Disease Detection: Steps to Diagnosing Plant Diseases and Disorders

As a kid, you might have dreamed of someday becoming a police detective, private investigator or even an archeologist exploring why dinosaurs perished from the earth. Life leads us on various paths, but some of us are fortunate enough to investigate alien lifeforms or mysterious happenings in the vast urban jungle (sounds a bit dramatic, I know).

Investigating plant problems can be challenging at times, especially if you do not have enough information, such as the history of the property or plant to help solve the problem. Observations may be your only option for gathering facts on site. Working off of a checklist will help keep you organized and ensure you don’t overlook a symptom or sign while making observations.  It can be easy to jump to conclusions after gathering preliminary information, but resist the urge to make your determination too quickly and take the time to dig a little deeper. Here are a few items that should be on your checklist when diagnosing a plant disease or disorder:

  1. Plant Identification. Identify the plant and determine what is ‘normal’ for that plant. Factor in the time of year and how seasonality may influence the symptoms you are observing.
  2. Describe the Symptoms. Do you see chlorosis, drying leaves, wilting, limbs turning purple, etc?. Take note of any other out-of-the-ordinary items that you observe  (e.g. oozing, premature leaf drop in the fall, late leaf-out in the spring). Always take plenty of pictures to help you revisit the site once researching the symptoms on your computer. It also helps to create a record of the progression of decline or improved health if treatments are to be made in the future.
  3. Define the Area of Impact. Are the symptoms seen on one species or multiple species? Is the concern property-wide or on random plant(s) on the property?  Did symptoms develop overnight or a slow progression? If problem is limited to a single plant or species, where is the plant impacted? Is it the entire plant or localized to one limb? Is it concentrated to one side, and which side? Is the top of the tree only showing symptoms? Pictures of the plant along with a comparison to adjacent plants of the same species can also be helpful during your investigation.
  4. Make Area and Property Observations. Inspect nearby walls, windows, water features and other property features.  This will help rule out abiotic disorders such as scorch from light or heat reflection of a building or chemical burn from splashing water on foliage.  Also, look over the fence. Is the adjacent property as clean as an operating table, as in ‘sterile’?  Tree roots don’t stop at property boundaries and at times, inappropriate herbicides may have been used next door. Be careful on this one: you might find yourself in the middle of a neighbor dispute and tort case!
  5. Examine Property History. Have there been recent changes to the area, such as construction in the last 3-5 years or recent fertilizer or pesticide application?  Has there been work on the building, road or parking lot?  I have seen road tarring singe 50% of the canopies of the street trees, symptoms which were only noticed a week after the work had been completed.
  6. Consider Weather/ Climate Impacts. Were there recent weather changes or other environmental changes that might have altered the growing conditions?  Sometimes it is helpful to draw out the past month of weather patterns and match up the progression of the plant problem to rule out weather as a possible cause.
  7. Evaluate Irrigation System. Is the irrigation system operating and delivering water as it should? Have you inspected the soil? Again, have you inspected the soil? In our region, irrigation is so very critical to our plants, it should be one of the first items you rule out on your checklist.

If after ruling out abiotic factors you are still stumped on what is plaguing your plant(s), start exploring biotic or biological-caused diseases and disorders. In the Desert Southwest, we are fortunate to not have many diseases that impact our plants due to our arid conditions. This makes it a bit easier to narrow down what pathogens could be damaging your plants. Doing a quick online search on your local cooperative extension’s website may also help offer ideas of which diseases can impact a specific plant species.

Physical evidence or a ‘sign’ of a disease can be very helpful in terms of identification. Signs, such as a fungal fruiting body or a conk at the base of tree, white powdery substance on leaf surfaces or even a spore mat that develops on the soil after a rain is very helpful to identify a pathogen. These signs are also helpful for pathology labs to make positive identifications.

When should you seek help identifying a disease? There are a handful of diseases prevalent in the Desert Southwest that can be diagnosed in the field (e.g. powdery mildew on foliage in the springtime, a Ganoderma conk sitting on a buttress root of a cottonwood and even the tell-tell symptoms of Texas Root Rot on elm trees, the sudden onset of crispy brown leaves remaining firmly attached to the limbs around monsoon season).  But even with these common diseases, enlisting the assistance of a pathology lab can provide you and your client with firm evidence that a pathogen is or is not responsible for plant decline.

Sampling plant tissue should be done carefully in order to preserve the specimen and make sure that it gets to the lab in once piece. If limbs or above ground tissue are being tested, look for tissue pieces that have a margin of diseased tissue into healthy tissue. If you send a rotted, soggy root mass or a dried branch to the lab, they likely will have difficulty isolating a pathogen. If the plant has been decaying for some time, there will likely be additional organisms in the sample, which were not the primary culprits. If testing, make sure to send fresh samples, preferably overnight, early in the week so they arrive promptly to the lab. If you know you will be pulling a sample during the summer, pack an icepack in a cooler and if you are unable to send off quickly, put it in your refrigerator overnight and ship the next morning. Talk to your lab about packing procedures for the disease which you would like to test. Filling out the lab’s pre-diagnosis questionnaire will also provide the pathologist clues to a dieases. You also may be required to fill out a permit to move living plants, specimen or soil across state lines or into other counties. Discuss this with your pathology lab to confirm you have all the details that need to be on or in your shipment.

If you need results more quickly than a pathology lab can provide them, then you might find field testing a good tool to have available. Currently, there are a few reasonably priced field kits available to test for fungal diseases such as Phytophthora, Pythium and Rhizoctonia. There are also kits available for testing select viral and bacterial diseases.

Whichever method works in your operation, the best path to discover your answer is making observations and asking questions. The more information you can provide your lab, cooperative extension agent or horticulturist, the better chance you will have at making a positive identification of the plant disease or disorder which is wreaking havoc on your property.


Kasey Billingsley is currently the Educational Coordinator for the AZ Landscape Contractors’ Association where she oversees 3 certification programs and over 70 classes annually. She also is the Horticultural Consultant at Harmony Horticultural Consulting, assisting landscape contractors, tree care companies and homeowners with diagnosing plant problems, developing irrigation and plant health care programs and training needs. In her spare time, she runs half marathons and chases her two young boys around the park.