(s) a threat to plant survival, or is it just appearance? If the plant's health is at risk, then you need to implement a management tactic immediately. If it is just a cosmetic issue, then maybe nothing has to be done other than educating your client.

If you reach the point where you need to take action, consider all of your options. Can you correct or stop the problem by changing any cultural practices such as re-planting, changes in irrigation, altering the soil pH, correcting pruning mistakes? What about beneficial insects? Some plant feeding insects have a complex of predators and parasites that feed on them. Many times they play a role in decreasing the population, and thus the amount of plant damage.

When cultural or biological options do not correct the problem, chemical pesticides may be a viable option. There are some very basic, yet important, steps to follow. Read the label. It is a legal document. Wear the personal protective equipment as described on the label. Use the least toxic pesticide that will be effective. Target or spot spray--treat only the area or plant affected. Finally, spray only at the proper time for the pest. Spraying too early or late does not provide optimum control.