Browning In Conifers – Hedges: Leylandii And Others.

Question
We have a stand of leylandii castlewelland about 7ft. high x 12 ft.Long which has been in our front garden for at least 15 years. Quite recently we have noticed a “browning” emanating from the centre – wider at the top than the bottom – and we would appreciate any advice on what may be the cause and if any treatment would solve the problem.
Answer
It sounds like Phytophthora.The only solution is to remove the brown areas feed the plants with a grow more fertilizer,add lots of organic matter around the plants in the soil i.e spent mushroom compost or garden compost, and water the plants in the summer. This will help the plants fight off the Phytophthora. I have included some notes below for your information.

“Phytophthora cinnamomi is a microscopic soil borne organism,invisible to the naked eye,which causes root rot of a wide variety of plant species including many native and introduced ornamental plants.Other species of Phytophthora may cause diseases on a wide range of plants but are generally less severe.The biology and control measures are very similar so this outline will concentrate only on Phytophthora cinnamomi.

Infection often results in the death of the plant,with earlier symptoms including wilting, yellowing and retention of dried foliage and darkening of young feeder roots and occasionally the larger roots.Phytophthora cinnamomi requires moist soil conditions and warm temperatures to be active,but damage caused by the disease most often occurs in summer when plants are drought stressed.The plant is unable to adequately absorb enough water from the soil because its roots are damaged and consequently may die.Small swimming zoospores are released which attach to and infect roots,normally behind the root tip. All spores and structures of Phytophthora are microscopic and cannot be seen with the naked eye.There is no way of visually telling if the pathogen is present in the soil.

Phytophthora grows through the root destroying the tissue which is then unable to absorb water and nutrients.Further zoospores are produced in sporangia, particularly when the soil is moist and warm, and are released into the soil.Consequently zoospore numbers can build up quite rapidly. Zoospores move in water and may infect neighbouring plants especially those down slope from a site of infection.These spores are easily transported in storm water,drainage water,contaminated soil and on tools,footwear and vehicles. A further two spore types may be produced,a chlamydospore and an oospore, which are survival structures produced when conditions become unfavourable such as when a food source is exhausted or in periods of low temperature or drought.These spores are capable of surviving for extended periods of time,and when conditions become favourable they germinate and renew the life cycle.This allows Phytophthora to survive in dead plant tissue for a number of years.

At present there is no one simple method for controlling Phytophthora cinnamomi.A combination of sanitation measures, good horticultural management,selective use of some fungicides and the addition of organic matter to soils can be used to retard the activity of Phytophthora.”

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