170 years after the famine, the late blight of potatoes remains

Dundee, Scotland, 1861

Despite the tools available to combat and control plant disease, the pathogen which caused the Irish famine continues to destroy potato crops worldwide


The famine wasn’t that long ago. I can trace my family back to Peter Lettice and his wife Mary Lowrie who left Ireland in the early 1840s, in their case for Dundee, Scotland, to avoid starvation. Many people can do the same. Knowing their names means that the headline figures that get used in connection with the famine - one million dead and one million emigrated - become very personal. Those figures get used whenever anybody talks about the famine, but they make the whole thing anonymous in a way. It's something that happened to other people and their families.

The massive global changes brought about by the famine are still evident in the large number of people claiming Irish heritage in North America, Australia and elsewhere. At home, the population of the island of Ireland (approximately 6.5 million in 2016) has only now returned to pre-famine levels.
Historians can rightly point to many contributing factors and causes for the famine. Political, social and economic issues all played a role, but the cause of the crop losses at the heart of the Irish potato famine ultimately was Phytophthora infestans. This pathogen comes from a group of organisms called oomycetes and can no longer be correctly called a fungus. In fact, it’s more closely related to the brown algae.

The ‘father’ of plant pathology, Anton de Bary, was the first to demonstrate experimentally that the pathogen caused the disease we now know as late blight and de Bary coined the name Phytophthora, meaning "plant-destroyer". English botanist Rev. Miles Berkeley had first observed that late blight was "the consequence of the presence of the mould, and not the mould of the decay" 15 years earlier (Journal of the Horticultural Society of London, 1846).

Phytophthora is an appropriate moniker. Symptoms of the disease include blackish lesions on the leaves and purple-brown lesions on the surface of the tubers themselves. When the disease is advanced, the tubers are rotten inside and there is a distinctive odour which must have struck fear into the heart of poor subsistence farmers all over the country during the famine.

As any potato grower will tell you, late blight of potatoes has not gone away. It remains the most economically destructive of all potato diseases worldwide. Typically, commercial potato growers in Ireland use between 15 and 20 applications of fungicide to control the pathogen every year and there are no commercially-viable resistant varieties available. 170 years after the famine, our potato crop is still as vulnerable as ever to destruction caused by Phytophthora. The difference now is the availability of chemical control options to keep the worst of the losses at bay.

Globally, many crops are vulnerable to diseases that have the potential to cause devastating losses. For example, rice blast fungus (Magnaporthe oryzae) is the most destructive disease of rice, a staple foodstuff that feeds half the world’s population. Diseases of cereals like Puccinia and Fusarium are a threat that require constant vigilance and we are regularly reminded that the much-loved and economically important Cavendish variety of banana (that’s the banana you had for lunch) is on the brink of extinction due to Panama disease caused by Fusarium oxysporum.

A major problem is our over-reliance on a small number of crops for much of the world’s food supply. Just 15 crop plants account for 90 percent of the world’s food with maize, wheat and rice accounting for over 50 percent of the world’s caloric intake (UN FAO). If even one of the top ten crops were to fail, the consequences could be catastrophic, especially for developing countries.
Increasingly though, we are running to stand still with a lot of the major diseases. Much like the Red Queen in Lewis Carroll’s Through the Looking-Glass, keeping one step ahead of emerging and evolving plant pathogens "takes all the running you can do, to keep in the same place".

In this context, it’s more important than ever that we are using all of the tools available to us to combat and control plant disease. That means an integrated approach to pathogen and pest management where one tool such as chemical control is not over-used. Such reliance on one control method runs the risk of forcing the pathogen to evolve to overcome the control measure, rendering it useless.

One of the tools that will certainly be in that toolbox is the development of resistant varieties. However, in the case of late blight, we’ve yet to breed a commercially-viable, fully blight-resistant potato. That’s not to say it’s impossible: Sarpo Mira, Sarpo Axona and Blue Danube are all potato varieties that are very resistant to late blight but they have not been commercially successful outside of the organic market.

Luckily, help is at hand in the form of modern plant biotechnology which has the capacity to quickly develop blight resistant potato varieties as well as resistant crops to various other diseases) A major problem with conventional potato breeding is the difficulty in crossing domesticated varieties with their disease-resistant wild relatives. Genetic transformation has overcome that problem by transferring a potato gene for resistance from wild to cultivated varieties. Such varieties were grown successfully in Ireland in recent years.

Gene-editing technology will allow even more precise changes to be made to plant genomes with the goal of introducing resistance for a host of important crop diseases. Whatever our personal views on such technologies, there is no doubt they will be an integral part of maintaining global food security and preventing future famines.

Recently, Ireland was named the most food-secure nation in the world. That’s an amazing turnaround, even if it has taken 170 years. In light of our remarkably journey from famine to world leaders in food security, surely there is a moral imperative on us to support other countries to boost their food security - and to advance the science that will prevent similar famines from happening to other countries in the years to come?

The National Famine Commemoration 2018 takes place at University College Cork on Saturday May 12th. The International Association for Plant Biotechnology congress 2018 (IAPB2018) takes place in Dublin in August

This article first appeared on RTE Brainstorm.

Famine and food

The ongoing debate regarding the efficacy of genetically modified (GM) crops to increase global food security goes on, while a recent study of US consumers indicates that opinions on genetically modifies crops are not swayed by specific arguments about plant disease and famine.

At least one million people died and a further one million were forced to emigrate during the Irish potato famine of 1845-1852. Those figures are so often repeated in undergraduate plant pathology classrooms that they lose their shock value. Those years seem so distantly removed from our lives in the 21st century that we occasionally fail to recall that it happened just a handful of generations ago. The famine had such a profound impact on the social, geographic and economic landscape of Ireland that the country still bears the scars. For instance, the population of the Republic of Ireland, as measured by the 2011 census, stands at 4.6 million compared to a pre-famine population of 6.5 million in 1841. Meanwhile, there are 39.6 million Americans who claim Irish ancestry, due in part to massive emigration to the US during and immediately after the famine.

In a recently published study, American consumers were asked their opinion of GM. Half of the sample group were first asked to read a short vignette describing the causal agent of the potato famine, the fungal-like potato disease late blight. The second half of the sample was asked to read a similar vignette, though not mentioning late blight and the Irish famine specifically.

For example, the late blight-specific vignette included:
"Late blight was a key cause of the Irish Potato Famine of the 1850s that led to the starvation of millions of people in Ireland and forced many Irish to leave the country. Late blight has re-emerged in recent years as a substantial threat to crops across the United States and around the world."

Ultimately, even when the question was contextualised in relation to late blight and famine, there was no significant difference in public views about the perceived risks, benefits or fairness of GM crops. This is an interesting finding; given calls in Europe and elsewhere to increase the cultivation of GM crops, particularly in traditionally GM-sceptical nations such as the UK.
This year, for example, the Council for Science and Technology in the UK, scientific advisors to the government, called for the EU to end its “dysfunctional” regulations on GM crop cultivation saying that if the country didn’t embrace GM “the risk is people going unfed”.

Even in Ireland, where one might expect the memory of the famine to linger long with consumers, limited trials of late blight resistant potato plants in recent years have met with some resistance. These EU-funded trials, conducted by Ireland’s agricultural development agency were described as “economic suicide” by opponents who called GM an “unwanted technology”. The scientists conducting the trials, which began in 2012, were keen to stress the impartial nature of the study and that it was not about “testing the commercial viability of GM potatoes” and was specifically concerned with their environmental impact.

In fact, there is a myriad of reasons why some consumers reject GM technologies in foodstuffs. Not all of them, of course, are supported by any real science, but that doesn’t negate the fact that they are real obstacles to overcome for those who would promote a sustainable food-production system which incorporates all aspects of biotechnology, including genetic modification of crop plants. What is clear now is that simply using the approach of emphasising the crop protection benefits of GM is not enough. Consumers are, rightly or wrongly, also worried about the environmental impact of such crops and no amount of appealing to their memory of past catastrophic crop failures will appease them.

One might argue that the passing of time between the Irish potato famine and the current advances in plant biotechnology can account for the lack of relevance and impact on consumer opinion. Perhaps, informing consumers about more recent plant disease outbreaks would be more beneficial.  One could point to the Bengal famine of 1943, when an estimated 2 million people died when the rice crop was attacked by a fungal pathogen. In truth, the vast bulk of food for human consumption worldwide is provided by just fourteen crop plants. Failure of any one of these could have a significant impact on global food security. However, we are in a very dark place indeed if we must look for a catastrophic crop failure to remind consumers of the value of plant biotechnology in protecting our food supply.

Potato Genome Sequenced

The potato genome has just been published in the journal Nature.

The Potato Genome Sequencing Consortium (PGSC), an international team of scientists including some from Teagasc in Ireland, has published the "genetic blueprint" for the world's third most important food crop.

Scientists say the information will help plant scientists and breeders to improve yield, quality, nutritional value and disease resistance pf potato varieties. The PGSC say it should also allow potato breeders "reduce the 10-12 years currently needed to breed new varieties".

The potato genome is the first sequence of an Asterid to be published - a group of flowering plants encompassing around 25% of all known plant species.

The consortium published a draft sequence in 2009 after a meeting of the consortium in Oakpark, Carlow in Ireland to plan the final phases of the project. The most recent publication is a refined version covering approximately 95% of all the genes in the potato - around 39,000 genes that code for proteins.

Worldwide, it's estimated that a loss of about €3 billion per year in the potato crop arises from diseases such as late blight and potato cyst nematodes. These problems are still largely controlled by frequent applications of fungicides and nematicides.

An indepth knowledge of the genetics of the potato should allow scientists to develop new varieties which show high levels of resistance against these pests and diseases.

The potato genome has 12 chromosomes and an extimate 840 million base pairs.

Speaking on the release of the first draft, Professor Jimmy Burke, head of Teagasc Crops Research Centre and leader of the plant biotechnology programme said: “Research such as this is incredibly important to the future competitiveness of Irish agriculture and puts Teagasc at the forefront of exciting developments in science”.

Combining our expertise in plant breeding with cutting-edge biotechnology-based research is enhancing our ability to develop plant varieties suitable for Irish conditions and agricultural practices. We are pursuing similar projects in other species important to agriculture here in Ireland, including perennial ryegrass, white clover and wheat, and energy crops, and we expect similar successes in these species in the future”.


The Nature paper is available here.
The full sequence is available here.

Christmas Science 18: New Mistletoe Species Discovered

In the run up to Christmas, Communicate Science offers you 20 Christmas Science Facts. We'll post one every day until the 25th December.

New mistletoe species among this year's new discoveries at Kew

As the UN's International Year of Biodiversity draws to a close, scientists at the Royal Botanic Gardens, Kew are celebrating the diversity of the planet's plant and fungal life by highlighting some of the weird, wonderful and stunning discoveries they've made this year from the rainforests of Cameroon to the UK's North Pennines. But it's not just about the new - in some cases species long thought to be extinct in the wild have been rediscovered.

Professor Stephen Hopper, Director of the Royal Botanic Gardens, Kew says, "Each year, botanists at the Royal Botanic Gardens, Kew, working in collaboration with local partners and scientists, continue to explore, document and study the world's plant and fungal diversity, making astonishing new discoveries from microscopic fungi to canopy giants. 

"This work has never been more relevant and pressing than in the current era of global climate change and unprecedented loss of biodiversity.Without a name, plants and fungi go unrecognised, their uses unexplored, their wonders unknown.

"On average, 2,000 new plant species are discovered each year, and Kew botanists, using our vast collection of over 8 million plant and fungal specimens, contribute to the description of approximately 10 per cent of these new discoveries. Despite more than 250 years of naming living plants, applying each with a unique descriptive scientific name, we are still some decades away from finishing the task of a global inventory of plants, and even more so for fungi.

"Plants are at risk and extinction is a reality. However stories of discovery and rediscovery give us hope that species can cling on and their recovery is a very real possibility. Continuing support for botanical science is essential if plant based solutions to human challenges, such as climate change, are to be realised."

This year's new showstoppers include;

From Africa with Love - Wild Mozambican Mistletoe …

This parasitic, tropical mistletoe was named in 2010, and was first discovered near the summit of Mount Mabu in northern Mozambique, a region which hit the headlines in 2008 when a Kew-led expedition uncovered this lost world bursting with biodiversity. Since then, the team at Kew have worked tirelessly sorting through the hundreds of specimens they collected, and they have described this new wild mistletoe (Helixanthera schizocalyx), just in time for Christmas!

It was spotted by the expedition's renowned East African butterfly specialist, Colin Congdon, while the team were trekking up the mountain, on a path that took them from the moist montane forest up to where the broad granite peaks break through the dense foliage. Colin quickly realised this species was different from anything he had seen on the mountains in neighbouring Malawi and Tanzania, and on closer inspection back at Kew it was confirmed a new species.

Tropical mistletoes, from the family Loranthaceae, are a great example of biodiversity and the symbiotic relationship between plants and animals. Birds play a vital role in both pollinating these mistletoes, and also distributing the seeds. As birds eat the small fleshy white sweet fruits, the seeds are then wiped on a branch to which they adhere. Once germinated the root grows into the living tissue of the tree to obtain the new plant's nutrients. Tropical mistletoes are also popular with butterflies and in particular the blue group Lycaenidae. These strong links between the plants, their host trees, and various birds and butterflies, shows the interconnected nature of forest species, and the need to conserve all elements in order to preserve the environment.


A Medicinal Wild Aubergine from East Africa…  

Commonly known as 'Osigawai' in the local Masai language, Solanum phoxocarpum was discovered by Maria Vorontsova on an expedition to Kenya's Aberdare mountainous cloud forests. Having researched specimens of wild African aubergines in RBG, Kew's vast Herbarium collections of dried plant specimens, Vorontsova, who was based at the Natural History Museum, London at the time, discovered some unusual unnamed specimens, some of which were unlike any she had seen before. Eager to discover more, Maria set out on an expedition with botanists and seed hunters from Kenya's 'Seeds for Life' project team, partners in Kew's Millennium Seed Bank Partnership.

Many of the old collection locations they visited had been stripped of native vegetation, but after four weeks, the team was successful. They spotted a wild aubergine shrub with distinctive unusual long, yellow, pointed fruits and deep mauve flowers that was indeed a new species. They collected its fruits and set out slicing them open to collect seed for banking. While spreading the fruit's yellow sludge onto paper, so the seeds could dry for long term storage in Kew's Millenium Seed Bank, one of the team noticed that the fruits began to emit a pungent odour and later that day they became ill. It is now believed that this species may be poisonous, and having consulted Kew's historic specimens, it also proves to be used medicinally by local people.


Cameroon Canopy Giant… 

A gigantic tree, Magnistipula multinervia, described excitedly by Kew's well seasoned plant hunter, Xander van der Burgt, as "the rarest tree I have ever found", has been discovered in the lush green rainforests of Cameroon.

Towering above the canopy at 41metres high this critically endangered tree was discovered in the lowland rainforests of the Korup National Park — a hot-bed for new discoveries in the South-West Province of Cameroon. Due to its height, rarity (with only four trees known) and the fact that the flowers hardly ever fall to the ground, it proved difficult to identify and collect in flower. After numerous visits to the four known trees over a period of several years to check if they were flowering and fruiting, the team were successful and using alpine climbing equipment, they managed to scale the dizzy heights, and make their collection, and identify it as new.

Smut and moon carrots - the rediscovery of extinct British fungi… 

The long-lost British fungus, bird's-eye primrose smut (Urocystis primulicola), recognised as a species of "principal importance for the conservation of biological diversity" (BAP review 2007) had not been seen for 106 years until it was rediscovered by Kew and Natural England mycologist, Martyn Ainsworth, during a two hour 'ovary squeezing' session.

Smuts are species of inconspicuous, microscopic fungi that are found inside living host plants, in this case the red-listed wild pink flowered bird's-eye primrose (Primula farinosa) found in the North Pennines. The bird's-eye primrose smut has co-evolved with the plant and hijacks its ovaries, replacing its seeds with a black powdery mass of smut spores. Concealed in the ovaries, it is only when the bird's-eye primrose seed-pods are squeezed in the late summer, when the seeds are ripe, that this rare smut can be found.

In a similar story, the moon carrot rust (Puccinia libanotidis) was rediscovered in England after it was believed lost for 63 years. Rust fungi are so called because their spores are often produced in brownish orange powdery masses on the leaves and stems of host plants. The moon carrot (Seseli libanotis), the plant that hosts this rust, is a red-listed wild plant confined in Britain to the chalky soils of the Chilterns, Gog Magog Hills and the South Downs.

Martyn Ainsworth, Senior Researcher in Fungal Conservation says, "It is always exciting to rediscover species thought to be extinct but to find one that has been lost for over 100 years, while carrying out a quick survey in a likely spot during a journey between England and Scotland, was an exhilarating 'Eureka' moment. To wipe these rare British fungi off the extinct list is a joy, and we hope that with further field surveying we can now provide a clearer picture of these species' current British distribution.
"Both these fungal species have been re-discovered on rare British plants, and therefore their conservation is dependent on that of their host plants and their habitats. I'd encourage all field naturalists to get out and start looking for so-called extinct fungi and find out about their relationships with other fungi, plants and animals so we can understand their habitat and conservation requirements better. There are so few of us doing this work, we need all the help we can get."

And finally the biggest new discovery of them all…

The biggest genome in a living species -bigger than Big Ben!

Scientists in Kew's Jodrell Laboratory, as part of their ongoing research into the causes and consequences of genome size diversity in plants, discovered the largest genome of all living species so far - found in Paris japonica, a subalpine plant endemic to Honshu, Japan.

With a genome size of 152.23 picograms, its genome is 50 times the size of the human genome, and 15% larger than any other found so far —it's so large that when stretched out it would be taller than the tower of Big Ben! However, having such a large genome may have direct biological consequences, as plants with large genomes may be more sensitive to habitat disturbances and environmental changes and be at greater risk of extinction.


All images are courtesy of Kew and are copyright of their respective owners.
  

Wheat and Apple genomes provide hope for food security

The recent sequencing of two major crop genomes is good news for plant protection and for food security.

With the full genetic sequence now available for wheat and apple, scientists will have more information at their fingertips for the improvement of these crops to fight plant diseases and to ensure that growing human populations have adequate food resources in the future.

On the 26th of August last, a team of researchers in the UK released the 'first draft' of the wheat genome. While further work needs to be done to produce a fully annotated genome, the work is a major step forward for plant science.

The team of researchers responsible for the the wheat genome publication were funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and come from the University of Liverpool, the University of Bristol and the John Innes Centre.

The first draft (basically the raw data from the sequencing work) will need to be further annotated and assembled into the individual chromosomes and is based on a reference wheat variety called Chinese Spring. Information on this reference variety will be key to unlocking the genetic information behind other commercial varieties of wheat.

Prof. Mike Bevan from the John Innes Centre says that "The sequence coverage will provide an important foundation for international efforts aimed at generating a complete genome sequence of wheat in the next few years".

The information should lead to improvements in current wheat varieties to ensure that high yields can be maintained in the face of changing environmental conditions and an increased threat from a variety of plant pathogens.

'By understanding the genetic differences between varieties with different traits we can start to develop new types of wheat' - Prof. Anthony HallProf. Anthony Hall of the University of Liverpool hopes that the new information will allow scientists to probe differences between wheat varieties with different characteristics: "By understanding the genetic differences between varieties with different traits we can start to develop new types of wheat better able to cope with drought, salinity or able to deliver higher yields. This will help to protect our food security".

Meanwhile, just this week, an international team of scientists announced that they had published a draft sequence of the domestic apple genome. The genome was published in the current issue of Nature Genetics.

Apple is the main fruit crop of the world's temperate regions and is a member of the plant family Rosaceae which includes many other important species including cherry, pear, peach, apricot, strawberry, and rose.

Much like with the wheat genome, this new information will allow scientists to identify genes which provide desirable characteristics to the crop such as higher yields and disease or drought resistance.

The work by scientists from Italy, France, New Zealand, Belgium and the US was based on the well-known golden delicious apple.

'the scientists were also able to delve into the apples mysterious past'As well as looking to the future of apple production, the scientists were also able to delve into the apples mysterious past. For years they have argued about where the domestic apple came from and now they know. The data published this week shows that the ancestor of the modern apple in Malus sieversii, a plant native to the mountains of southern Kazakhstan.

Prof. Doug Kell, Chief Executive of the BBSRC, who funded the wheat project, points out that "The best way to support our food security is by using modern research strategies to understand how we can deliver sustainable increases in crop yields, especially in the face of climate change. Genome sequencing of this type is an absolutely crucial strategy.

"Knowledge of these genome sequences will now allow plant breeders to identify the best genetic sequences to use as markers in accelerated breeding programmes" said Prof. Kell.

Both the apple and wheat genome work have been made possible by huge advances in sequencing technology. As Prof. Hall of University of Liverpool notes: "Sequencing the human genome took 15 years to complete, but with huge advances in DNA technology, the wheat genome took only a year. The information we have collected will be invaluable in tackling the problem of global food shortage".

'Sudden Oak Death' is here

Sudden Oak Death (Credit: sarcozona)

Department of Agriculture scientists have identified Phytophthora ramorum in Irish trees for the first time.

The organism was isolated from Japanese Larch trees in the Tipperary/Waterford area.

Just this month, the detrimental effects of sudden oak death has already been dealt with on this blog.

A statement from the Dept. of Agriculture said that they were investigating "a small number of cases" found when a special survey of Japanese Larch was ordered after the pathogen was found on the same trees in Britain late last year.

"In addition to the findings in a small number of larch trees, beech trees which were growing in proximity to the infected larch trees were also found to be infected as were two noble fir trees" according to the survey.


The Department of Agriculture say that Japanese larch "represents some 3% of the total forest tree population in Ireland.

"The bulk of the wood from infected trees can be used in the normal way provided the necessary hygiene measures are taken at felling and in sawmills."

The Department went on to say that they were taking "all necessary measures" to find out the extent of the infection and to control its spread.

From Clare to India: EJ Butler "The Father of Indian Plant Pathology"

Born on this day in 1874, Edwin John Butler had a remarkable career which saw the Irishman traveling the globe and becoming a plant pathologist of international renown.

Butler was born in Kilkee, Co. Clare where his father was the local Magistrate. He studied medicine at Queen’s College Cork (now University College Cork) and graduated in 1898.

In Cork, he came under the influence of Prof. Marcus Hartog who was Professor of Natural History and later Professor of Zoology at the college. Hartog was interested in the mechanics of Saprolegnia, a genus of water-moulds which he collected from ponds including that in the lower grounds of the college (where the Glucksman Gallery now stands). Butler began to use similar techniques to study the neighbouring genus Pythium.

Butler went on to study in Paris and London before being appointed as Imperial Mycologist to India in 1906. His work on aquatic Phycomycetes in India as well as his classical studies on the diseases of palms and sugarcane, on wilt of pigeon peas, on wheat rusts, on downy mildews and much more mean that he is regarded as the “Father of Indian Plant Pathology”. He was responsible for categorising nearly 150 species of plant pathogenic fungi.

In 1918, he published ‘Fungi and Disease in Plants’ on Indian plant diseases. He later adapted this book for a European audience and ‘Plant Pathology’ was published a number of years after his death in collaboration with S.G. Jones. It was the classic plant pathology textbook of its time.

Butler left India in 1921 and took up Directorship of the newly established Imperial Bureau of Mycology at Kew, London where he continued his work and became a distinguished figure in the world of plant pathology; travelling widely and founding a number of new journals.

The plaque awaiting installation at Kilkee Library

The Imperial Bureau of Mycology later formed part of the Commonwealth Agricultural Bureau which is now known as CABI and celebrated its centenary in 2010.

Butler was Knighted in 1939. In Butler's obituary, EW Mason notes that

"his most striking characteristic was perhaps his immense interest in fungi both as fungi and as the causal organisms of disease in plants, and coupled with this his power of transferring that interest to botanical and lay minds alike. His lifelong habit of wide and deep reading, linked with his accumulated personal experience, enabled him to present problems in their correct perspective and to recommend the line of attack that should best deserve success." (Mason, 1943)

Sir Edwin John Butler died of influenza on April 4th, 1943 in Surrey. He is commemorated by a plaque at Kilkee Library, Co. Clare (which is awaiting installation) as well as the Butler Medal which is awarded by the Society of Irish Plant Pathologists to individuals who have made a significant contribution to the field. The Butler Building at University College Cork was built in 2000 and is also named in his honour.

Sudden Oak Death: An impending threat?

Sudden Oak Death is caused by a fungal-like pathogen called Phytophthora ramorum and has hit the headlines again recently when the disease spread across the Bristol Channel to South Wales.

First found in Britain in 2002 (where it was infecting Viburnum tinus in a garden centre), it has been causing problems in the US since the mid-nineties.

In 2000, researchers at the University of California identified the cause of dying oak trees in many parts of the state to be a previously unknown species of Phytophthora. Another Phytophthora species, P. infestans, causes late blight in potatoes- a topic we have covered previously on this blog.

Alternative hosts for P. ramorum include rhododendron, viburnum, bay laurel, douglas fir, redwoods, yew, horse chestnut, beech.

The conclusive link between the pathogen and sidden oak death was not made until work published in 2002 (Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflora in California. D.M. Rizzo, M. Garbelotto, J.M. Davidson, G.W. Slaughter, and S.T. Koike.Plant Disease 86: 205-214. 2002).

By 2005, the pathogen had been found in Northern Ireland.

However, despite the problem in Europe and North America arising around the same time, research has shown that the mating-type from the American isolates (called A2 type) are different to those found in Europe (A1 type). This indicates that the pathogen did not come from North America to Europe or vice-versa. It is thought to have originated in an as-yet unidentified third country.

The pathogen can cause cankers of the bark or can cuase damage to the leaves. Depending on the host plant, differing symptoms are visible.

The pathogen may reproduce by sexual or asexual means. To reproduce sexually, both the A1 and A2 mating types (i.e. the European and American types) need to be present together and, apart from a single exception) this has not been the case up until now. Hence, the spread of the pathogen is generally via the production of asexual spores (called zoospores) which are spread by rain and wind.

In order to eradicate the pathogen in California, authorites cut and burn infected trees along with all other host plants within 100 feet. Controlling the spread of infected (or potentially-infected plant material) is crucial.

Control of the disease in Ireland is governed by European phytosanitary legislation and despite it being found on rhododendron and viburnum species in parts of the country, it has yet to be found on trees. With trees now infected Wales, how long until it spreads across the Irish Sea?

Spread of P. ramorum in Europe (2006)