Species of Phytophthora are responsible for a small number of significant, but very important plant diseases. These can severely damage the plant involved and can also lead to total dieback and death of the affected plant. The most significant are potato and tomato blight, phytophthora root rot, phytophthora bleeding canker and ramorum dieback.
Phytophthora root rot (several Phytophthora species)
Phytophthora root rot is the most common cause of root decay in a wide range of trees and shrubs. Root systems of even large trees can completely rot away.
Attacks: Numerous woody plants – trees, shrubs and climbers – as well as herbaceous perennials, bedding plants and bulbs.
Symptoms: Visible symptoms on the leaves and stems don’t often develop until the root decay is well advanced. These symptoms are typical of the plant being unable to take up water and nutrients from the soil by its roots, and include wilting, yellowing or sparse foliage and stem dieback. Waterlogging, drought, root pests and other root diseases cause similar symptoms.
Phytophthora bleeding canker (various Phytophthora species)
Attacks: Aesculus hippocastanum and A. carnea (horse chestnuts), Alnus (alders), Tilia (lime trees).
Symptoms: Bark infection producing cankers that bleed a dark, sticky fluid. Dead bark falls away on older cankers, exposing the wood. Rapidly spreading infections cause branches to die and even death of part or all of the tree. Sparse and pale foliage.
Horse chestnut bleeding canker is also caused by a bacterial disease brought on by Pseudomonas syringae pv aesculi.
Ramorum dieback (Phytophthora ramorum and P. kernoviae)
This disease is sometimes referred to as sudden oak death, but this name should be restricted to the USA, as it rarely attacks oaks in the UK.
Attacks: Camellia, Fagus sylvatica (beech), Kalmia, Larix (larch), Magnolia, Pieris, Rhododendron and Viburnum.
Symptoms: Symptoms are very variable, but include: brown, spreading lesions on the leaves; cankers or lesions form on stems, spreading onto leaves; wilting and dieback; areas of dead bark, known as bleeding cankers (see above), oozing a black or brown liquid; on larch the needles become purple or black and drop prematurely.
Potato and tomato blight (Phytophthora infestans)
This is the most devastating disease of both potatoes and tomatoes. See our page on potato blight and tomato blight for full details.
Several species of soil-borne Phytophthora cause damping off in seedlings.