Pest and disease threats accelerating

August 30, 2012

"Every invasive alien pathogen represents an uncontrolled, open ended experiment in evolution."

Nigel Bennet from Sappi receiving TPCP award
Nigel Bennet from Sappi received the prestigious 'TPCP Hands-Free Beer Slug Trophy' from Mike Wingfield. This is a highly complex maneuver requiring strength, flexibility, balance and nerves of steel. Well done to Nigel for wresting the trophy from previous winners NCT.
TPCP meeting 2012
Attendees of the TPCP meeting held at the University of Pretoria.


This quote from Dr Steven Woodward of the University of Aberdeen, attributed to eminent scientist Clive Brasier, provided food for thought for participants at the 23rd annual meeting of the Tree Protection Co-operative Programme (TPCP) and the Centre of Excellence in Tree Health Biotechnology (CTHB). The meeting was held at the University of Pretoria, and was once again well attended by industry representatives, students, researchers and scientists from around the world.

It was a theme that reverberated through almost all the presentations. The movement of pests and diseases around the world is accelerating due to increasing global trade, and when these alien pests and diseases arrive on our shores, they can behave in unpredictable ways, with unpredictable consequences. This process is exacerbated by climate change, which leads to the evolution of more virulent pathogens and the increasing potential for host jumps.

The TPCP, based at the University of Pretoria, is the largest programme dealing with insect pest and disease problems affecting plantation forestry in the world. It provides research direction and integrated pest and disease management services to South African forestry, through the Forestry and Agricultural Biotechnology Institute (FABI). Services include field extension, diagnostic services and monitoring of pests and diseases in South African plantations.

"The negative impact of pests and pathogens on forests and forestry is growing rapidly," said Prof Mike Wingfield, director of FABI, in his opening address. "Simply looking at graphs reflecting cumulative arrivals of new insect pests and diseases is cause for great concern, and there is little indication that this trend will change.

For plantation forestry, there are many technologies that can be used to reduce damage due to pest and pathogen problems, and clearly we must use these opportunities as actively as possible. But in the case of native woody ecosystems, the threats are great and the solutions are very limited," he said.

Dr Woodward provided some examples of the damage that alien pathogens can cause:

  • Chestnut blight, an alien pathogen that arrived in North America and, within 25 years, wiped out the giant chestnut trees on which a whole industry depended. This majestic tree now resembles a shrub.
  • Dutch elm disease: described as the most destructive plant disease ever recorded, it entered through one of Britain's many ports and in a few years killed all the elms in England and completely changed the landscape. Elms were a dominant species throughout the UK prior to the arrival of the pathogen.

There are many more examples, cypress canker introduced to Europe from North America, Phytophthora cinnamomi attacking old growth jarra forests in Australia, etc.

Even the world's top scientists acknowledge that they know very little about the thousands of species of fungi that live unseen within plants. They do little harm in their native range as they have evolved over millions of years alongside the host plants that they depend on to survive. But take them out of their native range where they have no natural enemies, and they can become unpredictable and highly destructive.

The same applies to insects that move out of their native ranges into new territories.

Puccinia psidii or eucalyptus rust, is a pathogen expected to arrive in South Africa soon that is a cause for concern. According to Professor Jolanda Roux of FABI, it is native to Brazil where it jumped onto plantation eucalyptus trees. The first outbreak was on eucs grown from seed sourced in South Africa.

She said it is a threat to eucalyptus but also to native trees. It releases huge volumes of spores that are very tough. It is present in North and South America and Asia and has recently been picked up in Australia where it is spreading fast and causing lots of problems.

"It's a matter of time before it reaches us in South Africa," said Jolanda. "We need strict quarantine to keep it out, but even the Australians – who have high quarantine standards – failed. So we need to get ready to tackle it when it arrives," she said.

Another pathogen causing concern is Dothistroma septosporum, aka needle blight. Scientists from 30 countries around the world are collaborating in research around this threat. Dr Irene Barnes of FABI said it affects over 80 pine species and causes severe needle defoliation. She said pinus radiata is very susceptible, while P. patula is more resistant, so it is not a huge problem in South Africa yet. However, the situation is changing – especially in Europe where new pine species are being affected, and it is jumping onto other species such us Norway spruce.

"The pathogen is spreading, and it could be caused by climate change," she said.

Professor Randy Ploetz of the University of Florida described the unpredictable nature of the Ambrosia and bark beetle-associated tree diseases, which have been responsible for the extinction of trees in some parts of the world. In less than a decade, it destroyed red bay trees in Georgia. He referred to them as 'black swan' events, i.e. they come as a complete surprise, with extreme consequences. The beetles carry the fungus with them, and then 'farm' the fungus in the bark.

"Some of these fungi are so potent that just one injection by the beetle is enough to kill a whole tree," he said.

Meanwhile, much work is being done in South Africa to manage tree diseases, like Fusarium circinatum, that are already here. The industry has a two-pronged approach to Fusarium: improve hygiene in the nurseries and breed more tolerant trees.

Dr Andrew Morris of Sappi is heading up the Pitch Canker Control Programme which has been set up under FSA. He says that the pathogen has been controlled in nurseries quite well, but the problem got worse as asymptomatic seedlings are planted out, resulting in unacceptably high mortality rates. P. patula has been the worst affected.

Andrew said that a key focus of the group's research is to try to understand the link between the disease in the nursery and in the field. New P. patula x P. tecunumanii hybrids being developed by growers are showing promising signs of resistance.

On the pests front, work continues to control the impacts of a number of insects.

Brett Hurley reports that the TPCP team is working on improving the effectiveness of the bio-control of the Sirex wasp as it continues to spread to new areas.

Leptocybe invasa continues to cause damage to eucalyptus plantations. FABI has received authorisation from DAFF to release a bio-control agent which is a parasitic wasp that attacks the larvae. The population of the bio-control insect needs to be bulked up in the FABI facility before it is released into the forestry areas of South Africa.

There are still a lot of questions around Leptocybe. For instance, many, but not all, G x U clones are resistant. It comes down to individual clones.

The bronze bug, Thaumastocaurus, is also still spreading to new regions and continues to cause damage. The FABI team has identified a bio-control and will be making an application to the authorities soon to release it.

The eucalyptus snout beetle has been around for a long time but a recent outbreak in Zululand caused concern. It seems there are two – and possibly three – species in South Africa. A bio-control agent, Anaphes nitens has been deployed, but it's effectiveness has been varied. According to Jeff Garnas, the FABI team is busy doing long term monitoring of parasitism levels.

The forestry industry is also bracing for the arrival of another gall wasp, Ophelimus maskelli, that has already been picked up in Mauritius. The biological control facility at the Pretoria University's research farm will be used to develop a bio-control for this new insect pest.

Meanwhile, mystery still surrounds the large-scale death of a eucalyptus clone in Zululand. To date, the experts have no clue as to why these trees, which are all a single clone, have died.

Fortunately, the grower company concerned limits the area planted to individual clones as a standard risk mitigation measure, so the losses in this outbreak are not too severe. But the cause of death is still unknown, a reminder of the unpredictable nature of the threats to tree health in the context of global trade and climate change.

Published in June 2012

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