In general, Leylandii are disease free. However, here are a list of diseases that can occasionally effect Leylandii.
Pestalotiopsis and Botrytis are fungal diseases that are spread by wet conditions. It generally only infects plants that have been weakened in some way, such as attack those trimmed too often or too hard, particularly late in the autumn. Under the right conditions, Pestalotiopsis and Botrytis can affect many plants including conifers and Leylandii.
In the past, diseases such as Pestalotiopsis and Botrytis were only found rarely on production nurseries where plants were grown very close together with overhead sprinkler irrigation making the leaves wet every day. There is evidence, however, that over-enthusiastic trimming of Leylandii creates a lot of dead material or "thatch" in a hedge. This thatch can act like a sponge and hold water, keeping the hedge wet for extended periods of time. The occasional attacks by Cypress Aphid can also make disease more common in hedges so it is important to check Leylandii hedges for Cypress Aphid in January or February and take action if they are found (see below).
The symptoms of Pestalotiopsis and Botrytis are the tips of the shoots turning brown and dying back causing random brown patches. However, these symptoms can also be caused by a number of other factors including Cypress Aphid, a blunt blade on a hedge-trimmer or trimming at the wrong time of the year.
Plant Leylandii properly (follow our instructions) so their roots get established well.
Avoid trimming in very wet or very dry conditions.
Trim from early spring to early summer.
Trim once (or twice a year). We do not recommend trimming more than twice a year.
Remove all dead material and "thatch" from a hedge after trimming using your arm or a rake or brush.
There are no fungicides recommended for specific use against Pestalotiopsis but products such as Sythane Fungus Fighter and Fungus Clear Ultra may be used at your own risk.
Check for Cypress Aphid in January and February every year and spray with Scott’s Bug Clear Ultra or Bayer Garden’s Provado Ultimate Bug Killer if they are found.
Honeyfungus is a common fungus that is often found in the soil. It attacks the roots of most plants including conifers and Leylandii. No plants are immune from attack but some are more tolerant than others. Privet hedges are particularly susceptible to the disease.
If whole plants within a hedge are dying and, over the years, the symptoms are spreading to neighbouring plants in the hedge, it is possible that the hedge is being attacked by a soil borne disease such as Honeyfungus.
Honeyfungus spreads through the soil by black ‘bootlace’ structures called Rhizomorphs. These can be seen in the soil but can be difficult to spot.
Examine dead material by scraping back the bark near the soil (up to 1 metre from the soil surface). If this reveals a white layer of fungal-like growth that smells strongly of mushrooms, then it is likely that it is honeyfungus.
Infected roots are dark brown or black.
Clumps of honey-coloured toadstools appear near the hedge.
Unfortunately, there is no chemical control.
Dig up and destroy the infected plants, including roots and stumps if possible. Do not recycle or compost the plant material.
The spread of honeyfungus may be inhibited by inserting a vertical barrier of heavy polythene or a pond liner (butyl rubber) in the ground to about 45cm (18 inches). The barrier should protrude above the ground 2-3cm (1 inch). This will hopefully prevent the spread of the rhizomorphs to other plants in the hedge.
Improving the growth of Leylandii by attention to soil drainage and nutrition, should render them less likely to succumb to honey fungus.
Phytophora is another soil-borne disease that can be introduced into the garden in various ways, such as on infected plants, in irrigation water or in soil or wind-blown dust. The fungus may be dormant in the soil for years and may only infect plants when the conditions are suitable. Whole plants will turn a grey/brown and then die.
Phytophora tends to infect plants that are under stress and especially on poorly drained soils. It can affect Leylandii when they are small but tends to have less effect on larger, established specimens. Many varieties of Lawson’s Cypress are more prone to Phytophora and should be grown on free-draining soils. Generally, once the foliage starts to turn brown, the infection is well advanced and the plant cannot be saved. Dig up the plant and remove as much of the soil, roots and stump as possible.