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Understanding the Causes and Classification of Tree Disease
Tree disease is identifiable as any unnatural change or malfunctioning of a tree due to a pest or agent. There are thousands of potential diseases affecting trees and plants, with different causes behind each of them.
Tree diseases are caused by one of two types of inducers: abiotic or biotic, which simply means non-living or living. Biotic or living diseases are then further split into smaller groups such as bacteria, viruses, fungi and phytoplasma. A lot of the potential causes of tree disease come from parasites, but this is not always the case. It is important to remember some parasites are actually good for the ecosystem and can be beneficial.
Classifying Tree Diseases
To classify the most common tree diseases there are a range of parameters considered:
- Cause or trigger
- Part of tree affected (leaf, bark, or root)
- Tree species
- Age of the tree
Tree disease identification also involves considering both signs and symptoms of any given disease. People working in forestry and tree management can utilize sophisticated detection technologies to see what changes trees are experiencing and how diseases are progressing. Below is a closer look at different types of common tree disease and potential action or treatments once they are discovered.
Tree root disease affects species of hardwood and evergreen trees, primarily attacking the root and lower stems. A root disease is often fatal as they prevent water and nutrients getting to the rest of the plant and as the roots are unseen, the problem can go unnoticed for many years and often until it is too late.
Tree root diseases are particularly difficult to identify and treat due to their under-surface development.
Pine Tree Root Diseases
Pine tree root diseases including Loblolly pine decline, Littleleaf Disease and Annosum Root Rot:
Loblolly Pine Decline
Loblolly pine decline is specific to the loblolly pine species. It happens when an unfavorable
- Poor land conditions
- Disrupted tree health
- Pest infestation
It is most commonly found in drought-affected territories in the southern and south-eastern US states. Drought stress leaves the trees more vulnerable to pests and the pathogenic Leptographium fungi is known to be spread by bark beetles and this fungus deteriorates and ultimately kills the trees’ roots.
The key to minimizing the decline of loblolly pines is looking for solutions to drought stress and pruning and removing dead and dying trees to leave more resources for those still living.
A common fungus, Phytophthora cinnamomi, causes the Littleleaf infection, often found in loblolly and shortleaf pines. It attacks trees living in infertile soils that are excessively moist.
The fungus damages the roots and root hairs but unfortunately is only detectable once it is visible on the upper body of the trees. Symptoms include pale-green needles, slow twig growth and a multitude of undersized and small cones. Littleleaf disease can be controlled and minimized through regular thinning or trees and the use of nitrogen-based fertilizers.
Annosum Root Rot
Annosum Root Rot is particularly common in Arkansas pines and spreads quickly in areas with sandy soils. It occurs due to the spread of the fungus Heterobasion annosum, which is identifiable through its sponge-like formations. It gets into the trees via open cuts and slices and immediately attacks the root.
Annosum root control relies upon regular thinning of the pines, spraying fresh cuts on stumps with borax and planning planting properly, with adequate spacing.
Hardwood Root Diseases
Hardwood root diseases are also hard to diagnose and treat because they happen under the surface of the soil. This means that, unfortunately, a lot more trees die due to root diseases than other types.
Common hardwood diseases including Texas root rots and mushroom white, caused by several different biotic pathogens and agents.
Unfortunately, once they set in they cannot be cured and the weakened trees will need to be cleared as efficiently as possible. It is important to remember these fungi can remain in the soil for several years, so planting should be postponed for at least two years.
Tree Leaf and Needle Diseases
Foliage, including both leaves and needles, are often attacked by fungi, the main culprit of this kind of disease. The signs and symptoms of foliage disease often resemble other types of issue such as insect infestation or chemical damage, so they can be hard to identify.
Most treatments for foliage disease involve removing and destroying leaves each fall, ensuring the pathogen does not remain in place during winter, returning in spring. Let’s look more closely at some common needle and leaf diseases.
Pine Needle Diseases
Pine needle infections and diseases do not usually result in a serious problem and usually don’t require any treatment. This differs, of course, if the pines in question are being grown for sale for ornamental or festive purposes. Common needle diseases include:
Needle Rusts: the least dangerous disease, which covers the whole needle, discoloring it.
Needle Casts: the fungi which cause needle cast to grow within the needle and cause defoliation, as the needles are cast from the tree. There is a range of different casts dependent on the fungi growing within the needle and there are approximately forty different pathogens causing needle cast in the USA.
Needle Blights: this is another infection that develops inside the need and results in partially killing it. Tree diseases within the needle blight category include brown spot, snow, and red band.
Hardwood Leaf Diseases
Diseases affecting leaves differ in many ways due to the wide variety of different trees and therefore, different leaves. The most effective way of dealing with leaf diseases is to remove and destroy contaminated leaves. Common leaf diseases include:
Leaf rusts are usually non-dangerous, although some can result in leaves shedding prematurely, which can stunt growth. Rusts look like yellowed spots with powder-like spores on the upper part of the leaf and they usually become apparent towards the end of August, affecting many types of trees including maple, birch, popular, willow, ash and cottonwood species.
Looking just like a sprinkling of flour or talcum powder, this tree disease spreads in patches or spots across the leaves. It is usually caused by Microsphaera fungi.
Unusual among fungi, it is a particularly persistent problem in dry areas and can colonize succulent plants and trees. It is most effectively controlled using chemical sprays, although it is also sensitive to sulfur dioxide and doesn’t occur in regions with high pollutant levels.
Tar spot is most common in maple species and is caused by the fungus Rhytisma. It begins with yellowish spots appearing towards the end of Spring, with tar-like formations appearing by late Summer.
Tar spots don’t kill trees, but they can result in the leaves shedding and stunt the tree’s development. It is common in the north-eastern regions of the US and fallen leaves need to be removed to stop the infection returning in the spring.
Anthracnose is an infection that can disfigure leaves and stems. It leads to leave necrosis in irregular shapes and burnt-looking foliage. It can cause extreme damage to the tree but is difficult to treat.
It can typically be found on tree species including oak, maple, birch, hickory, and walnut.
Leaf spots are one of the most common infections found in leaves and are usually brown in color. They are caused by many different species of fungi including Mycoshaerella and Actinopelte and can also be caused by parasitic algae.
Tree Bark Diseases
Diseases impacting the stem and bark of a tree are often induced by different fungi. Unfortunately, once an infection affects the bark, it is much more serious and difficult to deal with. Disease-affected branches are easier to treat, as the damaged branches can simply be removed but if the disease is in the trunk, there is often no way to save the tree.
Rusts are probably the most common bark disease in evergreen trees and particularly Arkansas pines. It can be particularly dangerous for younger trees as it can impact the trunk quickly. Rusts can be common on older trees and only affect their branches and never affect the trunk and become lethal.
If a tree is infected by wilt, it cannot be cured. It is a tree trunk disease that can be identified through leaves and foliage that appear burned but do not drop off. The tree dies as the fungus is living and taking over its vessels, stopping water from being properly absorbed. The most common species affected by this disease include Dutch elm, mimosa and oak.
Decays are typified by the growth of mushrooms, which cover the tree, alongside its discolored bark. The mushrooms, or conks, develop over many years before they become noticeable.
They find their way into the tree via wounds and lacerations and quickly take over deep inside. Removing outer conks does not solve the issue or treat the disease properly and often, a tree can handle and manage the disease itself thanks to compartmentalization.
Trees are able to release chemicals that get rid of the fungi and help to form calluses to cover any wounds where the fungi take hold.
Decays are thankfully not lethal but they can make the tree much weaker and its timber is not good for sale, so they need to be dealt with quickly in forests grown for timber.
Black knot is a fungal development found most commonly in fruit trees. It is a tree bark skin disease caused by the fungus, Apiosporina morbosa. This fungus can live on the tree for many years before causing any problems.
Black knots begin as a green-brown or brown swelling or formation on the bark but within a year turn black and can swell. After two or three years most of these formations will die off due to fungal colonization.
If a tree is covered in black knots, it is at risk of dying as the fungi spores can quickly spread to the trunk. Treating this disease is possible through simple pruning or you can copy for chemical fungicidal sprays.
Treating and Managing Tree Diseases
Identifying a tree disease as soon as possible is essential to efficiently begin managing and treating the problem. The following methods are all standard for managing and minimizing tree diseases:
Fungicide is commonly used to prevent diseases from even beginning. All fresh-cut stumps should be sprayed to minimize the risk of fungi growing or pest infestation.
Removing damaged branches and leaves is an essential part of forest management and even more important if any trees are infected with a disease.
To manage foliage and leaf diseases, all affected leaves need to be removed and destroyed. Leaving the leaves allows the disease to simply return once Spring comes round. Removing them minimizes the risk of further infection in the following season.
Taking time to improve soil conditions, fertility and minimize drought stresses or excessive moisture can ward off fungi that thrive in these conditions.
Other measures to consider include planting resilient species that tolerate or adjust to the pathogens impacting the forest in general. It is also important to consider delaying reforestation to ensure any infection is fully dealt with and new trees aren’t at risk of immediately succumbing to the same disease.
Many of the diseases that trees experience are unique to their particular species, and this can make identification and treatment difficult. Controlling tree disease is essential to avoid any risk of further spread and impact on the whole forest. The sooner a disease is identified, the sooner it can be treated and eliminated, keeping more trees safe from harm.