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Forestry

Tree Diseases: How To Identify Them And Protect Forest

23.04.2021
forest tree disease

Tree disease control is among the major activities in forestry since forests suffer from multiple pathogens, nutrient deficiencies, and pest invasions. Any tree disease, regardless of the inducing cause, threatens forest health and impacts any related industry. From harvesting to environmental needs – tree pests and diseases are an ultimate nuisance for any business involved. Besides, healthy trees mean a healthy population, so common people are affected as well. In this regard, tree disease identification and treatment is a regular activity of foresters, benefiting all.

Causes & Classifications Of Tree Diseases

A tree disease suggests any deviation or malfunctioning due to a persistent agent. There exist about a hundred diseases for each of thousands of plant species, with different triggers.

Causes

There are abiotic and biotic inducers (non-living and living). Biotic diseases are further grouped according to the pathogen type (bacteria, fungi, viruses, phytoplasmas, nematodes, etc.).

Here it should be noted that in most cases, pathogens are parasites. However, not each pathogen is a parasite, and not each parasite is a pathogen. Thus, some parasites cause no harm to plants and, correspondingly, don’t provoke any diseases. On the contrary, parasites may be beneficial. Also, some earth-dwelling bacteria don’t parasite on plants but produce harmful toxins that trigger tree root diseases.

Classifications

The most common classifications base on the following parameters:

  • Cause – depending on the trigger.
  • Host tree – according to the suffering species (poplar, conifer, pine, maple, etc.).
  • Tree part – specifying the affected area. There are leaf (foliage), stem (bark), and root infections.
  • Tree age – considering the degree of maturity. There are nursery, sapling, and mature tree growth diseases.

Tree disease identification takes into account signs and symptoms. Signs are changes produced by pathogen tissues (e.g., white rust). Symptoms are how plants suffer from infections (e.g., defoliation, crown thinning, decay, wilt, etc.). Using change detection technology, foresters can understand when woods are stressed.

Tree Leaves Disease

As the name suggests, these affect foliage. The main culprits of foliar infections are fungi. However, signs and symptoms can be similar to chemical injury of insect infestations, which complicates tree leaf disease identification and the choice of corresponding management. The problem eradication strongly depends on the reasonability of treatment costs and is not always possible due to favorable weather conditions for fungi development. In this regard, the most typical method of foliar tree disease treatment is removing and destroying the leaves in the fall. It prevents pathogen overwintering and relapse in spring.

Tree leaf diseases affect both conifers and hardwoods and differ by the degree of severity. While some cause little harm, the rest are rather dangerous and can cause mortality.

Pine Needle Diseases

Conifer foliage infections are typical but rarely represent a serious threat. Thus, in most cases, they don’t require treatment unless merchantability is in question. This refers to ornamental and Christmas trees, and spraying is the common method to defeat the pathogen colonization. In large forests, it is rarely implemented though due to a lack of feasibility and necessity. Generally, there are three types of pine needle diseases: needle rusts, casts, and blights.

pine needle rusts, casts, and blights

Needle Rusts

Needle rust covers the needles and refers to the least dangerous coniferous tree diseases.

Needle Casts

Needle cast fungi grow inside the needles forming long hysterothecia and typically cause defoliation (or casts, hence, the name derives). Depending on the pathogens, there distinguished lophodermium, elytroderma, rhabdocline casts, and many more. About forty pathogenic genera cause this tree disease in the US.

Needle Blights

Needle blight is also an infection inside the needle that causes its partial death. The most common types of tree diseases in this category include snow, brown felt, brown spot, and red band needle blights.

Sooty Mold

It is a soot-like substance on the leaves due to insects’ honeydew secretion. This condition is not a disease since fungi dwell in honeydew without penetrating the plant, yet the black sooty covering severely reduces Christmas tree merchantability. It is typical for the northeastern regions of the USA and affects different pine species.

Hardwood Leaf Diseases

Most infections of hardwood species are also caused by fungi, with no specific tree diseases treatment. Common management suggests removing and destroying contaminated leaves.

Anthracnose

The anthracnose infection reveals leaf necrosis of irregular shapes and burnt foliage that may also affect stems. This is a fungal pathology that may cause severe damage, yet it is difficult to tackle, especially in wet spring weather favorable for fungi development.

Anthracnose is typical for walnut, oak, maple, birch, hickory, among others.

infected maple leaves

Leaf Rusts

Leaf rusts are among common tree diseases and are typically non-dangerous unless they cause early leaf sheds and, thus, negatively impact growth. Rusts are yellowish spots with powdery spores on the upper leaf part. Typically, rusts cover hosts in the second part of August and affect maple, birch, poplar, ash, plum, willow, and cottonwood.

Leaf Spots

These tree diseases are infections in the form of spots, most of the brownish color. They are caused by some fungi species (like Actinopelte, Septoria, Mycoshaerella, Phyllosticta) and parasitic algae. Cool wet springs are particularly favorable for the infection spread. Poplar hybrids are especially prone to leaf spot infestations.

Tar Spot

This tree leaves disease is usually caused by the Rhytisma fungi colonizing the maple family (maple proper and sycamore). The symptoms start as yellow-green or light green spots in late spring-early summer with added tar-like formations by the late summer. Even though they don’t kill the trees, they cause leaves sheds, which may affect the plant development. The infection is common in the northeastern part of the USA. If the fallen contaminated leaves are not removed in the fall, a new cycle will start next season.

Powdery Mildew

The signs of this tree leaf disease look exactly like white talcum powder. It should be distinguished from dust or bird droppings. It spreads in spots or patches and is mainly induced by the Microsphaera fungi. Unlike other fungi infections, it particularly persists in hot dry weather and colonizes succulent plants. The most common treatment is chemical control. The fungi are sensitive to sulfur dioxide and are not common in SO2-polluted regions. The pathogens can be transferred by wind, animals, or rain.

oak leaves infected by powdery mildew

Leaf Blister

In this case, leaves are infected by the Taphrina genus that causes additional growth of the contaminated area (blisters, curling, expansion, puckering). The pathology starts with light green spots that acquire a white coating and eventually become brown. This tree disease is frequently found in the oak family, peaches, female catkins, and alder. It develops under cool wet weather conditions at the stage of leaf expansion. Leaf blisters do not result in defoliation and do not produce a serious impact on the suffering plant.

Tree Bark Disease

Stem pathologies are typically induced by fungi like leaf ones. However, these are more serious, depending on what part is affected. Tree branch diseases have less severe consequences for the plant since the infected branch can be removed. Little can be done with tree trunk diseases though when fungi reach the vascular system, the host dies.

Rusts

Rust is among the most common evergreen tree diseases, in Arkansas pines in particular. It is especially dangerous and can be lethal for young samples due to trunk galls. Mature plants can live with that as long as only branches are infected, and the disease does not destroy the central stem.

Black Knots

The black knot is a fungal pathology typical for the genus of Prunus, fruit and ornamental cherries, and plums in particular. This tree bark skin disease is caused by Apiosporina morbosa that may dwell on the host plant for several years. Black knots start as greenish-brown and brown formations (swellings) during the first year that grow into black hard galls during the second one. After two or three years, mature galls usually die and turn whitish or pinkish due to fungi colonization. Such galls can be numerous on a tree, and this is a danger.

black knot on the tree

The fungi spores spread to new branches in wet mild weather. The treatment includes chemical or mechanical control (fungicide spraying or pruning, correspondingly). The removed branches must be instantly destroyed because the spores continue to release up to four months. This tree branch disease becomes lethal when it reaches vital stem parts.

Cankers

Canker is a tree bark disease with necrotic areas. It occurs due to pathogenic fungi (e.g., Botryosphaeria, Hypoxylon, Phytophthora, Botryosphaeria, Cytospora) that penetrate through bark cracks or mechanical and natural injures (e.g., man-made wounds, frost cracks, fire burns, sunscalds). Healthy plants cope with the infestation, yet weakened ones cannot resist it.

The consequences may be different. In some cases, cankers only weaken the infected hosts. In others, multiple cankers kill them. Chemical treatment is not effective in this case. Pruning is the common method when infected branches have to be removed. However, the whole tree is cut if there are cankers on the stem.

Decays

This tree disease diagnosis is pretty simple – typically, it is identified through mushrooms (aka conks) covering the tree and discolored bark. The conks develop for many years before they can be noticed. They penetrate the plant through wounds and are located deep inside. For this reason, simple conk removal won’t solve the problem. Instead, the host can combat the conks itself thanks to compartmentalization. It is a natural process to release chemical compounds to get rid of the fungi as well as to plug the vascular tissue and generate callus. The success depends on the fungi’ ability to adjust to the change and the host’s health. Decays are not lethal, but they do weaken the plant and spoil the timber salability.

tree infected by decay

Wilts

Wilt is a lethal tree trunk disease that is diagnosed through burnt leaves with no defoliation. The plant dies due to fungi inside its vessels that hinder crown water saturation. Susceptible species include mimosa, oak, Dutch elm, and more.

Tree Root Disease

Tree root diseases affect the root and lower stem of both evergreen and hardwood species. Compared to leaf and bark infections, they have the highest tree mortality rates since they prevent water and nutrient absorption by the plant. Also, as it develops in the unseen tree part, it remains undetected until the damage becomes visible. Thus, it is more difficult to diagnose tree root diseases.

Pine Root Diseases

The most common root infections in conifers are annosum root rot, loblolly pine decline, and littleleaf disease.

Annosum Root Rot

This pathology is typical for Arkansas pines and widely spreads on sandy soils. It occurs due to the fungus Heterobasion annosum that generates sponge-resembling formations (hence the other disease name – root sponge). It gets into the host through fresh cuts and wounds and then attacks the roots.

tree disease Annosum root rot

The standard methods of annosum root control comprise:

  • Thinning of pine stands in summer when the temperature rises 70F and above (which is unfavorable to the fungus spores).
  • Checking the presence of bark beetles in the area since summer thinning may provoke their attacks.
  • Spraying fresh cuts on stumps with borax.
  • Planting with enough spacing.

Loblolly Pine Decline

This root infection is characteristic of loblolly pine, as the name tells. It occurs due to an unfavorable combination of circumstances, including poor land conditions, pests, and overall disrupted tree health. It is spread on drought-subjected southern and southeastern territories of the USA. Drought stress reduces plant resistance to pests, making it susceptible to tree bug diseases, and bark beetles in particular. Apart from destroying the host by feeding and breeding, they also transmit pathogenic Leptographium fungi that deteriorate the roots. The symptoms include shed yellow needles and thin crowns.

The control suggests:

  • removing declined trees;
  • planting more resilient species;
  • tackling drought stress.

Littleleaf Disease

The infection is common for shortleaf and loblolly pine and is caused by Phytophthora cinnamomi. The fungus is found worldwide, but it attacks hosts on infertile soils with excessive moisture. It damages tree roots and root hairs but can be detected only when it is visible in the upper tree part. Its symptoms are short pale-green needles, poor crown, slow twig growth, and multiple small cones.

Methods of control include:

  • thinning;
  • planting resistant species;
  • nitrogen fertilization.

Hardwood Root Diseases

Like any other tree root disease, rots are unnoticed as they happen under the soil surface. This complicates the diagnostics, and the outcome is often lethal.

These types of tree diseases include mushroom, white, and Texas root rots, with Amalleria mellea, Corticium galactinum, and Phymatotrichopsis omnivorum as causing agents correspondingly. They attack sensitive and weakened trees and cannot be cured.

The fungi remain in the soil for several years after infestations, so the next planting should be postponed for two to four years on average.

expansion
Scheme of how tree root disease expands

Tree Disease Identification And Treatment

It is important to identify tree disease as soon as possible to start its timely management and minimize losses. Remote sensing, specifically satellite monitoring, can help to detect the problem areas that are damaged, and particularly helpful to observe distant and hard-to-reach sites. The satellite images from LandViewer below show the forest area damaged by bark beetles, and how it spreads over three years without intervention.

satellite image of forest area damaged by bark beetles

Pathology control depends on the causing agent and degree of severity. Some tree diseases pass without any serious harm, and for some of them, there is no treatment at all. So it is essential to diagnose correctly and then choose the proper action plan.

The most typical methods of tree diseases treatment and management include:

  • Prevention. Fungicide leaf applications or trunk injections prior to bud burst. Besides, fresh-cut stumps should be sprayed to prevent fungi and bug infestations.
  • Sanitation and removal. Most tree leaf diseases are not cured but require foliage to be removed and destroyed in the fall to avoid further infections in the next season.
  • Pruning. Cutting damaged parts is a common technique to stop branch disease spread. However, it is efficient only when non-vital areas are affected. If the trunk is infected, the whole tree should be logged. Pruning dense leaves also improves air circulation.
  • Improving soil conditions. Plant susceptibility to infections depends on its overall health, so it is essential to improve soil fertility and minimize drought stresses or avoid excessive moisture to boost resistance. Healthy trees can often combat pathogens on their own.
  • Biological control. Bark beetles spoil trees and carry pathogenic fungi. The introduction of biological enemies reduces their populations (e.g., birds, mites, flies, wasps).
  • Chemical control. Spraying with fungicides, insecticides, etc., depending on the causing agent.
  • Planting resilient species that tolerate and adjust to pathogens.
  • Stand thinning and loose planting for better infection isolation.
  • Reforestation delay to ensure the complete decay of the infection source.

Forests and orchards suffer from thousands of tree diseases peculiar to each species. Some of them demand immediate treatment, and others have no cure at all. Any tree disease control, irrespective of its harm and severity, starts with proper monitoring. Further observation is needed to assess the damage scope, track the disease development, and make appropriate decisions. Remote sensing is an efficient method to get credible information on the fly and facilitate the most adequate and timely response.