At TCI EXPO ’24 in Baltimore, Maryland, we conducted a Pre-Conference workshop entitled Plant Health Care Pathways. This tripartite workshop included sessions on honing diagnostic and management skills, tree nutrition and soil management and pruning by the ANSI 300 standard. In this two-part article, we summarize information included in the first segment, honing diagnostic and management skills.

Photo 1: Trees stressed by poor installation and lack of irrigation. All photos courtesy of the authors.

We introduced four foundational elements useful in diagnosing problems with trees and shrubs, represented by the acronym SPSD. The first “S” stands for “site.” Understanding the site in which woody plants are located is a critical consideration when conducting a diagnosis. The “P” stands for “plant.” Recognizing the species of each plant under consideration will help inform the diagnosis. The second “S” stands for “symptoms.” Knowledge of signs and damage symptoms is a powerful tool in making an accurate diagnosis and developing an effective intervention. “D” stands for “date,” and can be defined by an actual Gregorian date such as May 15, or by using a proxy such as a growing-degree-day value or phenological plant indicator that is linked to the seasonal development of pests and plants.

In Part 1 of this article, we’ll look at S, site, and P, plant.

Abiotic and biotic factors
Before taking a deep dive into SPSD, let’s review two broad categories of plant problems and their underlying mechanisms. The causes of plant problems can be grouped into abiotic or biotic factors. Abiotic problems are associated with non-living factors, including soil conditions, drought, water logging, heat, improper culture – including rates of fertilization and irrigation – physical injury, exposure to sunlight, temperature extremes and pollution. Biotic problems arise through interactions between plants and pests, living factors including insects, mites, pathogens, weeds, mammals, birds and, of course, humans. Experience suggests that the majority of plant problems are linked directly or indirectly to elements of the abiotic realm.

Both abiotic factors and biotic factors create stress for plants. Plant stress has been defined as an external constraint that limits resource acquisition, growth and/or reproduction (Grime 1989). These constraints often manifest themselves as easily observable changes in the appearance and growth of plants. Trees in urban forests often experience more frequent and intense stress than trees in natural forests. (Photo 1) Underlying causes include water deficits associated with impervious surfaces that deny trees water, the critical raw material necessary for photosynthesis.

Compacted soils in urban locations may compound problems allied to impervious surfaces. Urban soils are often degraded, deficient in nutrients and organic matter vital to tree health. Human inputs of air and soil pollutants, de-icing chemicals, pet wastes and excess nutrients related to fertilizers compound other sources of stress. Activities such as weed control and lawn mowing often result in increased incidence of mechanical injury.

Never-ending problems associated with improper installation, such as improper planting depth, and maintenance abominations, such as volcano mulch, also contribute to abiotic plant stress. (Photo 2) Abiotic plant stress predisposes plants to attack by several guilds of insects, including boring and sucking insects and pathogenic microbes that can be lethal.

Photo 2: Tree with volcano mulch, which contributes to abiotic plant stress.

Differentiating between abiotic and biotic causes of plant problems can be challenging, but here’s a useful tip. Biotic causes related to infectious pathogens and insects often exhibit an isolated or random distribution of symptoms. They often affect only one plant or species in a planting. Not all plants show symptoms, and symptoms spread over time. (Photo 3)

Abiotic causes often have a more uniform distribution, often affecting different species of plants in a location. The symptom usually does not progress. (see Photo 1) Excellent references for identifying and understanding abiotic problems include “Abiotic Disorders of Landscape Plants,” by Laurence R. Costello et. al. (2014), and “Abiotic Plant Disorders Symptoms, Signs and Solutions: A Diagnostic Guide to Problem Solving,” by Robert E. Schutzki and Bert Cregg (2015).

Photo 3: Arborvitae with defoliation by bagworms. Biotic causes related to infectious pathogens and insects often affect only one plant or species in a planting.

“S” stands for “site” – the template for plant problems
The site is the template for either normal, healthy plant growth or a life sentence for a plant plagued with periodic or chronic problems affecting growth, development and appearance. Understanding the environmental site conditions found in a client’s landscape can help diagnose plant problems. How often have we seen azaleas or pin-oak trees planted in alkaline soils suffering with chlorotic foliage, or yews failing in waterlogged soils near a downspout?

The location of a planting site in a landscape will have consequences for the susceptibility of the plant to pests and abiotic problems. We found that some sites in a landscape were associated with more pest problems than others. Certain sites were more likely than others to be infested year after year by pests or suffer abiotic problems. These locations are called key locations, and they should be monitored regularly.

For example, azaleas planted in sunny, exposed locations are much more likely to be infested by lace bugs than those planted in shady locations (Raupp 1984). Warm, sunny exposures on southern-facing slopes or near heat-reflecting foundations are unsuitable locations for plants like cotoneasters, azaleas and hollies. Plants in these sites were more frequently attacked by small insects like lace bugs and spider mites, and required more frequent interventions than plants in less-exposed locations. (Photo 4)

Thoughtful matching of plants with suitable planting sites can greatly reduce the long-term problems and labor and material costs associated with pest management.

Photo 4: Bed of discolored cotoneasters with lace bugs. Warm, sunny exposures on southern-facing slopes are unsuitable locations for plants like cotoneasters.

Historical data
Historical information provides valuable insights into diagnosing plant problems, especially for new clients in a plant-health-care (PHC) program. Some of this information will be gathered during visits to the site and other information may come from the client, hence the need for good communication skills by PHC technicians and salespersons. The following information can be useful in building a historical database for diagnosing plant problems.

• Genus, species and location of current and earlier plantings. Were some removed because of poor performance?
• Previous and current treatments – fertilizer applications, pH adjustments, soil amendments, applications of insecticides, miticides, fungicide and herbicides. Pruning cuts made to tree canopies.
• Previous environmental conditions, including unusual winter or summer temperatures and unusual soil-moisture conditions related to floods or droughts.
• Changes in grade that may have affected historical drainage patterns.
• Construction and maintenance that may have compacted soil or damaged trunks and branches above ground or roots underground.
• Other service providers that may be applying pesticides, fertilizers or soil amendments.

Clients also are an extremely valuable source of information helpful for diagnosing plant problems. During the aforementioned PHC workshop, we asked participants to list five questions they would ask a client who had contacted them about problems with a tree. Here is a short list of questions they would ask a client about an ailing tree.

• How long has the tree been in the landscape?
• When did you first notice the problem?
• How long has the tree had the problem?
• How important is this tree to you?
• Do you have other service providers on your property – lawn companies, mosquito or tick abatement?
• Do you irrigate or water your plants?
• Do you or a service provider fertilize or apply soil amendments to your plants?
• Has any construction happened recently on your property?
• Armed with this information about the site, you can begin to generate hypotheses about the root cause of a problem. However, the next step in the diagnosis is a critical one not to be overlooked.

Photo 5: Cherry tree with white prunicola scale.

“P” stands for “plant” – know your plants
Why is plant identification important? The following reasons apply.

First, all plants are not created equal. Some have a large complement of insects, mites and pathogens that attack them. Others may have virtually none or only a few pests. Knowing what the plant is will help narrow the list of possible pests it may have.

Some plants will have a disproportionate number of pest problems or a disproportionate value in the landscape. These plants are called key plants (Raupp et al. 1985). We found woody plants in the rose family to have disproportionately greater incidences of insect and disease problems than other plants in residential landscapes. (Photo 5) Historic trees or trees that are rare or have sentimental or monetary value also may be key plants in landscapes.

Second, different species and cultivars of plants tolerate different ranges of temperature, moisture, exposure and soil conditions. Abiotic problems related to these factors can be readily ruled out or ruled in by knowing what species or cultivar of plant you are diagnosing. A late cold snap in spring often causes discoloration and death of tender leaves and blossoms on magnolias in Maryland. Correctly identifying these plants as magnolias and knowing about their freeze intolerance will help lead you to the correct diagnosis. (Photo 6)

Third, most technical books and digital resources organize pests according to plants on which they feed. A quick Google search for “insect pests of azaleas” produced several websites with images and descriptions of lace bugs, spider mites, scales, caterpillars and borers found on azaleas and rhododendrons. In a similar way, reference books, including “Insects that Feed on Trees and Shrubs,” by Johnson and Lyon (1988), and “Managing Insects and Mites on Woody Plants: an IPM Approach,” by Davidson and Raupp (2014), have indices and tables listing pests found on genera and species of woody plants. Learning to identify plants is one of the most important skills for PHC technicians and salespersons.

Several apps also are available to assist with plant identification. These are easily installed on cellular phones or tablets for use in the field. Here are a few we use.

• PictureThis – Botanist in Your Pocket
• Seek by iNaturalist
• PlantNet Plant Identification
• Google Lens

Photo 6: Magnolia with cold damage.

More to come
In Part 2 of this article, planned for the November 2025 issue of TCI Magazine, we’ll look at “S” stands for “symptom,” in which we’ll zero in on causes of plant problems, and “D” is for “date,” looking at seasonal activity that also can be a diagnostic clue.

Michael J. Raupp, Ph.D., is professor emeritus at the University of Maryland in College Park, Maryland. His writing, research and scientific outreach have received a dozen national and international awards. He is a regular guest on television and radio. His most recent book, “26 Things that Bug Me,” introduces youngsters to the wonders of insects and natural history, while “Managing Insects and Mites on Woody Plants,” published by the Tree Care Industry Association, is a standard for the arboricultural industry. Visit his websites at www.bugoftheweek.com and https://www.youtube.com/user/.

Paula M. Shrewsbury, Ph.D., is a professor of entomology at the University of Maryland in College Park, Md., specializing in ornamental integrated pest management (IPM), sustainable agriculture and invasive-species management.

This article is based on their Pre-Conference workshop entitled “Plant Health Care Pathways” at TCI EXPO ’24 in Baltimore, Md.

Resources and references
Costello, L. R.; Perry, E. J.; Matheny, N. P.; Henry, J. M.; and Geisel, P. M. 2014. Abiotic disorders of landscape plants. University of California Agriculture and Natural Resources.
Davidson, J. A., and Raupp, M. J. 2014. Managing Insect and Mite Pests on Woody Plants: An IPM Approach. 2014. Third Edition. Revision. Tree Care Industry Association.
Grime, J. P. 1989. The stress debate: Symptom of impending synthesis? Biological Journal of the Linnean Society.
Johnson, W. T., and Lyon, H. H. 1988. Insects that feed on trees and shrubs. Cornell University Press.
Koehler, C. S. 1987. Symptomatology in the instruction of landscape ornamentals entomology. Journal of Arboriculture.
Raupp, M. J.; Davidson, J. A.; Holmes, J. J.; and Hellman, J. L. 1985. The concept of key plants in integrated pest management for landscapes. Arboriculture and Urban Forestry.
Raupp, M. J. 1984. Effects of exposure to sun on the frequency of attack by the azalea lace bug. Journal of the American Rhododendron Society.
Schutzki, R. E., and Cregg, B. 2007. Abiotic plant disorders symptoms, signs and solutions: A diagnostic guide to problem solving. Michigan State University Press.