Business of PHC, Part 5: The Science – Host Species and the Things That Affect Them

Scouting is the essential first step of an IPM program. Shown here is Tammy Kovar, owner of Biological Tree Services, a nine-year TCIA member company based in Sarasota, Florida. All photos and graphics courtesy of the author.

Plant health care (PHC) is the science and practice of understanding and overcoming the succession of biotic and abiotic factors limiting plants from achieving their full genetic potential in our landscapes and urban forests. Plant health care has been practiced as long as modern arboriculture itself and, as a science-based concept, is an important component in overall integrated pest management (IPM).

Pest management in our urban forests is a moving target and sometimes is overwhelming, especially for those early-
career professionals. I remember from my early days in the field the overwhelming thought of needing to know every pest for every tree! I literally had a truckload (back seat of a king cab) of university publications, bulletins and articles ripped out of magazines for reference in the event I couldn’t figure it out quickly and on site. Just that fear of not knowing was often very stressful. Well, that has changed significantly.

Business of PHC Series at a Glance

This is the third article in a planned 12-part series called Business of PHC that will run in TCI over the next year, focusing on what a smaller company needs to know to launch a plant-health-care program and start offering PHC services. The various aspects of this lucrative profit center that we have covered or plan to cover include:

  1. PHC – It Could Be the Shot in the Arm Your Company Needs” [TCI, April 2021]
  2. Elements of a Plant-Health-Care Business Plan” [TCI, May 2021]
  3. How to Equip Your Business Without Breaking the Bank [TCI, June 2021]
  4. What people will you need? [TCI, July 2021]
  5. The science: Host species and the things that affect them. Get to know the trees in your area and their problems. Understand treatment selection.
  6. Diagnosing pest/abiotic problems [Scheduled for September]
  7. Simple soil science/use of soil amendments [Scheduled for October]
  8. Structural pruning [Scheduled for November]
  9. Licensing and regulatory requirements [Scheduled for December]
  10. Marketing/selling PHC contracts [Scheduled for January 2022]
  11.  Scheduling/fulfilling PHC contracts [Scheduled for February 2022]
  12. PHC resources – TCIA PHC Technician, soil-testing labs, pest-diagnostic services, etc. [Scheduled for March 2022]

The point is that first, you don’t have to know everything, and second, resources now are easily and readily available. Today, the smartphone and computing opportunities available on mobile platforms, apps such as the Purdue Tree Doctor and other web-based apps have improved diagnostics significantly, making it simpler for the technician to get a better idea of their pest issue and easier to find a control strategy.

Adult emerald ash borers typically take flight about the same time that black locust trees bloom, indicating a good time for treatment.

One of the basic and most important keys when starting a PHC program is just learning to recognize the concerns for common trees already in your care. Tree identification is critically important to determine whether the tree is even a host for any given disease or insect. As a technician, you don’t need to know every tree in North America; just focus on those that are commonly found and that you are called upon to investigate for pest issues with your clients and customers. Few things are as awkward as misidentification of a tree and the corresponding application. Recommendations for treating emerald ash borer on a European mountain ash (Sorbus aucuparia), which is not susceptible to EAB infestation, could be fairly damaging to your credibility and your company!

Get some help

All there is to know and what you need to know can be mind-boggling; however, it is more manageable when we are able to discover the resources available. Often overlooked, local extension services from state land-grant colleges provide a tremendous collection of experts trained and educated in pest management. They often have plant and pest diagnostic laboratories assisting with identification of those challenging diseases or insects, usually at a very economical cost, along with the appropriate management strategy to apply for control.

Each state has a land-grant college with resources to assist PHC technicians.

The next level in PHC tactics is recognizing other available science-based tools. These can include analyzing common host-pest interactions and applying concepts like phenology and growing degree days (GDD) in the diagnostic process. This helps make better-informed decisions on questions such as, “When is the optimal time to apply insecticides for control of bagworms or EAB?,” “When should I expect an outbreak of scale insects on the shade trees at the commercial park?” or “When is the best time to control mites downtown?” These are common questions that can be perplexing at times for any PHC professional.

A good way to start the diagnostic process is by recognizing plant- and pest-development thresholds. This can help deliver the most effective and timely approaches to managing pests. Pests and plants are dependent on temperature to develop. They can develop and emerge earlier in warmer years than in years with cooler temperatures. Also, plants progress and bloom earlier in those “early-spring” years. Monitoring plant phenology such as bloom times can be useful as a biological calendar to track growing-degree-day accumulation and to predict pest activity.

Look for apps that can provide quick and reliable help in identification and control.

GDD, while not perfect, is a more reliable method of predicting plant and insect development than relying on a calendar for when to expect outbreaks, and for timing the most vulnerable stage of the insect’s life for optimal control with lower amounts of active ingredients. Scouting for an insect outbreak may begin too early or too late if using just the calendar itself, resulting in wasted time and missed damage on trees and shrubs, or the most effective control windows for pests.

Phenology may be defined as the study of naturally recurring events in plant and insect lifecycles, such as bud expansion and bloom times, or when scale crawlers appear on plants. Also, it’s good to know how seasonal variations in weather, especially temperatures, affect the timing of those events. GDD is a tool in phenology measuring heat accumulation to estimate growth stages of plants and life stages of insect and insect-like pests.

Growing-degree-day tracking and calculations can be a challenge. There are at least a couple of ways to determine this temperature-based function, however, there are tools available that compute this information and provide the necessary metrics daily.

Phenological-episode tracking is tedious and time consuming as well, but quite effective in combination with these other tools for scheduling applications. Regardless of the time of year, bloom sequence is typically consistent with pest emergence, and is a reliable tool.

The information in phenology models originates from published literature, usually from nearby universities or pesticide manufacturers and distributors, and is verified by pathologists and entomologists. Also, a PHC technician can assemble their own pest-calendar model to include as part of their exclusive IPM resources. However, confidence in any model should be field tested in your area. This simply involves comparing the predicted generation time (degree days) for an insect using a phenology model and comparing it to the actual degree days for a generation using some sampling method, such as a pheromone trap.

GDD coupled with using indicator plants from the phenology charts can be a useful decision-making guide for applications. These examples are for north-central Indiana, and will vary with your area.

Pheromone traps use a scent produced by the female insect to lure their male counterparts. These traps assist in determining the mating activity of the insect pest by counting insects trapped over a specific time, enabling a better decision regarding application scheduling to target specific pest populations.

By observing the same events and comparing with model predictions over several years, it’s possible to develop a fine-tuned model for your client base. The model increases in accuracy and rigor in your pest-management program for each year of data collection.

Putting your tools to work

Now, let’s put some of these tools to work. First, it is necessary to find a reliable source for finding GDD for the target area. There are several websites available for obtaining this data, which include:


GDD Tracker

ISU Mesonet

Of course, there is an “app” for that! There are various smartphone applications that can provide GDD instantly and accurately on several web-based platforms.

Once the GDD has been determined, pest stage of development and plant phenology can be used to determine pest appearance and vulnerability for best pesticide-application times. GDD coupled with using indicator plants from the phenology charts can be a useful decision-making guide for applications. These examples are for north-central Indiana, and will vary with your area.

Timing is everything, and pest management is no exception! The application of GDDs and phenological events is one of the best strategies for scouting. Knowing when to expect the emergence of pests in your landscape and for timing pesticide treatments will help ensure effectiveness and minimize the many, many possibilities. This process can help take the guesswork and stress out of your scouting and application timing, and can create more effective plant-health programs.


So far, the focus has been on insect pests, which can be perceived as a bit less of a challenge than identifying and controlling disease problems. One of the major challenges with diseases is that often they have arrived and infected the plant long before any symptoms are expressed by the tree or shrub. The damage and the disease are already in progress and taking a firm hold of their host!

It’s important to recognize, as well, that some plant-health issues encountered in trees are merely an aesthetic concern, not always lethal to the plant, and don’t warrant any measures for control. Just like insect-pest identification, know the tree and the host to be capable of conducting an effective differential diagnosis, which is the process of differentiating between two or more conditions that share similar signs or symptoms.

It is important to distinguish between biotic and abiotic plant issues.

Similarly, timing is an important factor in when to expect certain diseases relative to spring and summer infectious agents. Some diseases are considered cool-weather problems and can be ruled out in the warmer summer months, such as anthracnose. This is the common name for a type of leaf-spot and canker disease that affects many trees, including maple, sycamore, birch, elm and dogwood. Symptoms are especially severe in years with cool, wet spring weather, whereas, for example, bacterial leaf scorch, a serious vascular disease on oaks, is typically a summer concern.

Understanding how some of these science-based tools work and assist us in our quest is quite an important asset for PHC professionals. Uncovering the effect of PHC products – such as fungicides, insecticides, fertilizer, biorationals, organics, etc. – well, that is another. Specifically, determining how such products impact a plant’s ability to survive and thrive long term is a key area of knowledge necessary in the PHC business. This is because the stronger and healthier the plant, the better its ability to withstand increasing environmental stresses to grow and survive.

If a plant is stressed by diseases, insects or the growing environment, its energy is diverted from growing naturally to simply survival mode. We need to recognize this when this happens and know what we can do as specialists to overcome the adverse effects of these external stressors. Knowing your tree is the basis of PHC. Understanding the growing environment has equal significance. Sunlight. Air. Water. Soil. Nutrition. These fundamental components are brought together through a complex web of plant processes to create a sustainable, healthy tree with an effective defense system.

When tree defenses against disease are compromised, the disease can gain an advantage in the tree, resulting in serious infection and sometimes tree death. Often, it is a matter of a compromised immune system in the plant that can predispose the tree to pest issues, especially diseases. Many environmental, or abiotic, factors can cause a tree to be stressed.

  • Environmental stressors, such as drought, planting too closely and damage to the stem or root system, are the most common inciting factors for increasing disease issues.
  • Trunk damage invites infection by creating points of entry for pathogens.
  • Root damage creates points of entry for pathogens and reduces the tree’s ability to transport water, which increases stress susceptibility to infection.
  • Drought, whether from lack of rainfall or from root damage, reduces the ability and the necessary energy resources for a tree to compartmentalize infections and prevent their spread through the tree.

Good cultural practices are the cornerstone of an integrated-pest-management approach. The PHC technician should be capable of identifying, analyzing and evaluating trees in the landscape for biotic and abiotic issues. The key is reduced stress as well as to avoid any mechanical damage to trees. These improvements will lessen the likelihood of disease problems with trees. Probably the greatest success stories for plant health care involve the number of insects and diseases managed by good cultural practices, above and below ground, with minimal impact to our environment.

Plant health care from an IPM approach is a holistic methodology for creating sustainable, healthy trees and landscapes. It requires not just that the applicator know the right solution or application but also the timing and response of the plant, as well as the impact of the applicator’s intervention to the environment.

It is equally essential to conduct any PHC business using best practices and management based on current research. Be sure the information provided to your client is accurate and well supported. The smartest person in the world doesn’t know all the answers; they just know where to find them. There are so many resources available for good science-based plant health care, and it’s a continual learning process, but it also is so much fun and so profitable!

Next up in this series, in the September 2021 issue of TCI, is abiotic issues and how to address them.

Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) registered consulting arborist (RCA) and an urban-forestry specialist in the Department of Forestry and Natural Resources at Purdue University in West Lafayette, Indiana.

Leave a Reply

Your email address will not be published. Required fields are marked *

Click to listen highlighted text!