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.
There are no effective fungicides approved for use by gardeners.