By Ryan Daugherty, Inyo-Mono Master Gardener Volunteer
This guide explores the common causes of leaf scorch and branch dieback in trees, emphasizing that these symptoms are usually signs of stress rather than specific diseases. It presents a diagnostic framework rooted in understanding resource flow disruptions—trees either lack essential resources, cannot access or transport them, or are consuming them faster than they can be replenished. Topics covered include poor drainage, drought, soil compaction, girdling, soil chemistry imbalances, extreme heat or light, winter desiccation, vascular diseases, pests, and herbicide exposure. The document encourages observation, environmental assessment, and prevention over exhaustive memorization of disorders.
Introduction
Every year, we receive questions about leaf scorch and branch dieback in trees. I thought it might be helpful to explore their most common causes.
Scorch refers to the browning of leaves, especially along the margins or between veins. Branch dieback occurs when a branch dies from the tip back to the primary stem. Much like a headache or body ache in healthcare, symptoms like scorch or dieback in arboriculture can stem from a dozen different causes—ranging from minor issues to fatal conditions. The advantage of these non-specific symptoms is that, while they may not be definitive, they are often more noticeable than the underlying conditions causing them. When considered alongside more specific signs and symptoms, scorch and dieback can help narrow down a diagnosis.
For brevity’s sake, these articles will focus more on the causes of these symptoms rather than remediation. However, I find that once we have a firm grasp of the problem and its cause, the remedy isn’t far behind. That said, if you still need advice, our friendly Master Gardener helpline is always just a phone call or email away. immg@ucanr.edu.
Potential Causes of Scorches and Dieback
When we see symptoms in a tree’s canopy, we often need to look down—not up. Poor root health can have several culprits, so let’s start there. Keep in mind that multiple factors affecting tree health can exist simultaneously. It may not be just one issue but a combination of several.
Poor Drainage
Quick soil science recap—don’t worry, this won’t be on a test! The mineral portion of soil consists of three particle types: sand, silt, and clay. The ratios of these define the soil’s texture. Over time, through cycles of wetting and drying, freezing and thawing, these particles arrange themselves into larger clusters called soil aggregates, which make up soil structure. If you’ve ever seen a TV show where small robots combine into a larger robot to fight the bad guy, you’ve got the gist of separates and aggregates.
The spaces between individual soil particles are called micropores, while the spaces between soil aggregates—being larger—are called macropores. Collectively, these spaces are referred to as pore space.
When it rains or we irrigate, water enters and fills these pore spaces. The water in micropores clings to soil particles through capillary action, resisting gravity and remaining available for root uptake later. Meanwhile, the water in macropores (gravitational water) drains deeper into the soil profile, allowing air to enter the macropores. This is crucial because oxygen is an often-overlooked but critical component of healthy soils and healthy roots. Root respiration (the process of breaking down sugars for energy) is aerobic, meaning it requires oxygen.
When soil drains poorly, water gets trapped, leaving no room for air. Anaerobic (low-oxygen), waterlogged soils often have a sour, boggy smell and may appear grayish or bluish. Tree roots in these conditions may grow toward the surface, where oxygen is more available. If anaerobic conditions persist, the tree’s roots switch from aerobic respiration to anaerobic fermentation. This process burns through the tree’s stored sugar reserves while yielding only a small amount of energy, ultimately starving the tree and leading to scorch and branch dieback. The big takeaway here is that good drainage is key. While soil texture (the ratio of sand, silt, and clay) determines how well soil holds water and cannot be changed, soil structure (how particles arrange into aggregates) affects drainage and can be managed.
Dry Soil
This may seem obvious, but as a general rule, plants need water to grow and survive. What may not be so obvious is that while we tend to take for granted that watering is a strictly quantitative decision–more vs. less– the biggest difference makers in the health of our landscapes as they relate to water are often more qualitative in nature– the how and why we water.
Compensatory root development is a very fancy name for a very unfancy concept: when one part of a plant’s root zone is in a favorable environment and another part is in a less favorable environment, the plant compensates by growing more roots in the better area. Makes sense, right? Since plants can’t move if they don’t like their conditions, they adapt by maximizing benefits and minimizing deficiencies.
We can encourage this natural response by watering deeply and infrequently. Since soil dries from the top down, watering just past the root zone and allowing the top layers to dry between waterings creates a favorable environment deeper in the soil while making the surface less hospitable. This induces trees to grow deeper, more extensive root systems, which helps them resist drought during hot, dry, or windy conditions—ultimately reducing scorch and branch dieback.
When trees suffer from drought stress, it’s often due to lack of irrigation or an irrigation system that hasn’t been adjusted as the tree grows. This issue is especially common with drip irrigation that people forget to expand outward as the tree grows.
As a tree matures, its irrigation zone should follow the root spread. The feeder roots of a tree typically extend as far as the canopy, while the roots nearest the trunk become more structural and less efficient at absorbing water. If irrigation remains concentrated near the trunk, it fails to reach the active root zone, leading to dry soil and tree stress.
Most often when we see soil dryness as a problem it is usually because irrigation isn't being provided or, (usually drip) has not been adjusted as the tree has grown. As the tree gets bigger the irrigation system needs to get bigger with it, expanding to follow the roots. As much as the canopy of the tree spreads, that's about how far your feeder roots spread underground, with the roots closest to the trunk becoming more structural and less absorbent as the tree matures.
We also see soil dryness being a problem with weed barriers. The plastic barrier stops water from getting to those roots and cloth barriers that are marketed as ‘permeable’ or ‘breathable’ will clog over time and cause the same problem. A mulch will provide a passive weed control and help prevent water evaporating from the soil. Just avoid the dreaded mulch volcano! Dont mound up mulch against the tree truck; it can keep moisture and pathogens in contact with the tree's surface and invite infection. Mulch volcanoes can also serve as handy cover for pests to gnaw on the tree. If a weed barrier has already been installed, periodically cut it back to follow the tree’s expanding root zone, ensuring water continues to reach the soil beneath.
It's worth noting that the water needs of a tree change throughout its lifetime and vary from species to species. Newly planted trees can lose up to 90% of their roots during transplant and need more frequent irrigation, while a healthy, mature, and established tree may only need to be watered in certain stressful conditions like heat or drought.
Soil Compaction
Growing trees in compacted soil is like trying to tap dance in flip-flops—frustrating and unproductive. Compacted soil creates a double threat: it combines the anaerobic conditions of poor drainage with the water stress of excessive dryness. In urban settings, soil compaction is one of the leading causes of tree decline. It destroys soil structure, reduces pore space, and prevents air and water from moving properly into and throughout the soil—ultimately suffocating roots. Additionally, compacted soils are denser and harder, making it difficult for roots to spread out and develop naturally.
Remediating soil compaction typically involves tillage or aeration in some form. However, most methods for breaking up compacted soil are easier to implement before trees are planted. This is because the majority of a tree’s feeder roots are located within the top 12 inches of soil. Because of this, any attempt to alleviate compaction must carefully balance the potential benefits against the risk of damaging existing roots.
It's also important to note that simply breaking up compacted soil isn’t enough–without additional steps, the soil will quickly become compacted again. Incorporating organic matter into the soil can help maintain decompaction and improve soil structure over time.
One method for improving soil conditions around existing trees is vertical mulching. This involves drilling 1- to 3-foot-deep holes around the root zone using an auger, then backfilling the holes with compost, loose native soil, or a combination of both. The goal is to provide a looser, aerated rooting environment for the tree.
However, the effectiveness of vertical mulching is variable. Its success depends on factors such as: The number of holes drilled, the depth of the holes, and the percentage of the root zone being amended. Additionally, since drilling has the potential to damage roots, the same cost-benefit considerations that apply to tillage should be weighed before proceeding.
A more advanced method for addressing soil compaction is radial trenching, which involves excavating the soil around roots using an air spade. Unlike traditional digging, air excavation more or less preserves the root system, minimizing damage. Once the trenches are dug, the excavated areas can be backfilled with aerated native soil, improving overall root health.
Radial trenching is best performed by professionals and is likely beyond the scope of most homeowners. However, in severe cases of soil compaction, or in the case that the tree is special to you, this method may be a worthwhile investment for long-term tree health.
Girdling
To fully understand girdling, it helps to know a little about the vascular system of a tree. The transport of water, nutrients, and sugars happens through two specialized tissues called the xylem and phloem. Xylem, which is what we commonly think of as wood, is responsible for moving water and dissolved nutrients—collectively known as sap—from the roots to the leaves. Phloem, located just beneath the bark, actively moves sugars produced through photosynthesis to various parts of the tree, supplying energy for growth and storage.
Girdling occurs when something restricts, compresses, or destroys this vascular system, preventing the movement of materials within the phloem and xylem. This can happen when stakes or guy wires are left on too long, when trees suffer mechanical damage from mowers or string trimmers, or when they experience sunburn or animal damage from chewing or scratching.
A particularly serious issue is girdling roots. This occurs when a root wraps around the trunk of the tree instead of growing outward. As both the tree and the root expand over time, they press against each other, eventually crushing the xylem and phloem. With the vascular system compromised, the tree struggles to transport water, nutrients, and sugars, leading to scorch, dieback, and a general decline in health.
Detecting girdling roots can be tricky since they often develop underground. However, there are signs to watch for. If a tree leans or appears unstable without an obvious cause like strong winds or soil erosion, a girdling root could be preventing proper anchoring. Asymmetrical growth is another red flag, especially if one side of the canopy appears weaker or less developed than the other. In severe cases, the tree may fail to develop a proper root flare, entering the ground straight like a telephone pole instead of widening at the base. This can indicate that the tree was either planted too deeply or settled into the soil over time, preventing roots from spreading outward. Instead, they remain confined to the original planting hole, increasing the risk of girdling.
Once a tree develops a girdling root, remediation is difficult. If caught early, the affected portion of the root can be removed. However, if the root has already grafted to the trunk or has become too large, removal could cause significant stress or destabilization. In some cases, the damage is irreversible.
Since girdling is often difficult or impossible to fix once it occurs, prevention is the best approach. When selecting nursery stock, it’s important to inspect the root system and avoid trees with circling roots. If minor circling is present, pruning these roots before planting can help prevent future problems. During planting, ensuring that the sides of the hole are not smooth or compacted will encourage roots to spread outward instead of circling within the hole. If the walls of the planting hole appear glazed from the back of your shovel, roughing them up with a shovel or garden knife can improve root penetration. Proper planting depth is also important– the root flare should always be visible above the ground, rather than buried.
After planting, trees should be monitored to ensure that any support stakes are removed after one growing season. If stakes are left on too long, they can constrict growth and lead to girdling issues. Mulching around the tree and keeping turf away from the base can also reduce the likelihood of mechanical damage from mowers and trimmers, which are common causes of girdling injuries.
By understanding how girdling occurs and taking steps to prevent it, many cases of tree decline due to vascular restriction can be avoided.
Soil Chemistry
Healthy trees rely on more than just nutrients—they need the right soil conditions to absorb and utilize them. A lot of tree health problems that appear to be caused by disease, pests, or environmental stress can actually stem from imbalances in soil chemistry. Since trees absorb nutrients through their roots, those nutrients have to be dissolved in water to be accessible. The chemical composition of the soil influences which nutrients are available and how easily they can be taken up, and when imbalances occur, symptoms like leaf scorch and branch dieback can develop.
One of the most important factors affecting nutrient availability is soil pH. The pH scale can be a little confusing for some people, in between prom and awkward social interactions it can be hard to remember what you learned in high school chemistry class so let's recap: The pH (potential of hydrogen) scale is a scale used to measure how acidic or basic (alkaline) a substance or solution is. The measurements range from 0-14 with 0 being the most acidic, 14 being the most alkaline (basic), and 7 being neutral. It can be a little confusing thinking of it as a scale of acidity or a scale that starts in the middle, so I find it helpful to think of the pH scale as a measure of alkalinity– the higher it goes the more alkaline (basic). The pH scale is also logarithmic, meaning that every step on the pH scale is a power of ten. For example, going from a pH of 7 to a pH of 6 means the soil is 10 times more acidic, and going from pH 7 to pH 5 means it's 100 times more acidic. Soil pH is a big regulating factor in nutrient availability and a small change in soil pH can have a big impact.
Most nutrients are available around neutral pH 6-7. Higher pH, or alkaline soils, bind nutrients like iron, zinc, and magnesium into less soluble compounds, making them harder to dissolve in soil water and thus less available to the tree. In lower pH acidic soils nutrients can become too soluble and dissolve in soil water too easily. Trees can't regulate how much of any given nutrient they’re taking up– whatever is dissolved in the soil water in any concentration will be absorbed into the roots and have the potential to cause toxicity.
Another limiting factor for nutrient availability is what we call the ‘cation exchange capacity’ or CEC of the soil. CEC is the measure of how well soils hold onto and supply nutrients to plants. Soils high in clay or organic matter hold an overall negative charge that attracts the plant nutrients that have an overall positive charge (called cations). Soils low in organic matter or clay have a low CEC and struggle to hold on to nutrients because they tend to hold an overall positive charge and thus repel plant nutrients, like trying to stick the positive sides of two magnets together. If you're curious about your specific CEC for your soils, you will need a soil test but the general takeaway is that nutrients hold on to heavy clay soils, and leach out of light sandy soils.
Inyo and Mono counties have a diverse array of soils influenced by their complex geology, varying elevations, and wide range of climatic conditions. That means it's hard to pin down hard and fast rules about our soil, but I'll try to give some broad strokes. There is some variation but a common characteristic of our relatively young Eastern Sierra soils is that they tend to have a high pH (alkaline soils) due to calcium in the soil. While our soils have great buffering capacity against extreme shifts in pH, they can bind up key micronutrients like iron, manganese, magnesium, and zinc. Sandy loams like our Inyo series of soils tend to drain rapidly but leach nutrients, while in other areas our dense alluvial soils hold water longer but can be prone to compaction. Other areas like Tecopa, Shoshone, and parts of Chalfant Valley can have sodic soils (high sodium) which interfere with soil structure and nutrient uptake.
There are things we can do to improve our soils. Sulfur can lower the pH of our alkaline soils. (See this fact sheet on adjusting soil pH.) Gypsum can displace salt in the soil, and watering deeply can flush salts past plant roots. Applying slow release fertilizers used in conjunction with organic soil amendments like compost can help keep nutrients in the soil longer. I would avoid using synthetic fertilizers in areas with salty soils as the nutrients are salt based. In those situations, I would recommend an organic fertilizer.
Keep in mind that all of these solutions are temporary and will need upkeep. Results may also be marginal so it's best not to think that you have the power to ‘change’ native soil characteristics. That doesn't necessarily mean that you can't have your trees, it just means you may need to select appropriate species for your specific soil conditions, commit to a soil amendments/fertilizer routine, or grow dwarf varieties in containers for example.
Thankfully, most communities in our area (aside of the low desert) have fairly inert soils. If anything they are usually nutrient poor due to sandy texture.
Excessive Heat
There’s a saying among plant nerds that humans perspire, and plants transpire. Transpiration is how plants cool down, they release water vapor from their leaves. So, just like how you’d be dripping sweat on a hot dry day, those same conditions have plants cranking up their transpiration.
But here's the thing: On a really hot day, or during a heat wave, when transpiration demand exceeds the tree's ability to absorb and transport water, a water deficit is created, and eventually the tree becomes drought stressed. When the tree is in this stressed state, it will start a sort of self-pruning cycle, starting with cutting off water to the outer branches and to older leaves causing branch dieback and leaf scorch. If conditions like this persist the tree will continue in this pruning cycle until there isn't anything left, triggering defoliation, early dormancy, and eventual failure of the tree. Think of it as a long-term gamble. The tree is betting that if it can cut back enough, it can survive and outlast these less favorable conditions until those conditions improve and it can regrow and thrive again. You gotta know when to hold ‘em right? But if conditions don't improve, the gamble doesn't pay off, the tree goes bust and it all becomes an act of self-sabotage.
When it comes to heat, it's not just about drought stress—there's more going on beneath the surface for trees. As I mentioned before, heat ramps up transpiration, leading to water loss, which can trigger drought stress. In response, trees try to conserve water by closing some of the stomata on their leaves. These tiny pores are the gateways for water vapor to escape during transpiration, so closing them to retain moisture is a smart play. The catch is that stomata are also how plants intake atmospheric carbon dioxide which is an essential component in photosynthesis. With fewer stomata open, the tree can't produce as much of the sugars it needs to function. This means that the tree has to start cannibalizing its stored sugars to keep up with the demand.
The heat doesn’t stop there—it also speeds up the tree’s metabolism. The enzymes that drive respiration and other metabolic processes work faster when it’s hot, because the increased temperature gives molecules more energy to collide and react. Heat also increases respiration rates. As a result, the tree burns through its sugars more quickly, not just for respiration but for all of it’s metabolic activities. This means that the tree isn't just breaking down sugars at a faster rate but also performing other functions more rapidly, all of which take energy, and thus more sugar. Long term, the pilfering of stored sugars, increased respiration rate, acceleration of other metabolic processes, and hampered photosynthesis cause a carbohydrate deficit in the tree. As time goes on the tree responds to this stress by downsizing the parts of itself it needs to allocate carbohydrate stores to, and it starts with branches and leaves, leading to branch dieback and leaf scorch.
Excessive Light
For this one we need to be clear on the distinction between adaptability vs. acclimation. Adaptability is a species’ built-in genetic flexibility for different conditions, while acclimation is how an individual tree gradually adjusts to its environment over time. For instance, Hackberry trees can adapt to grow in a wide range of environments but still may need time to acclimate to them. Coconut Palms on the other hand, lack the adaptive range that Hackberrys have and won't acclimate to certain environments no matter how much time goes by. The key to acclimation here is ‘gradual’, acclimation is a gradual process and generally happens alongside changes in the tree's growing environment. We put this concept into practice when we harden off plants before we plant them in our gardens but we tend to forget that trees acclimate too, just on tree time. Trees are longer lived than your pepper plants for example and so respond to changes more slowly and on a different timeline, even if they are ultimately adaptable to those changes.
Few growth responses are more dramatic than phototropism. Phototropism is a plant's growth response to a light source. The classic example of phototropism would be a plant developing a lean toward a light source, or away from a source of shade like a wall or larger plant, but there are other responses too. One response that trees exhibit when growing in shade is to develop broader, thinner leaves than a specimen of the same species that grows in full sun. This is to increase surface area and light penetration to maximize photosynthesis. But, if suddenly exposed to full sun those same leaves can scorch because they aren't built to handle the intense light. Growth to catch more light is called positive phototropism, but there's also negative phototropism or a growth response to minimize light exposure. A tree species that has limited adaptability to harsh light might grow smaller thicker leaves, or lean away from a harsh light source, you may also see foliage grow more on a shaded part of the tree with branch dieback occurring on the parts of the tree exposed to more intense light.
Diagnostically when we see scorched, curled, or bleached looking leaves, especially on one side of a tree, we're going to put the concepts of acclimation and phototropism to work by asking ourselves what's new. What could have caused light conditions to change so suddenly that the tree wasn't able to gradually acclimate to it? Is the tree newly planted in a place with harsh light? Was a larger tree removed or pruned that was shading the affected tree? In the case of group planting like a backyard orchard, were some border trees recently removed from the grouping that could have exposed interior trees to the elements? Was there recent construction like a wall, fence, or pavement that could be reflecting light and heat back onto the tree? If construction was involved, remember that your tree’s roots might have taken a beating too. Whether it was from trenching, compacting soil for a new foundation, or just heavy equipment doing a number on the root zone, these things can all cause scorch and dieback.
If you think light is the problem, ask yourself if your tree is adaptable to its new growing environment. Can it acclimate over time? Do you need to step in with some TLC like painting that white fence a less reflective color, erecting temporary shade or providing more water while it attempts to acclimate? Some trees just aren’t cut out for the spotlight and might be better off in a shadier spot or it may be time to swap it out for something a little less sun-sensitive.
Winter Desiccation
Winter desiccation is a common but often misunderstood cause of damage in evergreens– particularly in broadleaf evergreens like boxwoods, laurels, and hollies. Unlike deciduous trees that shed their leaves and remain dormant throughout the winter, evergreens keep their foliage year-round. That means plant transpiration (the process where water evaporated from leaf surface) is still taking place albeit at a reduced rate.
Winter desiccation occurs when trees lose more water through transpiration than they can replace from the soil and there are some key factors that make that possible. A lot of us live in a cold, arid climate and that means cold nights and sunny days. That sun heats up the surface of those broad leaves and triggers transpiration causing water loss, while the cold nights ensure that the water in the soil remains frozen and inaccessible to the plant roots. Even though the plant is losing water, it can't absorb more from the soil, leading to leaf scorch and branch dieback. Ironically winter desiccation works in a similar fashion to summer drought stress. Quite a few of us also live in windy areas that exacerbates the desiccation by pulling even more moisture from leaf surfaces, especially in open landscapes that may be on an exposed hill or near a roadside.
Leaves may be scorched or completely brown particularly on the south and west sides of the tree having the most sun exposure. New growth is also very susceptible to winter desiccation as it's too tender and doesn't have enough time to harden off before temperatures decline. We also see this a lot in newly planted trees because they lack the extensive root system needed to withstand desiccation. The best way to prevent winter desiccation is to mitigate water loss and provide moisture. Water deeply and infrequently throughout the year to promote deep root growth. Water thoroughly in late fall to ensure moisture is in the soil before it freezes. Pay special attention to soil moisture when dealing with new plantings. Apply mulch around the base of the tree to help delay freezing and retain moisture. Finally, avoid pruning evergreens in late fall to avoid inducing vulnerable new growth. Trees can recover from some mild winter desiccation and will self-correct in spring. For more severe cases of dieback, you may need to prune in early spring to remove dead branches. If the tree suffers repeated seasons of winter desiccation then it may be time to consider relocating it to a spot less prone to the conditions that favor winter desiccation or to replace it with a hardier species altogether.
Vascular Diseases
When diagnosing vascular disease in trees, one of the most important concepts to understand is the disease triangle. The disease triangle explains that three conditions must be met in order for disease to occur: A susceptible host, a pathogen, and a favorable environment for infection.
Susceptible Host: Not all trees are vulnerable to the same pathogens and not all pathogens are adapted to infect every tree. The first step to any diagnosis is going to be identification because misidentification can waste your time and lead to ineffective care decisions. For example, an internet search of symptoms may suggest sudden oak death, but if the tree in question is a maple, that diagnosis is irrelevant. Keep a list of trees in your landscape, include species, planting date, and maintenance history (pruning, pesticide treatments etc.) and include any past injuries or environmental changes like wind damage or construction.
Take a look at what a healthy specimen looks like either in your own landscape or around your neighborhood. If you have a terrible memory like me, take a picture of a healthy tree and jot down some observations. For example: Does a healthy specimen have a strong central trunk or does it branch off into several leaders? How would you describe the shape, color, and texture of the leaves? Is the bark smooth and thin, or thick and furrowed? This will make problems easier to spot early. The more you know about the trees in your landscape and the diseases common to them the better equipped you will be to identify and manage problems.
Pathogen: This seems obvious but I suppose it bears mentioning that in order for a pathogen to infect a tree, a pathogen has to be present. Some pathogens are widespread and only attack trees under stress, while others spread aggressively and infect even healthy trees. There are exceptions but often fungal infections like verticillium wilt or bacterial infections like bacterial leaf scorch cause blockages and sometimes decay in the vascular system of the tree and interfere with water and nutrients transport. A positive diagnosis of a specific pathogen takes lab testing but common symptoms that suggest the presence of a pathogen are wilting despite adequate soil moisture, brown or discolored sap wood when cutting into an affected branch (vascular staining), and a sudden decline after stress events like drought or root damage that may have weakened the tree and made it more susceptible to name a few. Knowing what pathogens are common in your area can help you make more thoughtful decisions about monitoring for infection and managing symptoms. Talk to your friends and neighbors that may have the same tree and see if they have experienced any tree infections.
Favorable Conditions: This is an important one because unlike the genetic susceptibility of a tree or the presence of an invisible pathogen, you have the most agency in this side of the disease triangle. Once infection occurs it's difficult or sometimes impossible to eradicate the disease, so most management strategies for vascular disease in trees rely heavily (or solely) on prevention. An ounce of prevention is worth a pound of cure as they say. You should do some research about your trees and common pathogens that they can be susceptible to so you can know what conditions favor infection. UC ANR has a database on several common trees and shrubs with work ups on common diseases and pests. In the unlikely event that you can't find your tree on the list some tips for preventing favorable conditions for the spread of disease include: watering sensibly– not too much or too little, drought can weaken trees but free water in the soil can be a pathogen breeding ground. Maintain good airflow in the canopy and around the tree. We can do this by pruning properly with good spacing between branches and keeping the interior of the canopy full but tidy. Finally, disinfect your tools! Many vascular diseases spread through pruning cuts, so sanitize tools between moving from tree to tree to prevent transmission.
Pests
What about pests? It is possible for pests to cause leaf scorch and branch dieback, but while it’s tempting to blame insect incursions for the decline of a tree, most pests are opportunistic. They target trees that are already struggling, rather than acting as the primary cause of decline. It may seem strange to think of trees as actively defending themselves, especially if you've been following this series of articles where trees often sound like passive victims of environmental stress. But trees have been evolving defenses for hundreds of millions of years—long before insects, fungi, or bacteria ever adapted to attack them. Over the millennia since, trees have acquired some incredible adaptations to defend against their natural enemies.
Trees don’t just sit back and take damage, they actively fight back using a combination of physical and chemical defenses. Some trees have hairy or thick waxy leaves, some produce phenolics to make their foliage poisonous or exude resin to trap boring pests, trees have adapted the ability to compartmentalize and wall off disease and disorder to prevent its spread. Trees are amazingly resilient and a healthy tree can usually shrug off the occasional pest without serious damage. However, when a tree is already weakened or stressed, its ability to defend itself declines, allowing pests to cause some significant problems.
If you see leaf scorch or branch dieback, your instinct might be to panic. You may assume that an outside invader is attacking your tree and needs immediate removal. But remember, these symptoms are often an adaptive response to stress rather than direct pest damage. A healthy tree is supposed to prioritize survival over growth, its supposed to shed leaves, limit growth, and redirect resources to vital systems during stressful conditions. All this is to say that the best way to deal with pests is to promote robust tree health with careful selection of a tree species suited for your conditions, cultivars resistant to known pests, placement that is mindful of the species requirements and vulnerabilities, and thoughtful cultural practices like watering, fertilizing, pruning etc.
Diagnosing insect related tree damage can be tricky because pests are often secondary stressors, meaning they take advantage of an already weakened tree rather than causing the initial decline. However, there are clear patterns and signs that can help distinguish pest damage from environmental stress, disease, or nutrient deficiencies. The first step is to identify the type of damage and see how the tree is being affected. Defoliating insects may chew holes in leaves either around the margins or through the middle giving a shot hole appearance, some defoliators skeletonize leaves where the leaf tissue is eaten with only the leaf veins remaining. Sap feeders have piercing sucking mouthparts so hallmark damage from them would be stippling on leaf surfaces appearing like tiny yellow or brown spots from feeding, leaves may also curl, wilt, or become distorted, and there may be a silvery sticky substance left on the surface of the leaves (honeydew). Boring insects attack the vascular system of the tree by boring into the branches or trunk of the tree to feed and lay eggs. Look for small round or oval exit holes, sawdust-like material either in the holes, bark crevices, or the base of the tree, and branch dieback usually starting at the top of the tree going down. Some pests can also form galls– round, swollen, or deformed growths on leaves, stems, flowers, and branches. Root feeders attack the roots of the tree and can be responsible for stunted growth or sudden collapse of tree health, trees will have wilting or yellowing foliage despite adequate waterings and roots may look chewed, deformed, or hollowed out when dug up.
If you determine that there is insect damage, identify the insect and determine the severity of the infestation. This will help you decide what management methods are best for your unique circumstances. Before you step in with insecticides, ask yourself if the damage is superficial or life threatening, if the pest population is winding up or if it has peaked and winding down, and if the treatment could harm beneficial insects like pollinators or natural enemies of the pest in question. Remember that even if you were somehow able to eradicate all of the pests in your landscape, without remediating the conditions that made the tree vulnerable in the first place, and altering the environmental factors that made your landscape favorable for the pest, you will be leaving a clear path for re-infestation and further damage to the tree.
Consult UC IPM’s site for more information on pests of trees.
Herbicides
When diagnosing leaf scorch and branch dieback, most people first consider factors like drought, nutrient deficiencies, or pests. However, a less obvious but increasingly common cause is herbicide exposure. Even when applied correctly, herbicides can unintentionally affect nearby trees and shrubs, causing symptoms that can look like environmental stress or disease. Exposure can happen in several ways e.g. over spray, root uptake from soil persistent herbicides, vapor drift, or contaminated mulch or composts.
Recognizing herbicide damage requires understanding the specific symptoms it produces, which vary depending on the herbicide type and method of exposure. Leaf scorch and browning, where leaf edges turn dry and crispy, often looks like drought stress and is commonly caused by soil-applied herbicides affecting root uptake. Twisted, cupped, or misshapen leaves are another frequent sign, typically associated with growth regulator herbicides like 2,4-D and dicamba. Trees exposed to herbicides may also exhibit chlorosis, where leaves turn pale green or yellow, often resembling iron deficiency but appearing suddenly after herbicide application. In some cases, new growth may appear stunted or distorted, especially when affected by amino acid synthesis inhibitors like glyphosate. In severe cases, trees may suffer progressive branch dieback, particularly if the herbicide was absorbed through the root system.
Diagnosing herbicide damage requires looking for unnatural patterns in affected trees. Damage is often asymmetrical, appearing on only one side of the tree, usually the side facing the herbicide application. Trees of the same species planted some distance from the affected tree may be unaffected, while trees of different species in the same area may show similar symptoms. Another important factor to consider is the timing of the damage. While most tree health issues develop gradually due to disease, drought, or pests, herbicide damage often appears suddenly, within days to weeks of exposure.
Preventing herbicide damage requires careful planning and application practices. Avoiding spraying on windy or hot days can help reduce drift and volatilization, while using low-drift nozzles and coarser spray droplets further minimizes the risk. Since tree roots extend well beyond the canopy dripline, herbicides applied too close to a tree can still be absorbed. Additionally, using tested, herbicide-free mulch and compost is essential to prevent contamination, as persistent herbicides from treated pastures or agricultural fields can remain active in organic material. Managing herbicide drift by increasing buffer zones between the tree and the application and using physical barriers like tarps can further reduce unintended exposure.
The extent to which a tree recovers from herbicide damage depends on the type of herbicide, level of exposure, and overall tree health. Trees with mild exposure, such as minor leaf distortion or yellowing, often recover on their own once new growth resumes in the following season. In cases of moderate exposure, where leaf scorch and slowed growth occur, deep watering, mulching, and foliar feeding can support recovery by helping the tree outgrow the damage. However, severe exposure, especially from root-absorbed herbicides, may lead to long-term decline, progressive dieback, and eventual tree removal.
Concluding Recommendations
If you've made it to the end of this guide, you may feel like you've been introduced to an overwhelming number of possible causes for leaf scorch and branch dieback, and this isn't even close to a complete list. But here's the good news– you don't have to memorize an exhaustive list of disorders to diagnose tree problems effectively.
Plants are the little logisticians of the natural world. They're entire lives depend on gathering, storing, using, and otherwise managing resources, and over the millennia they have gotten pretty good at it. Instead of reciting a list of possible causes, focus on the unifying theme in all of these articles: Trees decline when the resources they need (water, nutrients, sugar etc.) aren't reaching the parts of the tree that need them.
Excuse the pun but, it all comes down to three root causes:
- The resource isn't available in the tree’s growing environment. A tree can't use what isn't there and if an essential resource is missing, the tree will show signs of stress that often involve leaf scorch and branch dieback.
- Something is interfering with the uptake, synthesis, transport or utilization of the resource. Even if resources are available, the tree may not be able to access them due to some physical or biological interference/limitation.
- The tree is using the resource faster than it can replenish it. Tree’s have to balance growth and survival, but if supply can't meet demand, they start sacrificing parts of themselves.
Rather than memorizing dozens of disorders, a more effective approach is to use a simple diagnostic framework. First, check whether the environment is providing what the tree needs. Is the soil too dry, compacted, or nutrient-deficient? Has the tree experienced flooding, drought, or soil disturbance? Is it competing with turfgrass, weeds, or other trees for resources? If the tree’s environment seems suitable, look for signs that something may be interfering with its ability to absorb or transport resources. Are roots damaged, girdled, or buried too deeply? Are there indications of vascular disease, like brown streaking in the sapwood or oozing sap? Is there evidence of insect activity, like frass, boreholes, or abnormal leaf damage? If neither of these factors is the issue, consider whether the tree is simply outpacing its own ability to sustain itself. Has it recently experienced excessive heat, wind, or sun exposure? Has it been pushing out rapid new growth or producing an unusually large seed crop? Did a recent change in its environment alter its resource demands?
If you take one thing away from this series, let it be this: tree decline is almost always caused by resource flow disruptions—not just random bad luck. A tree is either missing a resource, struggling to absorb it, or using it too quickly due to stress. By focusing on how trees function, rather than trying to memorize every possible disorder, you can confidently diagnose and manage tree health issues no matter what new challenges arise.
Good Resources
- Abiotic Disorders of Landscape Plants. (This $47 guide to the topic is excellent.)
- Local tree lists for the Eastern Sierra. Start by planting the right trees.
- Find your soil type here.