Smart tree selection can beat exotic pests, seminar hears

The threat of oak processionary moth, horse chestnut leaf miner and ash dieback should not mean tree managers avoid planting trees of genera Quercus, Aesculus and Fraxinus entirely, a recent tree seminar heard.

Peter Thurman - image:Barcham Trees
Peter Thurman - image:Barcham Trees

Tree consultant and landscape designer Peter Thurman told the event at Barcham Trees in Cambridgeshire that an understanding of tree taxonomy can assist in selecting less vulnerable species.

Oak processionary moth (Thaumetopoea processionea) attacks only the sub-genus (section) Lepidobalanus, which includes Quercus cerris, Q. robur, Q. suber, Q. gallifera and Q. ilex, with the first two being particularly vulnerable.

It does not affect those of sub-genera Cyclobalanopsis, including Q. glauca, and Erythrobalanopsis, which includes willow oaks, black oaks, scarlet oaks, Q. agrifolia and Q. wislizenii

Likewise for Aesculus, of the five sections in the genus only those of section Aesculus, namely A. hippocastanum and A. turbinata, which are vulnerable to horse chestnut leaf miner (Cameraria ohridella).

Thurman recommended A. californica as a "very good small tree", A. indica 'Sydney Pearce' as "particularly floriferous", and A. parviflora as "a good choice for landscaping".

And with ash dieback (Chalara fraxinea) much in the headlines, he said that while Fraxinus excelsior and F. angustifolia are most susceptible, those of section Ornus are resistant. 

He recommended F. ornus, the manna ash, as "a good, but underused alternative for street and garden use", particularly for or drier, calcareous soils and sandy loams.

Thurman also highlighted several other tree pathogens of potential risk to British trees, particularly the pine processionary moth (Thaumetopoea pityocampa), the cause of "devastation" in Italian woods which he had seen first-hand. 

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