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Hammered shield lichen (Parmelia sulcata)

Hammered shield lichen (Parmelia sulcata) is the most common and widely distributed parmelia. It is a grey or bluish-grey leaf lichen, sometimes covered with light frost. Lichen thallus comes in a variety of sizes and shapes and appears wrinkled. Its lobes have a reticulate venation, the grooves of which are formed by soralia – places where the organs of vegetative reproduction, or soredia, develop. The underside of the thallus is black, with dark brown rhizines – dense hyphae threads, with which the lichen attaches to the substrate. This parmelia grows mainly on deciduous trees, treated wood, but sometimes also on rocks. Lichen is moderately resistant to polluted air.

Common orange lichen (Xanthoria parietina)

The common orange lichen (Xanthoria parietina) is one of the rare species of lichen that tolerates polluted air, so it can be found in populated areas on the walls of old buildings, concrete fence posts, stone monuments, and other surfaces, but it also thrives in forests on the trunks of deciduous trees. Lichen thallus is similar to a greenish-yellow or orange, scaly bark. The common orange lichen likes nitrogen-containing compounds and well-lit areas. There are usually many rounded fruiting bodies or apothecia on the surface of the thallus. The colour of the thallus is affected by the growing conditions – in shady areas it is not as bright as in well-lit areas.

Hymenoscyphus fraxineus

Ash dieback is caused by the fungus Hymenoscyphus fraxineus, which was first scientifically described in 2006 under the name Chalara fraxinea. Chalara fraxinea was later discovered to be the asexual stage of the fungus Hymenoscyphus fraxineus. The fungus causes the leaves of ashes to turn black and wither, and long, dark brown spots to appear on the bark of the branches and trunk. Young trees die within a few years of infection, but old trees survive with disease symptoms. Fruiting bodies of the fungus are formed on the leaves of diseased trees that have fallen from the previous year, the spores of which infect the new leaves and twigs in the spring. Since diseased ash trees have a weakened root system, they are easily blown over by the wind.

Caused by several Heterobasidion sp. fungi, mainly Heterobasidion annosum

Annosus root disease is caused mainly by annosum root rot. There are two annosum root rot species in Latvia – Heterobasidion annosum, which infects mainly pines, and Heterobasidion parviporum, which infects mainly spruce and larch. Fungal root rot causes trees to weaken, fall over in the wind, be attacked by trunk pests and tree decline, as it interferes with the transport of water and nutrients in the plants. Root rot spreads when the roots of diseased trees come into contact with the roots of healthy trees, and the hyphae of the fungus move to and infect healthy trees. Basidiocarps of the fungus are shaped like half-plates and grow one on top of the other. They are found both on the trunks of living trees and on dead trees and stumps.

Scots pine blister rust

There are approximately 5,000 species of rust fungi known. All of them are parasitic fungi that live on trees and shrubs. Scots pine blister rust is caused by two species of rust fungi – Cronartium flaccidum and Peridermium pini. Signs of the disease are black, resinous sores on pine branches and trunk. The areas of the bark affected by the infection appear burned and are greyish-black in colour. The branches and trunk do not thicken in diseased areas, but the unaffected areas do thicken, and the pine develops a cancerous scar that increases each year. Most trees that grow in humus-rich soil become ill and die.

Caused by fungus Ophiostoma ulmi

Elms and lindens are infected by Ophiostoma ulmi, which has small, black, rounded fruiting bodies that grow under peeling tree bark or in passages made by wood-boring insects. The mycelium of the fungus develops in the bark and vascular tissue of the tree during the next stage of fungus development (conidial stage). This stage of development of the fungus is called Graphium ulmi, therefore the disease is also often called graphiosis. The tree withers and loses its leaves first, then the branches dry out and the tree dies. Fungus spores are mainly carried by the wind and small beetles – bark beetles (Scolytus sp.). Tree decline is also cyclical because these beetles reproduce in large numbers every 4–6 years.

Caused by fungus – alder phytophthora (Phytophthora alni)

The alder phytophthora fungus (Phytophthora alni) is common throughout Europe, but it had not spread in Latvia and Lithuania until 2016. It mainly affects black alders, less often white alders. Phytophthora-infected trees have smaller leaves than healthy trees, black spots form on the bark near the base of the trunk up to 3 m high. Sometimes the leaves turn yellow and the crown of the tree becomes thinner. As the disease progresses, trees usually die. If black alder trees grow in flooded areas, fungal spores enter the water and spread the disease.

Caused by 2 bacteria: Gibbsiella quercinecans and Brenneria goodwinii

Oaks that are less than 50 years old can suddenly dry out and die within 2–3 years. The disease is caused by bacteria that have only recently been discovered by scientists: Gibbsiella quercinecans and Brenneria goodwinii. In the spring and fall, a dark, sticky liquid runs down the trunk of the bacteria-infected tree. Dark, wet spots are visible if the bark is ripped off in these areas. Larvae of the two‐spotted oak buprestid beetle (Agrilus biguttatus) are almost always found under the infected bark. Dry weather and beetle larvae damage may also contribute to the death of infected trees. The disease is still being researched.