plant pathogens

More and more emerging diseases threaten trees around the world

A new study published in NeoBiota reveals a troubling trend: the rapid emergence of new diseases, doubling approximately every 11 years, and affecting a wide range of tree species worldwide.

Diseases are among the major causes of tree mortality in both forests and urban areas. New diseases are continually being introduced, and pathogens are continually jumping to new hosts, threatening more and more tree species. When exposed to novel hosts, emerging diseases can cause mortality previously unseen in the native range.

Tar spot on maple in Frostburg, MD, USA. Photo by Andrew V. Gougherty

Although not all diseases will outright kill their hosts, some can dramatically affect host populations. In the 20th century, chestnut blight, perhaps the most well-known tree disease in North America, effectively eliminated chestnut as an overstory tree in its native range in the Appalachian Mountains. More recently, we’ve seen sudden oak death in California, ash dieback in Europe, and butternut canker in the eastern US, each having the potential to eliminate host tree populations and alter the ecosystems where they occur.

“The continued emergence and accumulation of new diseases increases the likelihood of a particularly detrimental one emerging, and harming host tree populations,” says Dr Andrew Gougherty, research landscape ecologist at the USDA Forest Service. Recently, he has been exploring where tree diseases have accumulated fastest, and which trees are most impacted by new diseases. This information could help researchers and land managers better predict where new diseases may be most likely to emerge.

Powdery mildew on maple in Vancouver, BC, CAN. Photo by Andrew V. Gougherty

The study, recently published in the open-access journal NeoBiota, analyzes over 900 new disease reports on 284 tree species in 88 countries and quantified how emerging infectious diseases have accumulated geographically and on different hosts. “The ‘big data’ approach used in this study helps to characterise the growing threat posed by emergent infectious diseases and how this threat is unequally distributed regionally and by host species,” the author writes.

Dr Gougherty found that globally, the number of emerged diseases has accumulated rapidly over the past two decades. “The accumulation is apparent both where tree species are native and where they are not native, and the number of new disease emergences globally were found to double every ~11 years,” he explains. Among the trees he assessed, pines accumulated the most new diseases, followed by oaks and eucalypts. This, he explains, is likely due to their wide native distribution in the Northern Hemisphere, and the planting of pine forests throughout the globe. Europe, in aggregate, had the greatest total accumulation of new diseases, but North America and Asia were close behind.

In addition, he found more emerging tree diseases in areas where tree species were native versus non-native, with the exception of Latin America and the Caribbean, likely because most of the trees he assessed were not native to this region.

“Unfortunately, there is little evidence of saturation in emergent tree disease accumulation. Global trends show little sign of slowing, suggesting the impact of newly emerged diseases is likely to continue to compound and threaten tree populations globally and into the future,” warns Dr Gougherty. “Climate change is likely also playing a role, both by creating more favourable conditions for pathogens and by stressing host plants.”

Research article:

Gougherty AV (2023) Emerging tree diseases are accumulating rapidly in the native and non-native ranges of Holarctic trees. NeoBiota 87: 143-160. https://doi.org/10.3897/neobiota.87.103525

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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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Study ranks potentially harmful invasive species in Ghana

Scientists ranked the 110 arthropod and 64 pathogenic species posing the greatest potential threat to the country if established.

A CABI-led study has conducted a comprehensive survey of nearly 200 potentially harmful alien plant species that could have a detrimental impact upon agriculture, forestry and biodiversity in Ghana once they enter the country.

Invasive Alien Species (IAS) continue to shape the global landscape through their effects on biological diversity and agricultural productivity. The effects are particularly pronounced in Sub-Saharan Africa, which has seen the arrival of many IAS in recent years. This has been attributed to porous borders, weak cross border biosecurity, and inadequate capacity to limit or stop invasions.

A farmer shows cassava root affected by cassava brown streak virus alongside a healthy root in a country where the disease is present – one of the 64 pathogens assessed by the scientists. Credit: CABI

The research, the findings of which are published in the journal NeoBiota, ranks 110 arthropod and 64 pathogenic species that pose the greatest threat but are not yet officially present in the country. However, they could arrive as ‘stowaways’ in cargo from other countries around the world, the scientists believe.

Dr Marc Kenis, Head Risk Analysis and Invasion Ecology at CABI, led on the horizon scanning exercise supported by colleagues from a range of institutions including Ghana’s Plant Protection and Regulatory Services Directorate (PPRSD).

Among the top arthropods prioritised by Dr Kenis and his team were the pink hibiscus mealybug (Maconellicoccus hirsutus Green) and melon thrips (Thrips palmi Karny) while the top pathogens highlighted include cassava brown streak virus and Maize lethal necrosis disease.

Cassava in Ghana, for example, is a main staple crop and contributes about 22% and 30% to the Agricultural Gross Domestic Product (AGDP) and daily calories intake respectively. The crop, however, can be at risk from cassava brown streak virus which can reduce yields by up to 70%.

Maize lethal necrosis disease, on the other hand for instance, can be a major disruptor of maize crops in Ghana where maize accounts for more than 50% of the country’s total cereal production. The disease can cause losses of between 50-90% depending on the variety of maize and the growing conditions of the year.

The scientists also found other species recorded in Africa that included 19 arthropod and 46 pathogenic species which were already recorded in the neighbouring countries of Burkina Faso, Côte d’Ivoire, and Togo.

Dr Kenis, who is based at CABI’s centre in Switzerland, said, “The ultimate objective of this research was to enable prioritization of actions including pest risk analysis, prevention, surveillance and contingency plans. Prioritisation was carried out using an adapted version of horizon scanning and consensus methods developed for ranking IAS worldwide.

“We have demonstrated that through horizon scanning, a country can identify potential invasive plant pests, both invertebrates and pathogens, and use the information to determine the risk associated with each.

“This will enable the country to invest the limited resources in priority actions such as preventing arrival and establishment of IAS, Pest Risk Analysis (PRA), surveillance and developing contingency plans.

“This study can serve as a model for future projects on plant pests’ prioritisation in Africa and elsewhere. It would be applicable for assessing the risk of invasive plant pests in any country or region, e.g. trade blocks, with minor modifications of the method, particularly in the mini-PRA protocol used to score species.”

The full lists of arthropod and pathogenic species surveyed can be found within the full paper which can be read online.

Mr Prudence Attipoe, Deputy Director Head Plant Quarantine Division, PPRSD, said, “The horizon scanning exercise for Ghana would give the PPRSD an insight into invasive pests which could possibly enter the Nation. The tool is timely and appropriate for conducting PRA for planning, training and future preparedness. The success of this exercise would pre-empt the introduction of these invasive pests into the country in order to protect Ghana’s agriculture, forestry and also cause staff of PPRSD to be more vigilant at the borders for these pests.”

Research paper:

Kenis M, Agboyi LK, Adu-Acheampong R, Ansong M, Arthur S, Attipoe PT, Baba A-SM, Beseh P, Clottey VA, Combey R, Dzomeku I, Eddy-Doh MA, Fening KO, Frimpong-Anin K, Hevi W, Lekete-Lawson E, Nboyine JA, Ohene-Mensah G, Oppong-Mensah B, Nuamah HSA, van der Puije G, Mulema J (2022) Horizon scanning for prioritising invasive alien species with potential to threaten agriculture and biodiversity in Ghana. NeoBiota 71: 129 148. https://doi.org/10.3897/neobiota.71.72577

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

Survival of soil organisms is a wake-up call for biosecurity

Tiny creatures in soil that attack plants have shown the ability to survive for at least three years stored in dry conditions in a recent AgResearch study, giving new insights into the biosecurity threats posed by passenger travel and trade between countries. The research article is published in the open access journal Neobiota.

The findings of the study also add to the discussions about how best to detect these creatures, called nematodes, before they cross borders and potentially reduce yields of important crops and pasture.

Nematodes are very small worm-like organisms. They can be extremely hardy and can have both beneficial and detrimental impacts. The harmful ones, the plant parasitic nematodes (PPN) include species that attack plants reducing their growth and survival.

In the study, funded by AgResearch via the Better Border Biosecurity collaboration, soil collected from a native forest and an organic orchard was stored separately in cupboards at room temperature for a period of 36 months.

Samples were then taken at regular intervals to see if any nematodes could be recovered from the soil and, if they could, whether they were able to infect plant hosts.

“In the study we used different methods to detect nematodes — including a water misting technique to draw them out of the soil, and a baiting method — where we grew white clover and ryegrass plants in pots containing a soil sample,” explain the authors.

“One of the PPN we looked at was the root lesion nematode. What we found was that lesion nematodes were able to successfully invade the roots of ryegrass even after 36 months,” says AgResearch nematologist Lee Aalders.

“They were also able to produce offspring at 13 months. Interestingly, no PPN were recovered from soil stored beyond the 13th month using the three-day misting technique.”

This means that given the right conditions, PPN in soil, which is carried on sea freight, footwear or used machinery, and protected from sun or extreme heat, will survive if they end up near a suitable host plant. This is a result that may not be detected using an extraction test like misting.

For quarantine officials around the world, this result is an important find, as it reinforces the risk associated with soil that, even though it may look sterile, unwanted nematodes may be present and undetected until paired with a suitable host plant.

“In the context of biosecurity, we think that the development of a generic test for plant parasitic nematodes – based around a molecular based bioassay — would enhance the probability of detection of PPNs and, therefore, prevent unwanted incursions beyond the border.”

Earlier this year, another AgResearch study into the survival rates of various transported soil organisms and published in Neobiota concluded that biosecurity risks from soil organisms are to increase with declining transport duration and increasing protection from environmental extremes.

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Original source:

Aalders LT, McNeill MR, Bell NL, Cameron C (2017) Plant parasitic nematode survival and detection to inform biosecurity risk assessment. NeoBiota 36: 1-16. https://doi.org/10.3897/neobiota.36.11418