Holly leaf blight is an infection of the leaves and stems of holly by a fungus-like (Oomycete) organism, Phytophthora ilicis. It has become an increasingly frequent problem in the past 20 or so years. Damage is often first seen in winter; fresh outbreaks of the disease tend to follow a period of wet, cool weather.

This is a disease specific to holly, although some species show resistance. See the section on 'Control' below for more information.

You may see the following symptoms:

• On leaves: Black or purple blotches appear, often quite difficult to see against the dark green glossy colour. Affected leaves drop prematurely
• On stems: Smaller stems can be attacked, blackening in the affected areas
• In hedges, the disease often causes ‘arches’ of defoliation, where the damaged area is widest near the ground

The leaf blotching caused by holly leaf miner can sometimes be mistaken for holly leaf blight.

Non-chemical control

Cut out infected areas as soon as they are detected, collect all fallen infected leaves and burn or remove to land fill.

Take great care not to buy infected plants from nurseries. Check if suppliers are aware of the disease, whether they have experienced outbreaks, and whether they use fungicides that suppress Phytophthora but do not kill it.

As a precaution, keep new hollies separate from others in the garden and under close observation for one or two months, to ensure they are really disease-free, before planting in their final position.

Information from the USA suggests the following Ilex species show resistance:  Ilex cassine, I. cassine  var. angustifolia, I. ciliospinosa, I. crenata cvs. ‘Convexa’ and ‘Hetzii’ and var. paludosa, I. glabra, I. intricata, I. latifolia, I. perado, I. pernyi, I. sugerokii and I. vomitaria.

In the UK, the disease affects primarily I. aquifolium, I. crenata, I. × altaclarensis, I. dipyrena and I. kingiana and has been found on I. colchica, I. pernyi var. veitchii and some clones of I. apaca.

Chemical control

There are no effective fungicides available for use by gardeners.

The causal organism, Phytophthora ilicis, is believed to originate in northern America where it was first described and to have been introduced accidentally to the UK and northern Europe, probably during the last century. It was first recorded in the UK in 1989 and was rarely recorded at that time, but there has been an upsurge in the past 20 or so years, for reasons unknown. Like other Phytophthora species, P. ilicis is a micro-organism more closely related to the algae than the true fungi.

It releases its infective spores into water, therefore wet conditions are required for infection. It is carried in run-off water, wind-blown rain, probably in contaminated soil and possibly by birds and other animals. It has also been detected in infected nursery stock.

In hedges, patches of infection are ‘arch shaped’, widest close to the soil, suggesting that infection may occur when spores are splashed upwards. Wounds in the leaves, which occur naturally from holly spines, appear to be necessary for the spores to gain entrance. The pathogen probably survives the hotter, drier seasons as resting spores in infected leaves and stems, breaking out in cooler wetter conditions, but many details of the biology of this organism remain uncertain.