The Hidden Threat: Tree Diseases – Causes, Types, and Ways of Dealing with Them

Forest diseases represent one of the biggest causes of global tree mortality which has dramatically increased worldwide. Environmental changes bring along extreme droughts and floods putting trees under stress and increasing their vulnerability. International trade aggravates the situation by making forests easy prey to non-native pathogens as they are unable to withstand attacks without a naturally developed resistance mechanism. 

While ground truthing remains the most accurate way of detecting sick trees at any stage of illness, remote sensing technology enables us to identify tree diseases’ signs at a greater spatial scale and within a shorter time. It gives us the upper hand in fighting and preventing tree diseases to preserve the world’s most precious forest biomes.

What causes forest diseases 

Tree disease refers to an infection or disturbance that causes abnormal structure or impaired growth of a tree. Some diseases are species-specific, while others can hit numerous types of trees. Sick trees may be recognized from reduced foliage, unusual color or shaping of leaves, crumbly wood and wilting foliage, bark abnormalities, bleeding cankers, etc. All of these can stem from infection caused by one of multiple pathogens, or the influence of abiotic factors. Thus, there are infectious and non-infectious causes of tree diseases. 

An infectious disease has several prerequisites: a tree susceptible to infection (host), a living organism – pathogen – able to cause infection, and favorable conditions for disease development. 

Environmental factors and human activities can also affect trees’ health directly. Weather extremes and human-caused air pollution are the most common stressors. Late spring and early autumn frost, sunscald, and winter drying are some of the natural phenomena causing damage to shoots, leaves, and bark of trees. While forested areas around industrial plants or croplands are often affected by fuming, agrichemicals, and “acid rain”. 

Common tree diseases

A single tree species can be host to hundreds of diseases. Infectious ones can be incited by various pathogens or parasites, such as fungi, bacteria, viruses, nematodes, and phytoplasmas. They also vary in symptoms, manifesting themselves as anything from leaf spotting to root decay to cankers. Often, two different diseases can show similar signs, challenging the process of tree disease identification.

Let’s look at some of the most typical tree ailments based on the type of pathogen.

Fungi usually produce spores that cause and spread infection when carried further by wind, water, or insects. Fungal diseases most often occur during wet seasons as they require an adequate amount of moisture to thrive.

1. Anthracnose

It is a group of fungal diseases that affect shade trees, such as sycamore, ash, and maple. Anthracnose usually starts on leaves and twigs of trees, causing leaf spotting and twig blight. The severity varies from unsightly looks or defoliation to tree death. Unlike most forms of this disease that require warm and humid conditions, Dogwood Anthracnose can develop in cool climates. 

2. Root Rot

This disease deteriorates a tree’s root system and impedes growth. Caused by multiple fungi species, such as Armillaria mellea and Ganoderma lucidum, root rot affects both hardwood and softwood tree types, at first manifesting itself as yellowing leaves, loss of vigor, and wilting. It happens because a host tree is unable to receive water and nutrients required for adequate growth. The most aggressive form, Armillaria root rot, can kill a healthy tree very quickly: when the infection reaches maturity, mushrooms emerge at the foot of a tree, signaling the coming death.

3. Oak Wilt

One of the most common tree diseases in oaks is incited by the fungus Ceratocystis fagacearum. It affects any oak species and is transmitted by insects and underground root connections. Oak wilt targets a tree’s main water “artery” – xylem – impeding the delivery of water from roots to leaves and branches. Infected trees can be recognized from crumbled and discolored leaves. The damage starts in the tree crown and gradually moves down affecting more and more branches until complete death. There are no known cures for oak wilt. 

In most scenarios, bacteria are beneficial to plant life, however, under certain conditions they also cause harm to trees. 

4. Fire Blight

It is a highly infectious and destructive ailment occurring in fruit trees, such as apples and pears.  Bees often transmit the infectious agent – bacteria Erwinia amylovora – during pollination and the first signs of infection are usually seen on new blossoms. Diseased leaves and twigs that look as if they’ve been scorched by fire speak for themselves. At later stages, cankers may show up, and in case of severe infection, fire blight might cause branch dieback and tree-kill.

Nematodes are tiny worms that feed on plant parts. Yellowing, wilting, and stunted tree growth might indicate the presence of these parasites.   

5. Root-Knot Nematode Galls

Root-knot nematodes feed off the tree’s nutrients and are recognized from the development of distinctive galls and root swelling. Most fruit and nut trees are susceptible to this disease. Above the ground, the infected tree may grow more slowly, produce smaller fruits, have yellowish foliage and wilting parts.

Prevention and cure for tree ailments

Early tree disease identification is the first step to planning effective treatment. However, it’s also important to take preventive action as a lot of known diseases can only be controlled, not treated. Here are some of the common techniques that help to keep trees healthy:

• Prevention: Spraying fungicides over leaf cover and freshly cut stumps can help fight off fungal pathogens.
• Selective planting: Planting tree species that are resilient to native pathogens is a good strategy for minimizing the risk of infections.
• Getting rid of the infected parts: In some cases, the only tree disease treatment possible is removing and destroying all the leaves, twigs, and other non-vital parts that have been infected, and protect the tree from getting infected the next season.
• Pruning: Cutting damaged branches or clearing overly dense foliage are forester’s best practices. However, when a trunk is infected, the tree should be felled to stop the spread of disease.
• Improving soil: Adequate soil conditions keep trees resistant to potential pathogen attacks. It’s necessary to maintain a healthy level of nutrients and moisture in the soil thus boosting the health of the trees that grow in it. 

EOSDA Forest Monitoring on guard of tree health

As tree diseases are globally on the rise, forest owners and managers are increasingly setting their sights on cutting-edge technologies. 

EOS Data Analytics develops satellite-based and AI-powered tools to make the extraction of valuable information from Earth observation data easier and more accurate. Its recently updated EOSDA Forest Monitoring platform is a custom solution for remote forest management that encompasses a wide range of features and promises great value to forestry businesses.

In the context of tree diseases, EOSDA Forest Monitoring can aid in monitoring tree health by assessing the vegetation state. Satellite data is used to distinguish between healthy and damaged leaf cover by the difference in the reflected radiation that sensors capture from space. By comparing the current vegetation index values with the historical ones, we can tell if forest health has improved or deteriorated and, in the latter case, take steps to further investigate the issue.

Another platform feature, Tree Species, can be instrumental in tree disease treatment and prevention. Considering that most diseases target certain species, an accurate map of tree types growing in your forest will add precision to health monitoring and disease prevention efforts. 

All solutions within the platform are scalable and custom-tailored, which makes it a viable tool for the timely detection of diseases, precise damage assessment, and effective treatment planning.