Diagnosis. It’s something that we do every day, from figuring out why the toast didn’t pop up to what that letter from the IRS actually means. For our purposes in the arboricultural world, it’s best to think of diagnosis as a process; a process by which many “influencers” are examined to determine the specifics of what appears to be a disease.
There are diseases and then there are non-diseases… at least not in the classical sense. Better stated, both living (biotic) and non-living (abiotic) factors can cause a diseased condition in trees.
So, why is so much emphasis put on the precursors and definitions? Well, if we skip to the bottom line, there are consequences of the result/determination of the cause or causes. The result of the process will determine the best course of action for a particular tree—fungicidal treatment, improvement in growing conditions, neither, or both.
First steps, second steps…
The process of diagnosis has many steps. As such, it pays to take the time to work deliberately in a step-by-step approach rather than being in a hurry and quickly jumping to conclusions. Overall, the procedure is general to specific, and described below:
Macro-look at the surroundings: The location of a tree in the landscape is of primary concern. As I was beginning my career, a wise arborist explained the value of walking a neighborhood to gain a sense of place and context for a tree’s malady instead of walking straight up to the specimen. The influential factors to consider in this first step are wind patterns, slopes, history/age of the development and client’s property, evidence of changes in grade, human and vehicular traffic patterns and condition of surrounding trees.
Macro-look at the tree: A tree is a large, living organism; it has roots, a trunk, flowers, fruits, stems, bark, conductive vessels, heartwood, sapwood and many other tissues. In order for the tree to be healthy, all must work together. After the macro-look at the surroundings, an “eyes-wide-open” circular loop around the specimen is a good second step. Walking 360 degrees is helpful; part B of the macro-look at the tree is the vertical, up-and-down view. This is best done with the help of binoculars, which helps to bring the far away up close, usually allowing you to see symptoms that ordinarily would not be visible such as cracks, insect holes and peeling bark.
Focus on the soil and rooting area: Half of the tree’s tissues are underground…half! If diagnosticians only look at the leaves and stems, they would be ignoring potential sources of pathogens and abiotic problems. In addition, the soil structure and fertility level is paramount for stability and providing essential elements for photosynthesis. If the area surrounding the tree is unsuitable, keep this in mind as the process rolls along.
A focus on the rooting area begs the question, “how big is the rooting area?” The quick answer is 1,000 square feet. Is that a real answer for every tree? No, but an average tree in an average landscape ought to have at least 1,000 square feet of healthy soil to grow in.
Hone in on the trunk: The trunk is the portion of the tree responsible for half of the tree’s water movement and structural integrity. If this area is compromised due to pathogens or abiotic issues, it becomes a major limiting factor. Cracks, which are serious because they are physical separations of bark, cambium and sapwood, usually lead to pathogenic diseases, such as cankers, and, often, insect invaders.
Move up to the branches: Next on the list are the branches. Crossing limbs, broken limbs (hangers) and oozing tissues are worthy of note and possible action. Binoculars are a must when looking into the tree canopy at the branches.
Finish at the foliage: When “tree disease” is mentioned, the foliage is usually the first thought in most minds…it should be the last, at least to ensure consideration of each of the first five steps. Foliage is important for photosynthesis, transpiration, respiration and shade for the property owner. Every leaf is significant for these reasons; if something out of the ordinary is observed, such as a spot, split or dark blotch, further investigation is necessary.
Read more: Pine Problems
Normally reserved for managing tree diseases, the disease triangle can be helpful for diagnosing them. Really? Sure, if you look at all three sides – Environment, Host, Pathogen – each has unique characteristics that can help determine what may be causing the tree in question to decline…or at least narrow it down some.
Environment: The weather conditions and elements in the landscape can have a profound influence on the likelihood of tree disease. For example, cool and moist conditions are conducive for foliar pathogens such as sycamore anthracnose. Likewise, hot, dry and windy conditions are the common cause of abiotic problems such as leaf scorch, which often is confused with a fungal disease.
Host: Each tree species has a set of infectious diseases commonly associated with them. When considering the host, it’s absolutely essential that you back up a step and are sure of the identity of the host species. Once the identity is confirmed, it’s very helpful to start with the common problems of the host. For example, crabapples are commonly infected by fireblight, apple scab and cedar apple rust, while Austrian pines often are diagnosed with dothistroma needle blight and diplodia tip blight. Information on specific diseases of each species can be found at university extension offices as well as nearby arboretums and botanic gardens.
Pathogen: Various pathogen groups – i.e., the cankers, the leaf spots, galls, wilts, root rots, decay organisms – have a characteristic appearance. As you consider this part of the disease triangle it helps to be familiar with the general form and shape of each. One of the best ways to do so is to simply go to an image search engine such as Google Images or Yahoo Image Search and type in “tree cankers” or “heartwood decay symptoms.” Such a search will provide a basic “look” of the pathogen group.
Abiotic factors: Roughly 50 percent to 60 percent of all of the “tree diseases” I encounter are caused by nonliving organisms. This is not to say they are unimportant and shouldn’t be treated; rather, it just means that there is often more than meets the eye. When abiotic factors are present, they often stress a tree sufficiently to lower its resistance to biotic diseases such as mildew, anthracnose, decay and cankers.
A few of the more common abiotic diseases are:
Girdling roots: Often caused by planting too deeply or a failure to loosen tangled roots that were created by a plastic pot/B & B root ball, roots that cross each other or press on the trunk basal plate create a weak point, sometimes leading to root starvation, heartwood decay, borer invasion and overall tree decline.
Lack of a root flare: Another good sign that the tree was planted too deep, the lack of a root flare usually is manifested by the tree entering the ground with the same trunk diameter as the lower scaffold limbs. Deeply planted trees often struggle due to lower-than-necessary gas exchange function, as well.
Soil compaction: Heavily trafficked sites often produce a root zone where soil particles are pressed together so much that roots cannot move throughout easily. These root systems do not develop sufficiently to extract moisture and nutrients and/or provide structural support.
Trunk bark damage: Whether from lawn mowers, weed eaters or juvenile delinquents, bark damage is highly consequential. When this occurs, decay organisms gain an entrance point to the cambium and sapwood, leading to degradation and the interruption of nutrient/water flow.
Irrigation system defects: Plugged, broken or misaligned drip and overhead irrigation system components can wreak havoc with the moisture and oxygen content in the soil, keeping the tree roots too wet or dry.
Springtime freezing or severe winter temperatures: New growth or dormant buds often are killed or weakened by these weather events. Recovery often is impossible; replanting or restorative pruning may be necessary.
High soil salts: Indicated by a soil test, high levels of soluble salts stunt root expansion and therefore limit uptake of nutrients and moisture as well as potential structural stability.
Again, remember that tree disease diagnosis is a process. It can be difficult and frustrating at times. Training from state arborists associations, university extension and the International Society of Arboriculture help to ease the path to success.
Read more: Could it be a nutrient deficiency?