 Fruit trees drape our landscapes throughout the Rocky Mountain Region, many having genetic links to wild and native plants that were here long before us. Fruit trees bring people together and touch all of our senses. Creating annual traditions from family photos with the backdrop of flowers to fruit picking and pie making; fruit trees are a part of our life experience and an invaluable component of the urban forest. Known as low maintenance, low cost, and low risk trees; fruit trees are a worthy investment and addition to the landscape. With an informed approach to care, you can be sure to receive the most from your fruit trees, for generations to come! To understand care for any species, we must first understand the stresses they face in their environment... Fruit trees - more accurately, anything in the Rosaceae family - are exposed to bacteria and fungi that colonize and terminate living branches. It is worth noting here that the Rosaceae family extends beyond just trees to also include our roses. For this reason, I may use the words 'plant' and 'tree' interchangeably. The most recurrent and pervasive agent affecting fruit trees in the Rocky Mountain region is a bacterium known as Erwinia amylovora, commonly known as fire-blight. Fire-blight Fire-blight, is a vascular disease that generally enters a plant through flowers in spring. The majority of transmission takes place via insects, such as bees and other pollinators moving from plant to plant. In the same manner in which they move pollen, they move disease. When fire-blight is active (spring-fall) it produces an oozing or seeping from branches that may not be visible to you in passing, though it is visible to the naked eye. This exudate or liquid contains high concentrations of sugars that attract insects. Within this glucose compound, there is also pathogenic bacteria, ready to be transmitted. Before we go reaching for insect sprays, remember: we need these insects to regulate the insect population and assist in fruit production. No insects, no fruit! Fire-blight can also enter through open wounds found along living branches. One of the more prolific episodes of fire-blight we will see is during growing seasons following a spring hail storm. The impact of hail wounds trees, opening large areas of living tissue. These wounds seep plant sugars that increase insect attraction and subsequently, disease transmission. Blight typically moves from the peripheral areas of a plant towards the interior, colonizing and feeding on plant tissue at a similar rate per square area. This means that blight appears to move rapidly in small diameter plant tissues and slower in large diameter tissue. In reality, it is moving at the same speed, there's simply less or more tissue to consume, respectively. With this understanding, we can surmise that blight proves especially detrimental to young trees within the Rosaceae family. And by this same logic, blight is capable of causing significant damage in the younger and outer edges of our mature trees. This disease characteristic results in quite an impact if we consider that the majority of our trees' leaves, flowers, and fruits exist in the outer edges of branches. Over time, if unmanaged, this progression stresses a tree's ability to make food and meet its basic metabolic needs. Disease Management Branch infected with fire-blight The good news is that this disease can be managed at nearly no additional cost, and does not require chemical treatment. Successful management to address fire-blight and other vascular diseases, involves the following steps:
Plant Resistant Species As with all things biological, some species are more resistant than others to hosting pathogens. There are several species of trees within the Rosaceae family that are listed to be resistant to disease. Taking this into account at planting time, can serve to alleviate concerns of plant loss or the necessity for intensive management from the beginning. Planting the right species in the right location, in the right way at the right time, is the collective first step to tree cultivation in the urban forest. These decisions and actions will dictate plant success, management practices and costs over the life term of the plant. Oversight during this phase results in the majority of tree removals and excess costs within the urban forest. Winter Pruning Pruning fruit trees can come with multiple objectives, apart from disease management. We prune fruit trees to train strong branches, reducing loss of plant parts when holding simultaneous loads of water inside the stem, mature fruit, and early fall snow. We might prune fruit trees to achieve clearance from a building, structure, or pathway; or to meet a certain aesthetic. Orchards will prune to keep young fruit trees short and stout, their fruit easy to reach. This objective maximizes productivity and safety for harvesters. When fruit can be reached from the ground, it keeps workers off of ladders and picking more fruit. This objective must be executed early in a tree's life. The main objective, the reason we are pruning in winter, is disease management. Pruning is wounding because it exposes living vascular tissue. Pruning in winter, allows us to cut into and expose living tissue when bacteria and fungi are dormant, thereby eliminating exposure to disease.  When cuts are made into living parts of a plant, vascular tissue is exposed to air. Air enters through the pruning wound, cellular tissue once dilated and turgid with water, contract air bubbles and cell walls collapse - this is the process of embolism via cavitation. As a result of this process, living conductive tissue becomes non-conductive (dead), and is no longer available to pathogenic agents like fire-blight when spring arrives. The simple approach of pruning in winter allows us to meet multiple objectives without exposing our trees to disease. A myth was once held that in order to eliminate disease transmission during pruning, pruning tools should be sanitized between cuts. The idea was that as we cut through diseased tissue, we could transmit this to other parts of the plant with subsequent cuts. This myth was debunked in lab studies that concluded bacteria and fungi cannot successfully adhere to metal pruning tools. Pruning in winter without chemical sanitation is perfectly safe for your trees. During winter, disease is readily identifiable to the trained eye, as is the difference between living and dead tissue. Current infection can therefore be removed, and steps can be taken to reduce likelihood of transmission in the future. Just as hail can wound and expose living tissue, so can any other object that regularly contacts living tissue. This can include branches rubbing buildings or fences, or even branches rubbing other branches. Pruning to create space and autonomy around healthy and viable branches can reduce wounding and the likelihood of disease transmission in the future. Disease can be quite evident and alarming to tree owners and managers during the growing season. The most evident symptom is known as flagging - branch tips with brown/black leaves (see photo above). It is advised to avoid any temptation to prune this out during this time. While you may successfully remove infected tissue, cuts into live tissue will leave your tree immediately exposed to the ingress of pathogens closer and further into your plant. Diseased tissue, while unsightly, does act as a barrier for healthy and uninfected tissue within the plant interior. In my twenty years of practice as an arborist, the worst cases of disease infection I have seen in fruit trees is when a tree owner or manager has attempted to prune in spring through fall to remove blighted branches. Remediation at this point necessitates removal of even more living tissue. I've witnessed cases so severe that it takes several years of unnecessary expense and stress for the tree to successfully arrest this situation.  Resource Optimization Resource optimization to maximize growth is the primary goal with all tree cultivation and management. This is always the objective for a tree grower! There are three simple truths I want all tree owners to understand: stress will come, loss will occur, and healthier trees are more resilient. Trees have no pre-determined lifespan. With unobstructed access to resources, a tree can live forever. Trees are autotrophic organisms, which means they produce their own food and use this food to generate brand new vascular tissue (xylem-cambium-phloem), perpetually. Ever notice that your tree is getting bigger - taller, wider, longer? This growth at the edges, is brand new vascular tissue that did not previously exist. And it will continue doing this until it runs out of resources. The outer most edges of a tree are where the living tissue exists. Scratch through the bark and there it is. The core of a tree is non-conductive (dead) tissue - the artifact of a once smaller and younger tree. When we are twenty years old, we have twenty year old organs - barring any major transplant operations. Whether a tree is twenty years old or one hundred, it has brand new vascular tissue. For a tree to die, some external agent has to obstruct or eliminate its access to resources. And there are a number of ways this occurs in both natural and urban forests. Larvae chews through vascular tissue inhibiting the flow of nutrients, lightning strikes and causes rapid expansion of conductive tissue, drought decreases stem pressure causing cellular loss; trees in a city hardscape grow to exceed their soil's ability to hold a sufficient supply of water and/or elements. And these are just to name a few! A tree never dies on its own, and rarely does a tree die from one factor, but rather compound stressors. To be a tree grower is to be a resource manager, which is to understand tree resources and the practical implications of resource optimization.  "Advanced Tree Biology: Primer On Essential Elements & Tree Health" - Dr. Kim Coder Understanding Tree Resources Trees rely on 19 essential elements to conduct life functions. Carbon, makes up about 19% of a tree's mass, making it the single most prevalent element found in trees. Carbon is airborne and absorbed through the leaves - we can't manage access to this element. 80% of a tree's mass is made up of water, and the remaining 1% is a varying combination of 16 essential elements absorbed and/or bonded with water and brought in through the roots. Here, is where we can affect change - both positive and negative. Soils in the urban forest are dense. If you've ever tried to dig in your yard, you know it's a challenging endeavor. It requires sharp metal tools and force. Yet, if we go to the forest and begin digging with just our hands, it's quite easy. Urban soils are about 5% void space by volume, as opposed to forest soils which are about 50% void space by volume. Forest soils simply have more open space. This means forest soils can hold more water and water moves around more freely and rapidly within forest soils. More water moving faster, means elements being absorbed and brought to roots faster. This results in more growth. It's really that simple! So then what's different about forest soils...  Forest floors have layers upon layers of decaying matter. And within this decaying matter is a whole community of micro-organisms feeding and excreting waste, replenishing nutrients. Larger organisms visit to feed on smaller organisms, also leaving waste. This is the nutrient cycle! As a result, forest floors create soils with more nutrients and more void space, capable of holding more water and moving more nutrients, faster and more freely. In contrast, urban soils hold less water, less nutrients are recycling, and everything moves just that much slower. When we consider that urban sites also leave trees more exposed to wind and sun, drying trees out just that much faster; we begin to understand just how challenging it is to be a tree in the urban forest. So then, how do we replicate the forest floor in our urban forest...  Practical Implications We mulch our trees! By mulching, we can simulate a rich forest floor that replenishes nutrients, holds more water, and moves resources around more freely. Studies have proven that no chemical application or fertilizer can rival the growth of a tree in mulch. In fact, fertilizer applications prove detrimental over time because they increase soil bulk density leaving less void space and less room for water to enter soils. To mulch your trees, establish a 5 inch deep mulch bed within the critical root zone, using organic, undyed woody mulch placed directly onto the ground. Do not use any cardboard sheeting or weed barriers. Mulch should not be touching or piled against the trunk of the tree. The critical root zone is the amount of root space that is vital to maintaining your tree's biomass. To determine your tree's critical root zone, walk up to your tree and measure the trunk diameter around chest height - doesn't have to be precise. If trunk diameter is around 12", the critical root zone is about twelve feet out from your trunk in every direction. As the tree grows, so does the critical root zone. Ideally, this radius is the space we mulch. This space belongs to the tree! For more information on proper mulching techniques, follow this link: https://www.treesaregood.org/Portals/0/TreesAreGood_Proper%20Mulch%20Tech_0321.pdf The next thing we need to do is monitor soil moisture at a 5 inch depth within your tree’s critical root zone, throughout the growing season. Apply water when the soil is dry, drenching the entire critical root zone. It is important to allow soils to dry before the next watering event. Maintaining saturated soil is a serious stressor! Bringing these two components to your tree care practices will result in trees that can withstand the stress of disease and loss of living tissue. In addition, it will reduce the cost of tree ownership over time.  Story Time Twenty years ago, you planted two apple trees. They were both planted perfectly in perfect locations at the exact same time in spring; there's an old picture of planting day somewhere in a drawer. One tree was planted in grass, the other was planted inside of a generous mulch bed extending to the edge of a mature apple tree's critical root zone (~12' radius). The one planted in grass was watered every time you watered your grass. You monitored soil moisture of the one planted in mulch and only watered when soil was dry. The tree in mulch has required about 25% less water because the soil was insulated from sun and wind, and experienced less desiccation, so it held moisture longer. The tree in mulch also grew about 300% more than the tree in grass because its soil is less dense. The mulched tree gets more water, which means it also receives more of its essential elements. The tree in mulch is much larger in every way! The branches are thicker, it has far more leaves, shows more flowers each year, more fruit, and produces far more shade. Then a big hail storm comes! Both trees are equally impacted. Both trees contract fire-blight. Both trees exhibit dead flagging branch tips throughout the canopy. You call Inner Tree to come prune these trees in winter. We specify that for both trees we will remove all existing signs of disease. You watch as we prune. You see us removing infected branch tips and looking into our cuts for further discoloration of vascular tissue; sometimes you see us prune back just a little bit more. Fire-blight moves at the same rate per square area. We end up pruning about two arm loads of branches from each tree. The tree in mulch loses about 2% of its photosynthetic material. You can't even tell that anything is missing after we've pruned. The tree maintains its same shape, form, and size. The tree in grass loses about 15% of its photosynthetic material. You notice! The next growing season, the tree in mulch fills out, puts on flowers, and fruit. The flowers are vibrant, the fruit is firm and healthy. Its new growth extends out about six inches all around. It makes as much food as it needs and has a very successful growing season. The tree in grass doesn't put on as many leaves, has pale flowers, puts on less fruit and the fruit it has is much smaller. Its new growth only extends out about two inches and not throughout the canopy. The loss to disease was significant; it simply has less places to hold leaves. It will make less food than it needs this year and rely on reserves from previous years. Then an early fall ice storm comes and breaks an equal amount of branches in both trees... Do you see where this goes? Our job as tree growers is to optimize resources, because stress will come, loss will occur, and healthier trees are more resilient!
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