Tree diseases & pests

CONTENTS

The implications of tree diseases & pests

Horse chestnut infected by leaf miner © Jennifer Hope-Morley

Diseases and pests are one of the biggest dangers to the future survival of many tree species.

Tree diseases and pests can affect the ability of forests and woodland to sequester and store carbon, reduce flood risk and purify water. They can affect the biodiversity supported by trees and the benefits to mankind’s well-being.

The outbreak and spread of a disease or pest can have disastrous impacts on forests, woodland and trees, which can then lead to the complete destruction of large areas, the loss or decline of crucial forest ecosystems. In some cases this results in considerable economic losses for forestry, threatening sustainable forest management. Countries where a single tree species has been extensively planted are at greatest risk.

The danger to trees from diseases and pests has greatly intensified due to the increasing international trade in agricultural and forest products and plant material. This has led to an augmentation in the introduction and spread of damaging pests and diseases into new environments.

When a pest or disease is introduced to a new environment outside their native range, native trees are unlikely to have any resistance or defensive means to combat them and weak and stressed trees are more vulnerable to attack.

Invasive pests spread rapidly and can take hold, particularly when the climate is favourable, there is a large number of suitable host species and they are outside their native environment – where they’ll have no natural enemies.

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The causes of tree diseases & pests

Trees are exposed to a wide range of pathogens (agents that cause diseases) and pests that can be caused by viruses, bacteria, fungi, oomycetes (algae-like fungi) and insect herbivores.

Tree diseases can also be caused by abiotic (non-living) factors such as air pollution, mineral imbalances and climate extremes, and by biotic agents such as viruses, bacteria and fungi, by nematodes (worms) or parasitic plants such as mistletoe.

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Why the increase in tree diseases & pests?

Trade in live trees, shrubs & horticultural plants

The increase in trade of live trees, shrubs and horticultural plants is a significant route for pests and diseases to enter countries. Many trees and shrubs are now being imported already semi-mature with large root balls. This provides greater opportunity for pests and diseases to be brought in at the same time.

Use of wooden packing crates

Pests and diseases can be introduced in anything made of wood such as: wooden packing crates, timber and wood-based packing material.

The biomass energy market

In the UK, for example, the demand for woodchip for power stations cannot be met by the country’s forests and woodland – this has resulted in the importation of woodchip, which is often stored over time, allowing any pest larvae to survive.

Human activity

Humans contribute to the spread of diseases and pests more than they might imagine, not only through the trade of plants and plant material but also by transporting spores or insects on, for example, live plants, clothing, shoes, machinery and tyres.

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Climate change impact on tree diseases & pests

Land surface temperature, Dec 2016 © NASA

Scientists predict that the impact of climate change will influence the spread of infectious diseases and pests, adding stresses to trees that will make them more susceptible.

A warmer overall climate, particularly warmer winters, will allow many pests to extend their current geographical range, as increases in temperature tend to affect flight behaviour and increase feeding opportunities.

Climate change is likely to impact native pests making them more damaging and problematic. The predicted changes may also increase the significance and profile of native species not currently perceived as pests.

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The consequences of tree infestations

Holly Ilex aquifolium insect-infested tree trunk © Ita McCobb

Tree diseases and pests can affect the ability of forests and woodland to sequester and store carbon, reduce flood risk and purify water as well as affecting the biodiversity supported by trees and the benefits to human well-being.

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Combating tree disease & pest infestations

Not all trees have the same susceptibility to certain diseases or invasive pests as others so it is important to build up the number of resistant trees.

In the UK the Millennium Seed Bank at Wakehurst Place in West Sussex is storing seeds from native trees with the aim of creating seeds with a resistance to pests and diseases.

Several options such as the planting of alternative species, the selection and breeding for resistant genotypes (with specific genetic constitution) and biological control could help counteract the impact of pests and diseases.

The International Plant Sentinel Network (IPSN) is being developed to facilitate collaboration among institutes around the world, with a focus on linking botanic gardens and arboreta, National Plant Protection Organisations (NPPOs) and plant health scientists.

The aim is for these institutes to work together in order to provide an early warning system of new and emerging pest and pathogen risks.

By building a better understanding of why certain pathogens and insects become major diseases and pests, and encouraging international scientific collaboration and strict trading restrictions, trees should have a better chance of survival.

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References: University of Michigan; UN-REDD programme; United Nations Environment Programme (UNEP); National Geographic; World Wildlife Fund; www.wwf.panda.org; Rainforest Connection (RFCx); The Forestry Commission UK; Forestry, An International Journal of Forest Research; US Department of Agriculture (USDA) Forest Service; The Woodland Trust; sciencedaily.com; US Environmental Protection Agency; Cornell University; The Forestry Commission UK; Cornell University; Food & Agriculture Organization of the UN; Institute of Chartered Foresters; Botanical Gardens Conservation International; International Plant Sentinel Network (IPSN); Zac Goldsmith in collaboration with The Countryside Restoration Trust, The Threat to England’s Trees; United States Department of Agriculture (USDA) Forest Service.