A good book or article about foliar tree diseases should read like a press release—outlining the details in terms of who, what, when, where, why and how.

With that being said, some of you older readers (or vintage TV series fans) might remember the series “Dragnet,” where Sgt. Joe Friday frequently told crime scene witnesses “All we want are the facts, ma’am,” as he carried out his investigations. Whichever example conveys the message of a thorough consideration of the pertinent specifics of biotic and abiotic maladies of tree leaves works for you, use it. Doing so will help prevent overlooking valuable information that could lead to better management of valuable specimens.

Browned leaf margins, also known as leaf scorch, are often caused by a disruption of water flow from roots to shoots.


Regarding the “who” of foliage diseases, actually, there are two. One, the property owners, who need to understand the importance of the tree, its value and benefits and to prioritize the tree in question compared with others on the property. Since the tree can’t speak for itself, the owner has to have its best interest in mind, to consider all of the factors involved with a treat/no treat situation.

The second “who” is the tree worker or arborist assigned to manage the account, the person who will be communicating with the owner about the tree’s health. These employees need to be well-trained in tree identification, malady diagnosis and treatment options in order to fully fill the bill in terms of the who.

What is it?

The “what” is rather straightforward—they are the classic pathogens, such as sycamore anthracnose, apple scab, diplodia tip blight, fireblight and the abiotic conditions of sun scorch and freeze injury. Each has distinct symptoms that are expressed after infection or the exposure to the insulting input.

The biotic diseases have a unique, four-stage life cycle. Knowing the most vulnerable life stage is crucial to preventing or curing the problem, with most falling into the prevention category.

Interior issues can cause exterior foliage problems.

For abiotic maladies, the most effective strategies are geared toward removing or altering the influence(s) that caused the injury to occur.

In order to become more familiar with the possible foliar diseases, it’s wise to enroll in an International Society of Arboriculture certification course, where you’ll be able to encounter first-hand each of the most common maladies in your area.

Most of these courses involve classroom study, as well as field trips and “walk and talk” pest identification sessions where the classic signs and symptoms are highlighted.

In addition to seminars and training sessions, several apps and online resources are available from university extension specialists and educators and industry manufacturers that can be very helpful in this regard.

What does it do?

Another aspect of “what” is what the disease does to the tree and the property. First and foremost, tree diseases affect tree health. The leaf lesions prevent leaves from photosynthesizing at their full potential, which weakens the tree. When severe infections cause leaf drop, there’s greater loss of photosynthetic potential. Additionally, in some cases where re-foliation occurs after initial leaf loss, a significant amount of carbohydrate reserve is used to create the new leaves, decreasing the reserves the tree can utilize to resist pests.

Unfortunately, trees are often planted without sufficient room for proper root development.

Along with the loss of vigor in the tree specimen, the loss of leaves causes a reduced aesthetic appearance. In other words, it looks ugly. A tree that is full of healthy green leaves simply looks better than one that is thin and lifeless.

A third consideration is the creation of leaf debris. Dropping leaves make a mess, generating worry in the mind of the customer and producing a less desirable, less valuable landscape – especially if the owner were interested in selling the property. Realtors tend to be much more interested in showing off a great house and healthy landscape than one with ailing plants.


The mantra of real estate may be “location, location, location,” but for foliage diseases it’s “timing, timing, timing.” When working to control pathogens, it’s important to consider that very few, if any, fungicide applications made after a full-blown disease outbreak will be effective. Most, if not all, are effective as preventive measures. Bottom line – in terms of when, the thinking should be before, not after.

Another aspect of “when” is to consider the actual pest, tracking with the historic outbreaks in the region. While many diseases, such as cedar apple rust and Diplodia (Sphaeropsis) tip blight, begin infecting a tree in spring, others are connected with summer and fall. Still others, such as sycamore anthracnose, are variable, and able to infect whenever conditions are favorable. It’s common to encounter a year with spring, summer and even fall outbreaks of anthracnose. Knowing the timing of previous disease manifestations can be helpful when communicating with customers about their needs for disease control.

The nonliving maladies, aka abiotic diseases, can occur at any time, and often do. These developments are often related to certain seasonal incidences, such as lawn herbicide applications, graduation parties and construction events. The key to success with these is to try to link them to the timing of the occurrence that led to the symptoms. For example, if a red oak begins to wilt and exhibit browned leaf edges, look around – if a new driveway was installed on the neighbor’s property, a significant number of roots may have been cut and caused a substantial reduction in water uptake.


This one might seem overly obvious at first glance. It’s… uh… on the leaves. But there’s usually more to it than that.


There are lower leaves, upper leaves, the crown, the periphery and interior leaves. As well, some abiotic maladies express themselves on one side of a tree, but not on the other. Getting specific on the location (again, the real estate analogy) is crucial to the overall success of managing foliage diseases, especially in diagnosis; determining the actual causal factor is often greatly enhanced by noting the specific location of the insult.

Why does it happen?

A focus on “why” should start with the classic disease triangle, comprised of a susceptible host, favorable environmental conditions and pathogen/biotic or nonliving/abiotic cause. In order for a disease to occur, all three must be conducive for disease development. A good example is with the apple scab disease of crabapple. There are many, many cultivars of crabapple, with some of them quite inherently genetically resistant to the fungus, others moderately susceptible and still others very susceptible. If a resistant crabapple cultivar is growing in a landscape, even though the environmental conditions that favor its development and the actual pathogen itself are present, very little actual infection will occur.

On the abiotic side, while they can occur just about any time, more often than not, hot and dry conditions tend to cause their symptoms to be evident. The effects of soil compaction, root disturbance, excessive or minimal mulching, excessive fertilization, overwatering, deep/shallow planting and stem-girdling roots tend to be more pronounced when temperatures are high and available moisture is low.

Cedar hawthorn rust symptoms in full force.

How to control

Control of biotic and abiotic diseases is a multifactor endeavor. There are many preventive and curative methods that can be utilized. Narrowing the scope a bit to foliage diseases of woody plants, three approaches stand above the rest:

  • First, the considerations of right plant, right place should be implemented to promote tree health. Planting with sun exposure, wind patterns and potential root zone in mind go a long way toward preventing a variety of maladies, including foliage diseases. Full sun conditions for large trees, slight to moderate shade for understory trees, proper air flow and providing sufficient room for the eventual root zone of various tree sizes are essential planting design considerations.
  • Next, regular scouting and monitoring programs are important in spotting disease outbreaks before they get out of control. If noted early enough in the disease expression cycle, many symptoms can be minimized through judicious fungicide applications. Scouting (thorough examination of all symptoms of trees on a property) and monitoring (a set of regular inspections made periodically throughout the growing season) are important for early detection and prediction of future maladies. If proper notes are made, a historical record can be created as a reference and useful tool in future disease management.
  • The third is the combined use of the first two to offer judicious fungicide application services to clients. In doing so, it’s important to base the recommendations on the relative priority of the tree to the property (shade value, historical/sentimental value, tree health), cost to the customer and potential for efficacy of disease control.

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