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Hypovirulence hope for management of sweet chestnut blight

| June 15, 2013

In 2012 sweet chestnut blight, caused by the fungus Cryphonectria parasitica, was found for the first time in the United Kingdom (UK) on trees imported from France in 2007 and planted at two sites, one in Warwickshire in the West Midlands and the other in East Sussex in southern England.

Hypovirulence 1 The attractive dark green and glossy foliage of sweet chestnut would be sorely missed

The attractive dark green and glossy foliage of sweet chestnut would be sorely missed.

If the UK plant health authorities fail to eradicate this fast moving disease the ‘game is over’ for Castanea sativa (European sweet chestnut) in the UK, a naturalised forest and orchard tree in Britain for 2000 years. Sweet chestnut was brought to Britain by The Romans for its high-starch low-fat nuts ground into a meal called ‘pollenta’ and used to sustain Rome’s legendary legions. Sweet chestnut never managed to fulfil its promise for nuts in Britain’s too cool climate but more than compensated in timber production and especially coppice poles.

Thousands of hectares of sweet chestnut coppice remain in South East England concentrated in East Sussex and Kent. Sweet chestnut has been undergoing a strong renaissance for timber and biomass energy as well as being an important habitat for increasingly uncommon spring flowers, butterflies and birds that respond to the positive light regimes within properly managed sweet chestnut coppice. Are all these to be lost to sweet chestnut blight?

Hope rests on hypovirulence

Not necessarily because scientists are pinning hopes on a form of naturally existing biological control called hypovirulence. Hypovirulence is a virus disease of C. parasitica that weakens and slows down chestnut blight disease by reducing the pathogenicity (virulence or aggressiveness) of the fungus. These fungal viruses which belong to the family Hypoviridae infect the fungus naturally in the field.

Sweet-chestnut

Sweet chestnut already struggles to produce edible nuts under UK climatic conditions.

Hypovirulence has provided measurable control of chestnut blight in parts of Europe and in Michigan in the United States but does not work everywhere. Hypovirulence relies on different interacting factors relating to the virus, the fungal pathogen population and host plant population, plus the environment. Any factor which reduces rate of disease spread will enhances the ability of the virus to invade the fungus. The naturally occurring virus infecting C. parasitica in Europe is the dsRNA hypovirus CHV1 which limits the ability of the pathogen to grow in chestnut bark or to produce spores.

Hypovirulence is used as a form of biological control in many European countries affected by chestnut blight. Sweet chestnut trees with virulent, growing cankers are treated with hypovirulent strains of the C. parasitica fungus. The action of CHV1 converts the virulent pathogen to a less aggressive form thus allowing trees to recover from infection. There are chestnut sites in Europe first infected 30 years ago and where CHV1 is now well established with disease severity correspondingly low.

sweet-chestnut-blight

Young lesion of sweet chestnut blight. Picture courtesy D. Rigling, Swiss Federal Institute for Forest, Snow and Landscape Research.

Ultimate success of hypovirulence depends on having populations of C. parasitica with only limited genetic diversity. Populations with high levels of genetic variation pose a major obstacle to the spread of the hypovirus and therefore limit its effectiveness. This appears to be the main reason why hypovirulence has only had a limited impact in the USA in the protection of Castanea dentata (North American chestnut). C. parasitica populations proved much more genetically variable in the USA where C. dentate, an imposing high forest tree from Maine in the north to Georgia in the south, was virtually wiped out with the loss of 3.5 billion trees.

North American scientists tried to manipulate hypovirulence in order to develop economically sustainable biocontrol of chestnut blight but there were several obstacles. Firstly the blight spreads very rapidly in nature while hypovirulence spreads very slowly. In addition there are many types of virulent strains in North American forests which resist transfer and reception of the virus responsible for hypoviulence. Some positive results have been achieved by using genetic engineering to transfer the debilitating genes of the virus into the fungus.

Hypovirulence 5.  All tissue above the lesion (canker) will die (Picture courtesy D. Rigling. Swiss Federal Insitute of Forest, Snow and Landscape Research)

All tissue above the lesion (canker) will die (Picture courtesy D. Rigling. Swiss Federal Insitute of Forest, Snow and Landscape Research).


Blight resistance

Genetic resistance to chestnut blight occurs naturally in populations of chestnut trees irrespective of species and is a feature well worth exploiting. Two strategies have been pursued in the United States to breed blight-resistant North American chestnut trees. They are breeding within the American chestnut gene pool and hybridisation of North American chestnut with Asian species of chestnut.

Hypovirulence 3 There are still high hopes that sweet chestnut will become one of  Britain's bulwark trees against climate warming

There are still high hopes that sweet chestnut will become one of Britain’s bulwark trees against climate warming.

Breeding within American chestnut populations was started by using the occasional surviving trees thought to possess some resistance. Enzyme studies on inner bark tissue revealed small resistance differences among trees. Cross pollinations were made among putatively resistant trees but failed to raise resistance to an acceptable level and consequently this approach was abandoned.

Resistance in populations of Asian chestnuts, especially C. mollissima (Chinese chestnut) and C. crenata (Japanese chestnut) was evident to plant breeders way back in the early 1900’s and breeding programmes began in the 1930’s.

However, first hybrids produced were nowhere near as blight resistant as the oriental chestnut parent. In an effort to increase the resistance level these hybrids were back-crossed to a resistant oriental parent, but the trees produced were both short in stature and branch structure making them uncompetitive in the natural forests of the eastern North America.

Later attempts at back-crossing managed to transfer the resistance while maintaining the desirable growth form and adaptability of the American chestnut. This work produced two first backcrosses – the ‘Graves’ and ‘Clapper’ trees that were partially blight resistant first generation hybrids. Overall these early breeding programmes failed to produce sufficiently blight-resistant trees but left behind a valuable legacy of germplasm and knowledge.

Real breakthrough came after the genetic mapping (finger-printing) of various Castanea species showing how there were just a few incompletely dominant genes controlling resistance in Chinese chestnut. A genetic map was subsequently drawn up and used to screen newly germinated seedlings for blight resistance. Back in 1997 The American chestnut foundation estimated that this year (2012) would see nuts being produced from the most blight-resistant breeding lines that can be used in reforestation.

Some of the most successful work has involved an integrated approach through combining blight resistance and hypovirulence. In Virginia’s Lesene State Forest American chestnut trees budded (grafted) with blight resistant stock and inoculated with hypovirulence have been thriving for years, although still surrounded by non-blight resistant chestnuts perpetually killed back by the disease.

The American chestnut

Some may wonder why the Americans are still going to such lengths to retrieve and resurrect this native chestnut which as a maiden tree is only remembered by the oldest people in the community.

Before the pathogen arrived in 1904 the American chestnut was the tallest and most dominant hardwood tree across the Eastern United States growing in vast stands from Maine to northern Florida.

Every fourth tree in the forests of The Appalachians was once an American chestnut. However, such was the severity of its fall that it is now regarded as an endangered species in states like Tennessee and Michigan.

For its imposing presence, the tree was dubbed ‘the Redwood of the east’. Such was the reliance of wildlife, including white-tailed deer and wild turkeys, on American chestnut trees and their nuts for food that populations of these animals crashed along with the blight stricken stands of trees.

With spores spread by wind or on the beaks and feet of migrating birds and by insects the fungal pathogen spread rapidly. Within 40 years virtually the whole of the Southern Appalachians including Virginia, Tennessee, North and South Carolina and parts of Georgia was infected with most chestnut trees dead.

Published in International Pest Control – January/February 2013 issue.

Author: Dr Terry Mabbett.
Director,  Dr Terry Mabbett Consultants

Category: Forestry