Research

Cypress dieback

Leyland and Lawson cypress are extensively used to make quick-growing hedges. Regular trimming is needed to avoid excessive growth of these hedges and brown patches (dieback) are becoming more common on trimmed hedges. Feeding damage by conifer aphids can cause browning on conifers, but it is uncertain if aphids are responsible for most of the damage seen.

In April 2006 the RHS and East Malling Research (EMR) received funding from the Horticultural Development Council for a two-year project to investigate the causes of brown patches in Leyland cypress hedges.

The first step was to conduct a survey to gather more information about the extent of the problem. After our call in the May 2006 issue of The Garden, more than 300 RHS members enthusiastically responded. The questionnaire was also circulated on RHS Online, at the RHS, EMR and the School of Biological Sciences at The University of Reading. We asked questions about the hedge's age, growing environment and management etc. In addition, data from the RHS Advisory Service database for 2005 and 2006, relating to enquiries about browning and dieback in Leyland cypress, were collated.

Main conclusions from year one results

• Analysis of the responses to the survey suggests that, of the factors that were analysable, only trimming the hedges in the autumn months (predominantly October) was significantly linked with the occurrence of foliage dieback in Leyland cypress.

• Although watering hedges that experienced progressive dieback coincided with increased hedge recovery in some cases, water availability alone was not related to the occurrence of the problem.

• There was no correlation between the appearance of dieback and any individual or combined environmental or soil factor (e.g. hedge density or size, proximity to the road or a barrier, type of soil etc.).

• Questionnaire respondents were unable to provide detailed information about the occurrence of possible pests or diseases on hedges affected by dieback, and it was not possible to draw any conclusions on the impact of non-environmental factors.

• Analysis of the causes of foliage dieback in Leyland cypress hedges in the 78 samples sent to the RHS Advisory Service in 2005 and 2006 determined that in 24% of the samples damage could be attributed to aphids. Other significant contributors were honey fungus (in 18% of the samples) and physiological causes (in 14% of the samples) e.g. aftercare problems, dry soil, dense planting of the hedge or excessive shoot pruning.

Aphid damage

Also in year one, scientists from the EMR collected and examined plant material from 21 locations around England for presence of the conifer aphid Cinara cupressivora. Samples of plant material were taken from the hedges at each location and inspected for live aphids or signs that aphids had been present (cast skins or honeydew).

• In the hedge sampling programme, not all hedges that were reported as damaged showed signs of aphid presence on the plant material examined. Evidence that the aphid was or may have been present was seen in samples of plant material from only 11 of 21 locations sampled (52%).

• In the locations inspected, there were no differences between damaged hedges that had signs of aphid presence and those that did not with respect to the variety of conifer sampled, the time of first appearance of symptoms, the amount of foliage left after pruning, or in worsening of the dieback symptoms over time, all factors that might affect or be related to aphid presence.

• Not all damage symptoms seen on the vegetation at sites where aphids were present appeared to be caused by aphid feeding.

• Plant material from five damaged and two healthy hedges was examined to determine if a fungal pathogen could be responsible for the dieback symptoms. No correlation was found between the fungal spores present and the state of health of the plants examined.

Main conclusions from year two research

In year one, the project demonstrated the widespread occurrence of dieback symptoms on Leyland and Lawson cypress hedges in the UK, and has shown that not all the symptoms reported by gardeners are associated with the presence of aphids. However, pruning in autumn does appear to be associated with dieback symptoms.

The aphid species present on selected Leyland hedge samples was identified by East Malling Research entomologists during 2007 as Cinara cupressivora (right): it feeds by sucking sap from the woody part of branches, with maximum numbers found 6-10 cm from the shoot tip.

Year two experiments were undertaken on potted plants and on established hedges to clarify the biology and behaviour of C. cupressivora and the effects of aphid feeding on the occurrence of plant damage symptoms.

In potted plant experiments with several cultivars of cypress, which were inoculated with aphids, there was evidence that aphid feeding caused browning damage on plants.

In a mature ‘case study’ hedge, which was identified as exhibiting dieback and where aphids were detected, general signs of hedge yellowing were seen between April and July, when adult aphids and obvious honeydew (aphid excrement) were present. Hedge browning was seen in areas with both low and high aphid numbers.

In assessments of hedges in 2007, most damaged branches with a firm (non-wilted) appearance were associated with aphid presence; brown wilted branches were not associated with aphid presence.

Information from the RHS database on RHS members’ enquiries into the causes of damage to conifer hedges for 2007 was added to data obtained from 2005 and 2006. In only 5 percent of the enquiries in 2007, C. cupressivora was identified. Although the causes of damage could have been as varied as root disease, management issues etc. it is likely that in some cases inadequate sample size or time of sampling prevented the identification of the aphid. This highlights the importance of timely sampling (aphids are present on the plants mainly from April to July) of the appropriate part of the branch (from the shoot tip to the woody part of the branch).

Dr Tijana Blanusa (RHS)
Dr Jean Fitzgerald (EMR)

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