fungal disease

The “Sooty Bark Disease”, harmful for maples and humans, can be monitored by pollen sampling stations

The hyper-allergenic spores of the fungus causing Sooty bark disease in maples were detected in six European countries.

Sycamore maples destroyed by the Sooty bark disease. Photo by Dr Miloň Dvořák

Especially after the last few COVID-affected years, nobody doubts that emerging infectious diseases can threaten the whole world. But humans are not the only ones at risk! With intensive global trade, many tree parasites are accidently introduced to Europe in packaging or directly on goods. Traveling in the wood, on plants or in the soil of their pots, they can remain undetected for a long time.

“Forms of life of parasitic fungi are extremely diverse and very often practically invisible,” says Dr Miloň Dvořák of the Department of Forest Protection and Wildlife Management at Mendel University in Brno, Czechia. “An infected tree may look completely healthy for some time, which complicates the control of the disease enormously. It reminds me of the ancient Trojan Horse, where European trees are so surprised, defenceless, and later defeated, like the Trojan warriors.”

How can an infected tree look healthy and then suddenly get sick? “Like in the human body, in trees too, the trigger can be stress,” explains Dr Dvořák. The tolerance of trees to a pathogenic fungus turns lower under the conditions of changing climate and so the tree starts to die of the disease.

One typical example of such a disease is the Sooty Bark Disease (SBD) on maples, caused by a microscopic fungus called Cryptostroma corticale. “The fungus was probably introduced to Europe during the Second World War and for the rest of the 20th century we did not hear much about it,” says Dr Dvořák.

Sooty Bark Disease (Cryptostroma corticale) on Sycamore. Photo by gailhampshire used under a CC BY 2.0 license

The situation has changed and over the last twenty years the fungus has been reported more and more often. After dry and hot periods, the trees start to die of the infection, which is accompanied by the creation of brown-black masses of “soot” under the peeling bark of the maples.

The “soot” is in fact spores, which help the fungus spread and infect other trees. It is harmful for wounded trees, but it can also cause hypersensitivity pneumonitis in humans.

So, the species became a target for a group of phytopathologists gathered by an European HORIZON 2020 project entitled “Holistic management for emerging forest pests and diseases (HOMED)”. Scientists from six countries (Czechia, France, Italy, Portugal, Sweden and Switzerland), including Dr Dvořák, decided to develop a precise, DNA based (real-time PCR) diagnostic method to detect and monitor the pathogen in air samples. They published their method, the outcomes of its use, and their new findings about SBD epidemiology in the open-access journal NeoBiota.

Volumetric air sampler installed in Brno, Czech Republic, sampling pollen for allergen forecast. Photo by Aneta Lukačevičová

How to look for DNA in air samples? Simple devices called volumetric air samplers can suck the air against a piece of sticky tape, where every particle gets stuck and can be analyzed. “These devices are not really cheap, moreover, they demand regular maintenance,” explains Dr Dvořák. “But, actually, they are in common and regular use in the whole of Europe – remember the weather forecast, particularly that part about the “pollen report” for allergic people. This forecast is based on data of more than 600 stations united by the European Aeroallergen Network (EAN). Every station permanently maintains one volumetric air sampler and keeps an archive of the samples.”

The HOMED team got in contact with their national EAN collaborators and processed their samples with molecular techniques (real-time PCR).

Thanks to this sensitive detection method, the survey among samples was very successful. The “sooty” fungus was found in air samples from countries where the disease has been reported, and, in a more detailed study in France, the pathogen was found in the air 310km from currently diseased trees! This result suggests that the fungus can disperse long distances by wind.

Black stromata – source of billions of hyper-allergenic spores. Photo by Dr Miloň Dvořák

“Our results show that the SBD disease is at an exponentially increasing phase in France and Switzerland with an increase in the magnitude of the number of disease cases that peaks following a marked water deficit,” the researchers write in their study. They hope that early aerial detection of C. corticale in disease-free countries could help implement more efficient measures for SBD detection and eradication in the field.

“This European experiment fully confirmed the potential of this approach to monitor the pathogen’s outbreaks in early stages of its spread,” concludes Dr Dvořák.

Research article:

Muller E, Dvořák M, Marçais B, Caeiro E, Clot B, Desprez-Loustau M-L, Gedda B, Lundén K, Migliorini D, Oliver G, Ramos AP, Rigling D, Rybníček O, Santini A, Schneider S, Stenlid J, Tedeschini E, Aguayo J, Gomez-Gallego M (2023) Conditions of emergence of the Sooty Bark Disease and aerobiology of Cryptostroma corticale in Europe. In: Jactel H, Orazio C, Robinet C, Douma JC, Santini A, Battisti A, Branco M, Seehausen L, Kenis M (Eds) Conceptual and technical innovations to better manage invasions of alien pests and pathogens in forests. NeoBiota 84: 319-347. https://doi.org/10.3897/neobiota.84.90549

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Cultivated and wild bananas in northern Viet Nam threatened by а devastating fungal disease

For over 100 years, Fusarium, one of the most important fungal plant pathogens, has affected banana production worldwide.

Fusarium is one of the most important fungal plant pathogens, affecting the cultivation of a wide range of crops. All over the world, thousands of farmers suffer agricultural losses caused by Fusarium oxysporum f. sp. cubense (referred to as Foc for short), which directly affects their income, subsistence, and nourishment.

As a soil-borne fungus, Foc invades the root system, from where it moves into the vascular tissue that gradually deteriorates, until eventually the plant dies. What makes it particularly hard to deal with is that, even 20 years after all infected plants and tissue are removed, spores of it still remain in the soil.

One industry significantly affected by Foc is global banana export, largely dependent on the cultivation of members of the Cavendish subgroup, which are highly susceptible to some of the Foc strains.

For over 100 years, the fungus has affected banana production worldwide. Researchers predict it will continue spreading intensively in Asia, affecting important banana-producing countries such as China, the Philippines, Pakistan, and Viet Nam.

For Viet Nam, predictions on the impact of Foc for the future are dramatic: an estimated loss in the banana production area of 8% within the next five years, and up to 71% within the next 25 years. In particular, the recent rise of the novel TR4 strain has resulted in worldwide anxiety about the future of the well-known Cavendish banana and many other cultivars. Fusarium oxysporum f. sp. cubense is, however, not limited to TR4 or other well-known strains, like Race 1 or Race 2; it is a species complex that plant pathologists are yet to fully disentangle.

In Viet Nam, like in the rest of Asia, Africa, Latin America, and the Caribbean, most bananas are consumed and traded locally, supporting rural livelihood. This means that any reduction in crop harvest directly affects local people’s income and nourishment.

It has thus become necessary to find out what are the individual species causing the Fusarium wilt among Vietnamese bananas. Only by understanding which species are infecting the cultivated bananas can concrete measures be taken to control the future spreading of the disease to other regions.

Using DNA analyses and morphological characterization, an international team of researchers from Viet Nam (Plant Resources Center, Vietnam National University of Agriculture), Belgium (Meise Botanic Garden, KU Leuven, Bioversity Leuven, MUCL) and the Netherlands (Naturalis Biodiversity Center) investigated the identity of the Fusarium wilt infections. They recently published their joint research in the open-access, peer-reviewed journal MycoKeys.

The study shows that approximately 3 out of 4 Fusarium infections of the northern Vietnamese bananas are caused by the species F. tardichlamydosporum, which can be regarded as the typical Race 1 infections. Interestingly, Foc TR4 is not yet a dominant strain in northern Viet Nam, as the species causing the disease – F. odoratissimum – only accounts for 10% of the Fusarium infections. A similar proportion of Fusarium infections is caused by the species Fusarium cugenangense – considered to cause Race 2 infections among bananas.More importantly, Fusarium wilt was not only found in cultivated bananas: the disease seemed to also affect wild bananas. This finding indicates that wild bananas might function as a sink for Fusarium wilt from where reinfections towards cultivars could take place.

Research article:

Le Thi L, Mertens A, Vu DT, Vu TD, Anh Minh PL, Duc HN, de Backer S, Swennen R, Vandelook F, Panis B, Amalfi M, Decock C, Gomes SIF, Merckx VSFT, Janssens SB (2022) Diversity of Fusarium associated banana wilt in northern Viet Nam. MycoKeys 87: 53-76. https://doi.org/10.3897/mycokeys.87.72941

New pathogen threatens fennel yield in Italy

A new fungal genus and species Ochraceocephala foeniculi causes fennel yield losses of about 20-30% for three different cultivars. It damages the crops with necrotic lesions on the crown, root and stem.
International research group makes the first step in handling the new fennel disease by publishing their paper in the open-access journal Mycokeys.

A new fennel fungal disease caused by a new genus and species – Ochraceocephala foeniculi, was observed for the first time in 2017 on 5% of the “Apollo” fennel cultivar grown in the sampled localities in Catania province, Italy. Now, it has spread to 2 more cultivars: “Narciso” and “Pompeo”, causing crop losses of around 20-30%. The new pathogen damages the fennel with necrotic lesions on the crown, root and stem.

Fennel, a crop native in arid and semi-arid regions of southern Europe and the Mediterranean area is massively used as a vegetable, herb and seed spice in food, pharmaceutical, cosmetic and healthcare industries with Italy taking the world-leading production. It is an important and widely cultivated crop in Sicily (southern Italy).

Symptoms caused by Ochraceocephala foeniculi on fennel plants
Credits: Dalia Aiello
License: CC-BY 4.0

Worldwide, fennel crops are affected by several fungal diseases. In Italy, amongst soilborne diseases, there have been reports of brown rot and wilt caused by Phytophthora megasperma and crown rot caused by Didymella glomerata.

International research group, led by Ms. Dalia Aiello from the University of Catania, made the first step in handling the new fennel disease by identifying the causal agent obtained from symptomatic plants and publishing the results of their research in the open-access journal Mycokeys.

In order to understand the origin of the causal agent, scientists collected 30 samples during several surveys in the affected areas in Sicily, and studied the consistently grown fungal colonies from symptomatic tissues.

“The fungal species obtained from symptomatic tissues was identified based on morphological characters and molecular phylogenetic analyses of an ITS-LSU-SSU rDNA matrix, resulting in the description of the fennel pathogen as a new genus and species, Ochraceocephala
foeniculi,”
shares Dr. Dalia Aiello.

According to the pathogenicity tests, O. foeniculi causes symptoms on artificially inoculated plants of the same cultivar. Preliminary evaluation of fennel germplasm, according to the susceptibility to the new disease, shows that some cultivars (“Narciso”, “Apollo” and “Pompeo”) are more susceptible and some are less susceptible (“Aurelio”, “Archimede” and “Pegaso”), but this is a subject yet to be confirmed by additional investigations. More studies are required in order to plan further effective disease management strategies.

Holotype of Ochraceocephala foeniculi
Credits: Mr. Hermann Voglmayr
License: CC-BY 4.0

“On the basis of the disease incidence and severity observed in the field, we believe that this disease represents a serious threat to fennel crop in Sicily and may become a major problem also to other areas of fennel production if accidentally introduced,”
concludes Dr. Dalia Aiello.

***

Original source: Aiello D, Vitale A, Polizzi G, Voglmayr H (2020) Ochraceocephala foeniculi gen. et sp. nov., a new pathogen causing crown rot of fennel in Italy. MycoKeys 66: 1-22. https://doi.org/10.3897/mycokeys.66.48389

New fungus found to cause cankers and declines in pistachio trees in Sicily, Italy

Starting in the spring of 2010, farmers from Sicily – the major pistachio production area of Italy – have been reporting a previously unknown disease on the trees. Characterised by cankers and declines, it sometimes leads to the collapse of the entire plant.

When the research team led by Salvatore Vitale, Centro di Ricerca Difesa e Certificazione, Italy, studied plants from a total of 15 pistachio orchards in Catania, Agrigento and Caltanissetta provinces, they identified cankers associated with vascular necrosis and tree decline on twigs, branches and stems, alongside abundant gummosis. There were also localised, sunken lesions with several central cracks. These lesions would deepen into the woody tissue, where discolouration and necrotic tissue were also present.

Additionally, the scientists conducted a series of pathogenicity tests on 5-year-old potted pistachio plants (Pistacia vera), which successfully reproduced the field observations. As a result, a previously unknown pathogenic fungus, which colonises the woody plant tissue, has been isolated.

*Timelapse of the symptoms reproduced in a potted plant.*

The aetiology of the disease and the description of the new species, named Liberomyces pistaciae, are published in the open access journal MycoKeys. Despite cankers and subsequent decline of pistachio trees having been observed in Sicily for several years, the paper is the first work to successfully determine the causal agent.

“On the basis of the high disease incidence and the frequency of this species observed in several orchards in the last years, we believe that L. pistaciae represents amenace to pistachio production in Sicily,” say the researchers.

*Symptoms caused by the newly described pathogenic fungus observed in the field.*

Out of the 15 surveyed orchards, the scientists detected the presence of the fungus in ten of them. Most of the observations occurred in the winter period and during late spring, but the authors found the pathogen in asymptomatic trees as well, which suggests that the fungus has a dormant growth phase.

When already symptomatic, the plants begin to exudate gum. Often, the bark on their trunks and/or branches would scale, appearing as if cracking and peeling. The initial pale circular areas present in the bark turn dark and sunken with time. Later, the infected patches were seen to expand in all directions, yet faster along the main axis of the stems, branches and twigs. When the scientists examined beneath the bark, they saw discoloured and necrotic tissues. Once the trunk of the tree is encircled by a canker, they report, the whole plant collapses.

Other symptoms include canopy decline as well as wilting and dying inflorescences and shoots growing from infected branches or twigs.

The newly described fungus is characterised with slowly growing colonies. With time, they turn from white to pale to dark brown with a whitish slightly lobed margin.

The researchers warn that essential hazard for the further spread and promotion of the infection is the use and distribution of infected propagation material taken from nurseries and mechanical injuries or pruning wounds.

Further research and studies are currently in progress aiming to extend the survey to other areas in order to eventually formulate effective disease management strategies.

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Original source:

Vitale S, Aiello D, Guarnaccia V, Luongo L, Galli M, Crous PW, Polizzi G, Belisario A, Voglmayr H (2018) Liberomyces pistaciae sp. nov., the causal agent of pistachio cankers and decline in Italy. MycoKeys 40: 29-51. https://doi.org/10.3897/mycokeys.40.28636