What is Dieback? 

Life cycle

Phytophthora dieback spends its entire life in the soil or in plant tissue. The life cycle of Phytophthora cinnamomi depends on moist conditions that favour survival, sporulation and dispersal of the pathogen. The highest levels of the pathogen occur within soils after significant rainfall and remain at high levels for a significant period of time. The exact period of time is unclear and is dependant on a number of variables including soil type, presence of vegetation and temperature.

Click here to view the lifecycle diagram of Phytophthora cinnamomi.

The pathogen extracts its food from living plant tissue because it is incapable of photosynthesis. Extraction of food is completed by the mycelia, which form the body of the organism and grow through the host tissue. Mycelia will continue to grow in the host tissue when the moisture content is greater than 80%. The transfer of mycelia or diseased plant tissue from one area to another during favourable environmental conditions can start a new infestation. After infection the pathogen invades root bark and forms lesions that may extend in to the plant's stem collar.

Under ideal environmental conditions, P. cinnamomi reproduces vegetatively. It is also capable of producing millions of tiny spores that reproduce the fungus. Two kinds of spores are likely to be found, zoospores and chlamydospores.

Zoospores

P. cinnamomi infects the roots of healthy plants with microscopic spores called zoospores. The spores are attracted to the roots of susceptible plants leading to the infection of the root system. This infection leads to a progressive dry rot of the plants roots that deprives the plant of water and nutrients, eventually resulting in the death of the host plant. Zoospores are fragile and short-lived and are produced in large numbers under favourable conditions. They can be carried in water over large distances.

Under ideal environmental conditions (high soil moisture & warm daily temperatures) millions of zoospores are produced that are all capable of infecting a new host plant. In the south-west of Western Australia these conditions are typical in spring and autumn. As a consequence the majority of new infections occur in between September to November and between April to May.

Chlamydospores

Chlamydospores are larger than zoospores and are also much tougher and long-lived (within dead plants and soil). They are produced under unfavourable conditions and are the resistant long-term survival stage of P. cinnamomi . They can be transported in soil or roots and then germinate to cause a new infection when exposed to favourable conditions. They germinate to produce mycelia and zoospores