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Many species of fungi can harm trees in forests

The primary factor that indicates and predicts the attack of forest stands by fungal pathogens is the weather in the spring months. Higher temperatures and lower rainfall can lead to the development of complex diseases and wood decay fungi, such as root rot. Extremely high spring temperatures can speed up the onset of many diseases, particularly mildew and rust, by up to 1-2 months.

If the trend of consistently higher temperatures and more frequent extreme temperature and precipitation events continues in the coming years, we may continue to see a weakening of tree species. This could increase the occurrence and significance of diseases and pathogens associated with this weakening, especially honey fungus, root rot, complex diseases, and white mistletoe.

Photo: massive occurrence of pine blight (Jindřichův Hradec region, May 2023), VÚLHM archive

The prevalence of fungal pathogens in our forests in 2023 varied from previous years. During the spring months and early summer, their occurrence was relatively high and expected due to high air humidity and suitable temperatures. However, in the second half of the year, fungal pathogens were observed less frequently due to high temperatures and lower rainfall. Nevertheless, the significance of complex diseases associated with drought stress has increased.

The data in the press release regarding the volume of wood or the area of stands affected by fungal pathogens represent the total from reports sent by forest owners to the Forest Protection Service VÚLHM, v. v. i. These data cover approximately two-thirds of the forest area in the Czech Republic.

In 2023, the incidence of pine blight was initially very similar to the previous year. The Lophodermium genus was the most frequently found on pine; i. e. species L. pinastri and L. seditiosum, covering an area of approximately 1.1 thousand hectares nationwide (compared to 1.2 thousand hectares in 2022). The massive occurrence was mainly observed in southern Bohemia.

The pine blight (Cyclaneusma minus) was also abundant, often on drying pines. In case of Douglas fir, the Swiss and Scottish blight (Nothophaeocryptopus gaeumannii and Rhabdocline pseudotsugae) and the spruce blight (Lophodermium piceae) on Norway spruce were represented there, too.

Photo: Detail of pine blight (Jindřichův Hradec region, May 2023), VÚLHM archive

After a long time, multiple occurrences of pine needle rust (Coleosporium tussilaginis) on pine trees have been identified. This fungus has elderberry, sedum, or related herbs as its secondary hosts. In several instances, the infestation had a nearly catastrophic impact.

In early September, several types of rust became abundant. This included poplar rust (Melampsora larici-populina) on poplars, crown rust (Puccinia coronata) on alder buckthorn, and pear rust (Gymnosporangium sabinae) on junipers and pears. These rusts affect different host plants, such as larches, grass, and junipers.

Last year, oak mildew (Erisyphe alphitoides) was the most common cause of leaf spots, affecting approximately 735 hectares (compared to approximately 840 hectares in 2022).

The incidence of wood decay fungi was similar to previous years, but it may increase significantly due to long-term drought, especially on former agricultural land.

The most significant representatives were the honey fungus (Armillaria spp.), particularly in the case of Norway spruce with A. ostoyae, and in the case of conifers (mostly spruce, pines, and firs), as well as honey fungus on deciduous trees, especially ash trees, oaks, and poplars. In 2023, approximately 123,000 m3 of honey fungus wood were reported nationwide, which is an increase of about 8,000 m3 compared to 2022.

Occurence of root rot (Heterobasidion spp.) was important in 2023 too, especially on former agricultural land of the first generation.

Photo: Ashes infested by necrosis (Lomnice and Příbram regions, VULHM archive)

On dry spruces and firs, there is still a lot of hairy stereum (Stereum hirsutum), as well as tinder fungus (Fomitopsis pinicola), which was also very often recorded as a parasite in spruce and fir stands.

Apart from honey fungus, there were found also found sedges and fireflies (Phellinus spp.), rusts (Inonotus spp.), and root rot (Kretzschmaria deusta) on deciduous trees. In the case of common birch, there often was found fungi Piptoporus betulinus, together with Ganoderma applanatum fungi on deciduous trees as well.

Complex diseases

Necrotic diseases were frequently observed in 2023. The decay of ash trees caused by Hymenoscyphus fraxineus (also known as Chalara fraxinea) was often accompanied by root rot agents such as Armillaria spp. and Ganoderma applanatum. In total, approximately 44,000 m3 of wood matter were recorded in ash stands affected by this complex withering.

In 2023, the sooty bark disease, caused by the fungus Cryptostroma corticale, was monitored. It was mainly observed on sycamore maples in the region of Ústí nad Labem. This disease was often accompanied by the fungus Prosthecium pyriforme.

In the Ore Mountains, there were numerous instances of die-offs of whooping wild service trees. These die-offs were associated with necrosis, the presence of fruiting bodies of fungi belonging to the genus Cytospora, and secondary infestation by the honey fungus (Armillaria sp.).

Photo: Dying larches in non-secured young stand (Olomouc region, June 2023), VÚLHM archive

The dieback of young larch trees, which emerged after many years in 2022, significantly increased. Throughout May and June 2023, the Forest Protection Service handled numerous instances of this decline, particularly in Silesia, Moravia, and the Bohemian-Moravian Highlands.

The most frequently affected trees were larches in young forests. The issue was observed in young stands with saplings typically between 1-5 meters high, from both artificial planting and natural regrowth. The affected larch trees showed symptoms such as rusting of needles (often before complete sprouting), gradual shedding of needles, death of individual branches or entire crowns, and in some cases, only the lower branches remained green.

Larch stems typically displayed uneven growth, noticeable resin leakage, cracks, necrosis, and the presence of fruiting bodies of the fungus Fusarium fujikuroi (formerly known as Fusarium subglutinans, which is one of the causes of necrotic lesions). Additionally, other fungi were occasionally present.

The larch trees had a lot of sucking insects, mainly the green gall aphid (Sacchiphantes viridis) and to a lesser extent the larch woolly aphid (Adelges laricis). There were also ants present due to the leaf beetles.

The die-off of the larch trees is likely caused by a combination of factors including the weather conditions, attacks by rusts, and fungal pathogens.

The dieback of oak stands remains at the same level as last year. Fungi of the genus Ophiostoma, related to tracheomycosis, were found on the withering trees. This is confirmed by the report of extracted wood (approximately 1,000 m3).

Photo: left – a dying larch in an unsecured culture (Olomouc region, June 2023), right – a dying larch in a mixed young stand (Bruntál region, June 2023), VÚLHM archive

Pathogens of the genus Phytophthora were observed to a similar extent as in 2022. For example, alder mold (Phytophthora alni complex) was observed again after two years in the Loděnice area.

The genus Phytophthora remains probably the biggest risk in the category of complex diseases and invasive pathogens.

The situation with the drying and dying of pine forests has worsened dramatically due to high summer temperatures and low rainfall. In August, a significant die-off of Scots pine was observed in the Middle Povltaví region. Additionally, abundant occurrences of the pine blights Sphaeropsis sapinea and Cyclaneusma minus were recorded here.

Bud blight (Gemmamyces piceae) may still be observed in its latent phase even in 2023 at the epicenter of its occurrence in LS LČR Klášterec and Litvínov. Around 6,000 m3 of infected wood have been reported, particularly in the districts of Most and Chomutov. However, the actual amount is much higher, surpassing the levels monitored in recent years, across the entire Ore Mountains.

Processed according to data from the publication Forest Protection Bulletin, Supplementum 2024, which can be downloaded here.

Prepared by Ing. Jan Řezáč, VULHM, v. v. i., e-mail: rezac@vulhm.cz