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New plant pests and diseases threaten UK gardens

January 17, 2014

Dealing with plant pests and diseases is part of everyday gardening. The likes of whitefly, aphids and powdery mildew will be familiar to many gardeners.  But some new and unfamiliar pest and diseases have the potential to devastate UK gardens, changing our historic landscapes forever.

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Photo: Citrus Longhorn Beetle (Anoplophora chinensis)

(Credit: Iwatebud at en.Wikepedia under License 3.0)

The Citrus Longhorn Beetle is a wood-boring pest that has found its way into Europe from its native Asia on imported Acers and causes severe damage to fruit and amenity trees.  Adult females lay their eggs in slits cut into the bark of branches, which hatch into larvae that bore their way through the wood, leading to potentially serious structural weakening of the tree.

The beetle was, in fact, first intercepted in the UK in the 1920s, but it is the more recent increase in the international plant trade that has the experts concerned.  Acers, Cercis and Malus infested with the beetle have already been imported from Asia into Europe, leading to potentially thousands of affected trees.  It is not only the trees  that can harbour the pests, but also the wooden pallets that trees are packed in.

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Photo: Oak Processionary Moth (Thaumetopoea processionea)

(Credit: Kleuske at en.Wikepedia under License 3.0)

The Oak Processionary Moth is already causing major concern in the UK.  The larvae (in the form of caterpillars) feed on the foliage of English, Sessile and Turkey Oaks and also Fagus, Castanea and Betula species.  The moth gets its name from the way the caterpillars form ‘processions’ along tree trunks to their nests.  These pests pose a serious risk to human health, as the late stages of the larvae have hairs that contain a toxin that can cause skin irritation and allergic reactions.  Nest of larvae have been found in an increasingly large number in West London locations. London borough councils such as Richmond now have annual programmes to destroy the nests

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Photo: Oak twig showing Sudden Oak Death caused by Phytophthora ramorum

(Credit: Joseph O’Brien, USDA Forest Service at en.Wikepedia)

Phytophthora (Greek for ‘plant destroyer’), a fungus-like plant pathogen with over 100 species, has been having a major impact on UK gardens.  Two species in particular have become an issue – Phytophthora ramorum (otherwise known as Sudden Oak Death) and Phytophthora kernoviae.  These attack many tree species, but are a particular problem where there are plantings of Rhododendron ponticum, which acts as a ‘host’ plant to the disease.  The symptoms vary depending on the plant infected; on Beech it takes the form of lethal bleeding cankers on the main trunk and on Rhododendrons necrotic (dying) foliage.

Phytophthora has particularly affected gardens in the South West of England, where Ponticum has become part of the landscape.  Some of the preventative measures are drastic, such as the complete removal of Rhododendron ponticum.  A range of hygiene and biosecurity precautions can reduce the spread of the disease, such as avoiding replanting on infected areas, using mulch to stop plants picking up infections from the soil, using clean water for irrigation (the pathogen is spread in water) and making sure vehicles are clean when entering and leaving gardens.

Large imported trees are a potential carrier of these new plant pests and diseases. Oak Processionary Moth, for example, arrived in West London on oaks imported from Italy.  Other types of imports also pose a risk, such as cut flowers, exotic fruit and vegetables and seeds.  It is an illuminating exercise to look at the Food and Environment Agency’s online lists of  plant pest and disease interceptions at UK ports .  Climate change has also played a role in expanding the geographic range of pests and creating conditions conducive to the spread of diseases, as Phytophthora thrives in wet conditions.

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