Above-ground symptoms often do not develop until the root decay is well advanced. They are not specific to Phytophthora root rot, and merely indicate that the plant is having trouble taking up water and nutrients through a poorly-functioning root system. Other factors causing root problems, such as waterlogging, drought or other root diseases (e.g. honey fungus) will cause similar foliar symptoms.
Symptoms include wilting, yellow or sparse foliage and branch dieback. In many cases the symptoms get progressively worse until the plant dies. A common symptom in conifers is a gradual fading in the colour of the foliage, from a vibrant to a dull green, through to greyish and finally brown.
Below-ground examination of the roots, collar and stem base of an affected plant will reveal a poor root system. Many of the fine, feeder roots will have rotted away. Some or all of the larger roots will also show evidence of decay – they will be brown or black internally, softer than normal and may break easily. Because Phytophthora species are microscopic organisms there will be no evidence of the pathogen itself in association with the decay, unlike with honey fungus where a prominent white fungal growth may be found below the bark. Very similar root symptoms to those of Phytophthora infection can be caused by prolonged waterlogging, and the situation is further complicated by the fact that Phytophthora root rot is frequently associated with waterlogged soils. Laboratory examination is often required to determine whether root decay in these situations is due to waterlogging or Phytophthora root rot.
In severe infections Phytophthora invades the collar or stem base of the plant, causing a brown or black discoloration below the bark (often seen at the stem base as an inverted ‘V’). This area of infection is sometimes visible externally as bark discoloration and/or weeping, although once again such symptoms can be caused by other factors such as drought, waterlogging or pest attack.