Tree Pests and Pathogens

Examples of Major Tree Pathogens and Pests

Bacterium

Candidatus Liberibacter asiaticus (citrus greening)

Host and Distribution

citrus greening

Acknowledgements: Photos from Florida Division of Plant Industry Archive, Florida Dept of Agriculture and Consumer Services and Jeffrey W Lotz, Florida, Dept of Agriculture and Consumer Services, Bugwood.org.

Ca. L. asiaticus is a bacterial pathogen originating in Asia that has recently been introduced into the Americas (1). It causes citrus greening, which now presents an unprecedented threat to citrus production worldwide. It is transmitted by the phloem-feeding citrus psyllid insects (Diaphorina citri). The bacteria has been detected in citrus plants in many coutries including China, Japan, Thailand, India, the Philippines, Arabian Peninsula, Africa and Brazil and the USA where it was first detected in Florida in 2005 (2) and is now present in a number of states.

Impacts on Ecosystem Services

Citrus greening is characterised by blotchy mottling of leaves, stunting shoots and branch dieback as the disease progresses (see photo). The fruit of diseased trees is small and lopsided with poor colour. Citrus greening is a major threat to commercial citrus production and is the most destructive disease of citrus trees worldwide (3).

Control Methods

With the citrus industry of Florida and elsewhere at stake, there is an urgent need to control Ca. L. asiaticus but this brings significant challenges. At present there is no effective control method to manage the disease and no resistant cultivars are available although there are differences in the host response and sensitivity of citrus genotypes to the disease. Thus rapid diagnosis and immediate removal of infected trees is normal practice. PCR systems have been used for molecular detection and antibody systems have been developed as convenient, low-cost detection methods for farmers and quarantine officials (4). A complete circular Ca. L. asiaticus genome has been obtained by metagenomics and this is the first genome sequence of an uncultured a–proteobacteria that is both an intracellular plant pathogen and insect symbiont.

Helminth

Bursaphelenchus xylophilus (pine wood nematode)

Host and Distribution

Bursaphelenchus xylophilus, pine wood nematode

Acknowledgements: Photos from USDA Forest Service, Region 2, Rocky Mountain Region Archive, Bugwood.org.

Pine wood nematode B. xylophilus (5, 6) is spread by Monochamus beetles and with the movement of infested live or sawn wood. B. xylophilus can live in the wood of all species of conifers other than Thuja, Tsuga and Taxus although North American pines in the nematodes’ natural distribution are not damaged. B. xylophilus originated in Canada, Mexico and the USA and is now widely distributed across China, Hong Kong, Japan, North and South Korea, Portugal and Taiwan. It has recently been eradicated from Spain.

Impacts on Ecosystem Services

There are massive losses of susceptible trees, especially pines, in areas where wilt expression occurs. Such mortality causes economic and environmental damage. Both the nematode and its vector need to be present to establish an outbreak.

Control Methods

Where wilt expression occurs, the spread of infection can be prevented by early detection and felling of infested trees before emergence of the vector beetle. Destruction or treatment of infested material is necessary to kill both the nematode and its vectors. The nematode can also be present in trees not showing wilt since Monochamus breeds in weakened or freshly killed trees and the nematode can develop saptrophically. No control measures are applied in North America where the nematode is native and does not kill native tree species.

Oomycete

Phytophthora cinnamomi and other phytophthoras

Host and Distribution

Phytophthora cinnamomi

Acknowledgements: Photos from Edward L Barnard, Florida Department of Agriculture and Customer Services and Elizabeth Bush, Virginia Polytecnic Institute and State University, Bugwood.org.

Phytophthora spp. are oomycetes (water molds) that cause some of the most highly aggressive plant diseases affecting a wide range of tree species globally (7). P. cinnamomi is present in over 70 countries around the world, with serious damage (sudden death of a number of native species) being caused in the United States and Australia (8, 9). Recently, P.ramorum has caused sudden oak death in the USA and is killing Larch in the UK (10). There are significant threats from other Phytophthora species e.g. P. kernovie and P. lateralis.

Impacts on Ecosystem Services

Because of its wide host range and the damage caused to native species (e.g American chestnut, Fraser fir, oaks and Jarrah, Banksia in Australia) the impacts on biodiversity and the natural environment are often the major concern although the impacts on provisioning services can also be substantial.

Control Methods

Control methods are well developed with eradication, containment and disease management being the usual options. Photosanitory procedures are important to prevent spread and some fungicides are available.

Insect (bark beetle)

Dendroctonus ponderosa (mountain pine beetle)

Host and Distribution

mountain pine beetle

Acknowledgements: Photos from Ronald F Billings, Texas Forest Service and USDA Forest Service, Region 2, Rocky Mountain Region Archive, Bugwood.org.

The mountain pine beetle, D. ponderosa attacks a broad range of pine species. It is native to western North America from Mexico to central British Columbia. In western North America and the Rocky Mountains, D. ponderosa and its microbial associates has destroyed millions of hectares of lodgepole pine (Pinus contorta) and ponderosa pine (Pinus ponderosa) (11-13). In early 2013 the beetle was devastating forests in 19 western states of the US and in Canada. Warm summers and mild winters result in better insect survival and the continuation and intensification of outbreaks. Recent weather patterns and perhaps climate change are associated with the current increases of populations and damage.

Impacts on Ecosystem Services

Wood can be harvested from dead stands but there is a significant problem because of the unprecedented area of damage. The presumption is that the large areas of dead pine stands present a fire hazard and there are concerns that the extent of the problem have significantly increased CO2 emissions moving north American forests from being a carbon sink to becoming a net source of greenhouse gases (13). Such a large scale of damage will also impact biodiversity and water resources.

Control Methods

Management of infections by harvesting the leading edge trees, removal of single or groups of infected trees and felling and burning infected trees (i.e. sanitation felling). Pheromone trapping and breeding for enhanced tree resistance have been employed and the pesticide chitosan, carbaryl, permethrin and bifenthrin have all been used against the beetle and can be effective.

Insect (borer)

Anoplophora glabripennis (Asian longhorn beetle)

Host and Distribution

Asian longhorn beetle

Acknowledgements: The inset shows a cut trunk with feeding damage by the larvae and an adult beetle. Photos from USDA Agricultural Research Service Archive, Bugwood.org (main picture) and Forestry Commission Picture Library (inset).

The most destructive non-native insect attacking US trees in the Asian longhorn beetle Anoplophora glabripennis (14, 15). As with the closely related Citrus longhorn beetle (A. chinensis), A. glabripennis attacks a wide range of Asian, North American and European tree species (esp. maples, chestnuts, sycamores and poplars). Asian longhorn originated from China, Hong Kong, the Democratic People’s Republic of Korea and the Republic of Korea. It is now recorded and under eradication in Austria, Canada, France, Germany, Italy, Poland, Slovakia, the UK and USA.

Impacts on Ecosystem Services

The beetle causes dieback of foliage during the early phase of attack with sustained attack resulting in tree mortality. Breeding galleries and larval feeding weaken the stem and branches and reduce timber value. The major impact has been to street trees.

Control Methods

Asian longhorn beetle is under EPPO and EU control measures to prevent the movement of infected wood and plants. Infestations are controlled by early detection and felling/ destruction of infested trees.

Insect (defoliator)

Thaumetopoea processionea (oak processionary moth)

Host and Distribution

oak processionary moth

Acknowledgements: Photos from David Williams, Forest Research and Forestry Commission Picture Library.

The oak processionary moth (Thaumetopoea processionea) has gregarious larvae that can defoliate oaks (Quercus) and which move between trees together in characteristic mass migrations or "processions". T. processionea has a native range in central and southern Europe and has recently spread with outbreaks in more northerly countries (i.e. Austria, Belgium, The Netherlands and UK).

Impacts on Ecosystem Services

The hairs of the larvae detach and are intensely urticarial and can become a significant public health problem. Oak processionary moth affects oak in woodland, forests and in urban areas. Caterpillar feeding will defoliate oaks over several seasons leading to dieback, long term decline and mortality. There has been a significant impact on oak productivity in central and western Europe and the public health risk is a major concern in parks and woodlands used for recreation.

Control Methods

Currently under EPPO and EU control measures to prevent further importation of live plants. Visual inspection for eggs, larvae and nests followed by insecticides (Bacillus thuringensis) use from the ground and where site conditions allow by aerial application (e.g. non-urban woodlands). Older larvae in nests can be removed using vacuum equipment.

Insect (phloem feeder)

Agrilus planipennis (emerald ash borer)

Host and Distribution

emerald ash borer

Acknowledgements: Photographs taken in July 2013 (Nigel Straw, Forest Research).

The emerald ash borer is native to north-east China, Korea, Japan, Taiwan, Mongolia and the Russian far East. In its native range the beetle behaves as a secondary pest, attacking severely stressed ash trees of the local species Fraxinus mandshurica and F. chinensis. However in the early 1990s A. planipennis was introduced to North America in solid packaging material from Asia but was only detected once ash trees started to die on a large scale. It has caused extensive dieback and death of native ash, particuliarly F. pennsylvanica, F.americana and F. nigra (green, white and black ash) initially in southeast Michigan in the United States and in Ontario, Canada (16). The population has continued to increase and spread and by 2012 infestations had been recorded in 19 states of the USA and in southern Quebec in Canada. In 2007 A. planipennis was reported in Moscow (Russia) to which it may have been introduced with infested planting stock or in wood packaging from China and where it is attacking and killing large numbers of F. pennsylvanica in parks, gardens and along street. The photographs above show a row of green ash killed by emerald ash borer in Moscow. The undamaged trees with a green canopy in the background are box elder trees (Acer negundo), which are not attacked by the beetle. The insert shows the larval galleries of A. planipennis under the bark of the dead ash trees.

Impacts on Ecosystem Services

This beetle has killed tens of millions of ash trees over the last 10 years causing significant economic, environmental and social damage (16-18), with high costs associated with removal and replacement of street trees in particular and is raising concern over the future of ash in North America and Europe.

Control Methods

North American species of ash appear to be highly susceptible to emerald ash borer because they have not been in contact with the beetle over a long period of evolutionary time and thus have not evolved chemical and physical mechanisms of resistance. A similar lack of co-evolution of the beetle suggest that European ash F. excelsior is also at risk and on this basis the European plant Protection Organisation have added A. planipennis to its list of A" quarantine pests. Import restrictions on ash planting stock and new international standards on the quality and treatment of wood packaging (ISPM 15) had been thought to greatly reduced the chances of emerald ash borer being introduced into Europe. However its presence in Moscow and its continued spread from the original area of infection present a serious risk. Experience in North America suggests that it is very difficult to prevent the spread of this beetle. New infestations were found beyond the ash-free cordon sanitaire possibly because of hitch-hiking on vehicles, The long-term impacts of A. planipennis on European ash are expected to be significant particularly if the beetle spreads to areas where trees are weakened from Chalara fraxinea.

Fungus

Cryphonectria parasitica (chestnut blight)

Host and Distribution

chestnut blight

Acknowledgements: Photos from Andrej Kunca, National Forest Centre, Slovakia, Bugwood.org and Joseph O’Brien, USDA Forest Services, Bugwood.org.

Cryphonectria parasitica is an ascomycete fungus that caused widespread blight in the eastern United States during the 1900s leading to the almost total elimination of American chestnut (Castanea dentata) (19). The fungus was introduced to North America from its national range in Japan. The first European report of C. parasitica was from Italy in 1939 and it has spread so that today scattered stands in the Netherlands and in the whole UK are free of the disease.

Impacts on Ecosystem Services

Almost total elimination of C. dentata in the eastern deciduous forests of North America had major impacts on forest structure and biodiversity. The elimination of street trees was also a serious loss to the amenity value provided by this tree species. Symptoms are slight on species other than North America and European Castanea.

Control Methods

Little could be done to slow the original epidemic of C. dentate in North America and subsequent efforts focused with some success on breeding programmes incorporating resistant germ plasm from Chinese and Japanese chestnut (20). In Europe a hypovirulent isolate of C. parasitica was identified which could infect European chestnut without producing lethal infection. Subsequently surviving blighted C. dentate were found in North America from which hypovirulent isolates of C. carasitica were isolated. Hypovirulence has been found to be caused by a fungal virus (a single or multi-segmented dsRNA genome and protein coat) (21), is transmissible between isolates of the fungus and can be used successfully as a biological control agent. The identification and use of the fungal virus in biological control have allowed the reintroduction of C. dentate in some sites in North America.

Fungus

Chalara fraxinea (ash dieback)

Host and Distribution

ash dieback

Acknowledgements: Photos from Forestry Commission picture library, show dead and dying trees in Denmark and insets of foliar symptoms and H. pseudoalbidus fruiting bodies on leaf rachis.

Chalara fraxinea was identified as the fungus responsible for ash dieback of European ash (Fraxinus excelsior) in 2006 and was found to be the asexual phase of a newly identified fungus Hymenoscyphus pseudoalbidus in 2010 (22). H. pseudoalbidus is closely related to H. albidus a saprophytic ascomycete which lives on ash leaves (23). The origin of Chalara/H. pseudoalbius is likely to be East Asia and it has been found in Japan. It is now widespread across Europe (UK, France and across to the Ukraine and Russia).

Impacts on Ecosystem Services

Affected trees usually die although the period between early foliar and stem symptoms and tree mortality can be several years. There is a range of susceptibility to Chalara across different ash species. In addition c. 1 to 2% of native European ash show some level of resistance (24). Thus, although the current wave of infection will have serious impacts on biodiversity and production services where ash is a major component of broadleaved and mixed woodland there are reasonable prospects for the survival of European ash.

Control Methods

Destruction of infected trees, restrictions on plant movement i.e. on containment strategy. It has provided difficult to prevent spread once the disease is established and Chalara has spread rapidly in Europe. The current approach includes containment and sanitation measures, silvicultural measures and a breeding programme which is likely to be the most effective measure in the medium term. As for other pests and pathogens the targeted use of chemical control can protect particularly valued trees.

Insect-fungus symbiosis

Ophiostoma ulmi and Ophiostoma novo-ulmi (Dutch elm disease)

Host and Distribution

citrus greening

Acknowledgements: Photos show an English church yard before and (inset) after the epidemic. Photos from Forestry Commission Picture Library.

Dutch elm disease can be caused by Ophiostoma ulmi, O. himal-ulmi and O. novo-ulmi. O. himal-ulmi (25) is endemic to the western Himalayas and novo-ulmi probably arose as a hybrid disease between the other two. O. ulmi caused problems in Europe from 1910 onwards, reaching North America in imported timber in 1928. O. novo-ulmi is extremely virulent and has devastated elms (Ulmus spp.) in Europe and North America since the 1940s (26). Dutch elm disease has also occurred in New Zealand. The fungus is spread by a number of species of elm bark beetle (Hylurgopoginus rufipes, Scolytus multistraitus, S. scheuyrewi and S. scolytus).

Impacts on Ecosystem Services

Dutch elm disease has eliminated a number of elm species from the European and North American landscapes.

Control Methods

Initial attempts to control the disease were by the removal and burning of diseased trees. There has also been control of the beetle vectors using insecticides (DDT and dieldrin) but concerns over impacts on birds shifted the focus to the potential use of fungicides (carbendazin phosphate or thiabendazole hypophosphite – injected into the base of the tree).

There have been a number of major programmes to breed disease resistant cultivars and hybrids of elms, often involving a genetic contribution from resistant Asian elm species. Such programmes have had significant successes with the commercial release of resistant cultivars e.g the Columella, Lutèce and Vada clones. There has been one attempt to genetically engineer English elm Ulmus procera by transferring antifungal genes but the programme was abandoned.

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