Impacts of oriental gall wasp and chestnut blight

SWEET chestnut (Castanea sativa) is not native to Britain and was almost certainly first introduced by the Romans. The tree’s high-starch nuts were valued for making polenta and used as a high-energy, low-rancid ration for the Roman Empire’s legions. Sweet chestnut in Britain never reached its full fruit-bearing potential, but it did find an important niche as coppice to produce poles which cleave easily and neatly for fence posts and other applications.

Sweet chestnut had 2,000 years of essentially pest- and disease-free life in Britain – but it was not destined to last. The now naturalised species, generally regarded as an honorary native tree, was recently hit in  succession by the world’s worst disease and worst insect pest of the Castanea genus.

Chestnut blight was identified in 2011 on sweet chestnut trees which had been planted on a Warwickshire farm in 2007 using material from a nursery in France. Oriental chestnut gall wasp (Dryocosmus kuriphilus) was discovered four years later in 2015 in a north-Kent woodland. Introduction from Europe via the fresh fruit trade is thought to be the most likely avenue of entry.

A steady stream of blight outbreaks since 2011 relate to consignments of sweet chestnut planting material, some imported from the same nursery in France, missed during the trace-forward exercises conducted from 2011 onwards due to a lack of documentation related to planting material imports.

Forest Research says that since 2011 the disease has been found on a small number of sites in the UK. However, Forest Research simultaneously says the plant health service is currently dealing with sweet chestnut blight on 65 sites in England, widely spread in Buckinghamshire, Nottinghamshire, West Sussex, Derbyshire, Kent, Devon and Cornwall.

READ MORE: DEFRA announces plan to combat Oriental chestnut gall wasp

Within a year of the first finding in woodland in north Kent and on street trees in St Albans in Hertfordshire, OCGW was active across London and its environs with infestations found during 2016 in 42 1-km grid squares. Safe to say the insect pest is now endemic in south-east England at least.

OCGW and chestnut blight are respectively regarded as the world’s worst insect pest and disease of the genus Castanea. With both now loose across England, long-established reports from Europe that physical damage delivered by OCGW can pave the way for infection by Cryphonectria parasitica must be cause for concern. The exit hole produced by the female wasp in the gall is thought to be a potential entry point for the fungal pathogen.

Forest Research has carried out preliminary research to see if there is any relationship between the insect pest and the disease in this respect on sweet chestnut in the UK. Initial research was carried out on 26 sites in London and West Sussex where chestnut blight and OCGW had been identified ‘together’ on sweet chestnut trees.

Twigs bearing abandoned galls of OCGW were collected to confirm the presence or absence of C. parasitica. After surface disinfection of the abandoned galls, necrotic gall tissue was plated on agar. DNA was extracted and tested for Cryphonectria parasitica using a real-time PCR assay technique. Of the 17 twigs carrying abandoned OCGW galls taken from five different sites, 10 tested positive for the presence of C. parasitica.

Authors say this is the first report of colonisation of abandoned OCGW galls by C. parasitica in the UK and further research is required to determine the exact nature of any insect pest–pathogen interaction. They speculate that as the number of new sites which are co-infected/co-infested with insect pest and fungal pathogen increases, the implications are for higher incidences of sweet chestnut blight in areas where both insect pest and fungal pathogen occur together. Chestnut blight and OCGW individually cause damage to sweet chestnut trees, but in terms of a lethal condition and associated economic loss chestnut blight is the most dangerous and damaging.

These findings must be cause for concern for the future of sweet chestnut in the UK. Chestnut blight and OCGW are not the only pest and disease problems currently faced by sweet chestnut. There are increasing reports of Phytophthora ramorum and ink disease, the latter associated with other species of Phytophthora including P. cambivora and P. cinnamomi. Throw grey squirrels into the mix and this vitally important woodland tree could be under real threat in future.

AUTHOR’S NOTE
Examples of an insect pest paving the way for a fungal pathogen and disease are not uncommon. I have seen such an occurrence in the budwood gardens of rubber tree nurseries in West Africa. Abrasive damage to the undersides of rubber leaves (Hevea brasiliensis) by yellow tea mite (Polyphagotarsonemus latus) increases susceptibility to anthracnose disease caused by the fungus Colletotrichum heveae. Nearer to home, puncture marks to the undersides of willow leaves caused by the stiletto-like mouthparts (stylets) of aphids facilitate infection by Marssonina salicicola, the causal pathogen of willow anthracnose.