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Cypress Leylands gradually dying

We live in the country and have planted Cypress Leylands around the house boundary for wind protection. I keep them clipped to about 8 feet high and the majority are healthy, but there is the odd one dying. The foliage on one branch gradually dies then over time it spreads, killing the whole tree. I hope you can advise what the problem is.

 

The main threat to cypresses in New Zealand is the fungal disease cypress canker. The fungus attacks the cambial layer of trees, affecting twigs, branches and whole stems. If the main trunk is infected, the whole tree can die. Symptoms include cracking, dark patches on the wood, sunken patches, resin bleeding and death of the foliage. Often one branch will die, sometimes followed by the entire tree. Macrocarpa (Cupressus macrocarpa) and Leyland cypress (C. leylandii) are quite susceptible, while Mexican cypress (C. lusitanica) has some resistance. Outbreaks of disease come and go, and vary in severity.

Canker damage can be minimised by making sure that trees are well-sited and spaced, and are generally healthy. Weak trees are much more susceptible to attack, and should be removed as a preventative measure. Avoid high levels of nitrogen in the soil and avoid over pruning as the canker seems to enter through pruning cuts. When pruning off infected branches, remember to clean equipment between trees. Remove severely infected trees. Fungicides can help where tree size and numbers make this feasible.

UnitecAdvice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

Reproduced with permission from NZOOM Home and Garden content,
from the previous website of  TVNZ News

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