Phytophthora sp.

Phytophthora is a soil borne, root decaying Oomycete (water mould). It is a major threat to Australian biodiversity and many horticultural industries. The disease affects thousands of plant species with some being more resilient than others. Many native Australian plant families are particularly susceptible to infection, exposing vital ecosystems to risk of irreparable damage.

Plant roots are the primary site of infection. The chemical signals released into the soil by the roots attracts the disease, which then starts attacking the feeder roots inhibiting the plants uptake of water and nutrients. Early symptoms are consistent with water stress: wilting of leaves, lack of vigour and dieback of new shoots.

Trees will then often exhibit classic stress responses: development of epicormic growth and heavy production of undersize fruit. Healthy specimens that are particularly susceptible may die in a short period of time. More tolerant species may die back over the course of years, with crown thinning and the stress induced epicormics dying off.

Phytophthora favours moist soil with temperatures ranging between 15-30˚c. It can lay dormant in adverse conditions for many years and proliferate when the ideal conditions are present.
Previously, Ph ytophthora, was referred to as fungi, it is now excepted to be more closely related to Algae.

Identification

Observe for symptoms that are consistent with phytophthora (Die Back, lack of vigour). These symptoms are also associated with root damage and other plant conditions, so check for recent excavations around trees and monitor for other conditions. Check the surrounding area for signs of dieback in other trees, phytophthora will often have a front that travels down hill with the flow of water.

For a confirmation of diagnosis, soil and root samples would need to be collected and sent off to a lab for analysis.

Transmission

The movement of soil and organic matter by humans is the main method of transmission. Locally, phytophthora moves through the flow of free water in soil and organic matter, although movement uphill is possible through root-to-root contact.

Host Species

Many plant species are susceptible: native plants in the PROTEACEAE, ERICACEAE and MYRTACEAE families are particularly at risk and their loss presents the biggest threat to loss of habitat and biodiversity in Australia.

Symptoms and Damage

Early symptoms: Dieback of branch tips, wilting, lack of vigour, stress induced epicormic sprouting, heavy production of undersized fruit.

Median symptoms: Dieback of stress induced epicormic growth, more substantial branch dieback.

Advanced Symptoms: large sections of tree have completely died-back, tree has completely died.

Damage – Decays feeder roots – Which inhibits the uptake of water and nutrients, eventually killing the tree.

Control

Once the disease is established on a site it is very difficult to eradicate it, therefore the main method of control is to minimise the spread.

Humans are the biggest vector for the spreading of phytophthora. Appropriate care should be taken to avoid introducing contaminated material or water to new sites, especially in sensitive areas.

When entering sensitive areas, a decontamination protocol should be implemented (cleaning and disinfecting of vehicles, tools, equipment, and clothing)

Phosphite treatments are useful on sites that are infected with phytophthora, it involves either stem injection or foliar spray of host plants to prevent the spread of the disease. This method does not eradicate the disease from the site but provides protection for host plants. Plants should be monitored and retreated. Stem injection generally provides protection for up to 10 years, foliar spray may only be 1-2 years. Surrounding plant communities should be considered as plants that grow in low phosphorous environments may be adversely affected.

Images courtesy of Brisbane Insects and Spiders Home Page and the European Institute Of Planted Forest.