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Dieback Disease of Sydney Red Gums
Introduction
Scientists from the Plant Disease Diagnostic Unit at the Botanic Gardens Trust in Sydney, have been carrying out a survey of Sydney Red Gums (Angophora costata and Eucalyptus species), around Sydney Harbour. The gums are showing signs of dieback - progressive loss of leaves, death of upper limbs and eventual death of the whole tree. Dieback can be caused by different factors, or several factors working together. One of these is an introduced plant pathogen known as Phytophthora cinnamomi or Pc.
The Pathogen
Phytophthora cinnamomi (Pc) is a microscopic, soil-borne, water-mould that causes root rot in a wide variety of plant species, including the Sydney Red Gum and other eucalypts. Initial symptoms include wilting, yellowing and retention of dried foliage, loss of canopy and dieback. Infected roots blacken and then rot, limiting water and nutrient uptake. Severely infected plants will eventually die. The symptoms are often more obvious in summer when the plants are also stressed by drought. Changed nutrient loads and drainage patterns from urban run-off may also stress the trees and make them more vulnerable to Pc infection.
Pc requires moist soil conditions and warm temperatures to be active. It produces minute organs called sporangia, from which small swimming spores (asexual spores) develop. These move through moist soil and attach to plant roots, usually just behind the root tips.
Once it has infected the host plant, Pc grows through the roots, destroying root tissue and stopping the plant from absorbing the water and nutrients it needs to live.
Pc spores can travel from an infected plant to neighbouring plants, especially those on the down-slope from a site of infection. The spores are easily transported in storm and drainage water. They encyst on the roots and then germinate, causing infection. This requires high moisture levels. Once inside the host, the pathogen grows readily and is independent of external moisture conditions.
Sporangia and spores survive for short periods. However, the pathogen can also produce thick-walled, resistant, survival structures known as chlamydospores. These can survive in plant tissue for long periods and are the major mechanism for long-term survival in infested sites. They can also be involved in long-distance disease spread in transported soil, on footwear, and on vehicles.
The Team
Dr Brett Summerell
Dr Brett Summerell (PhD): Brett has been a Plant Pathologist at the Royal Botanic Gardens for 14 years. He is responsible for liaising between the various organisations interested in curbing the spread of Pc, particularly around Sydney Harbour and arranging the funding to allow the project to continue. Brett also keeps these organisations and the general media informed of the results of the disease surveys and tells them the best way to avoid spread of the disease.
Bob Makinson
Bob Makinson (BA (Biol): Bob is the Coordinator of the Centre for Plant Conservation. His role is to liaise between community groups and scientific institutions to develop public awareness of the effects of Phytophthora outbreaks and their control measures. He has organised three workshops, gathering people together to discuss and plan strategies.
Suzanne Bullock
Suzanne Bullock (MSc): Suzanne is a Senior Research Officer in Plant Pathology at the Botanic Gardens. She works part time, for three days a week. She is responsible for testing soil samples that are sent to the Diagnostic Laboratory as part of Pc surveys, and determining if the pathogen is present. She maintains a collection of Pc and other Phytophthora species for studies on the variations in plant host resistance and the pathogen population.
Chris Howard
Chris Howard (B. Biotech Hons): Chris has recently started working on a PhD study of Pc. He is hoping to increase our knowledge of the genetic variability of the pathogen from various places in South Eastern Australia. This may help us understand how this pathogen behaves in Australian natural ecosystems and how we can best control its spread. Chris completed his degree at Wollongong University and has worked as a Research Assistant in the medical field.
The Survey
Collecting Samples
Soils, to a depth of around 20 cm, are collected using a small hand trowel, from around the bases of suspected infected trees, and placed in carefully labelled plastic bags. It is very important to sterilise the trowel between the collection of samples with 5% bleach to prevent the contamination of samples or accidental spread of the pathogen.
Laboratory Testing
Back at the lab, the soils are mixed with de-ionised water in disposable cups. The mixture contains 1/3 soil to 2/3 water.
Cubes of Pc-specific agar are added to the soil slurry. Agar is a clear gelatinous material derived from marine algae and is often used as a base for culturing micro-organisms. The agar has nutrients that the Pc needs for growth added to it as well as antifungal and antibiotic agents that inhibit the growth of other fungi and bacteria.
Once the agar cubes are added to the soil slurry it is left for two days to allow the Pc spores to be attracted to the agar and separate from the soil. The slurry is then refrigerated. The shock of the cold stimulates the Pc to release its spores which then settle on the agar and start to germinate on the nutrient-rich surface.
The agar is then removed from the soil slurry, rinsed, blotted to remove excess water and placed on a plate of fresh, selective agar. It is then left for two-three days to allow the spores to germinate into a colony of Pc. The plates are then examined under a microscope to identify any Pc that may be present. Pc has thick hyphae with characteristic swellings that make it easy to identify.
Conclusion
At present there is no one single method for controlling Pc. A combination of sanitation measures, good horticultural management, drainage control, control of other plant stress factors, selective use of some fungicides and the addition of organic matter to soils can be used to retard the activity of Pc.
What you can do to help
Control measures for bush regeneration
Any activity that involves soil disturbance has the greatest potential for movement of the pathogen. Therefore:
- Always assume that the area you are about to work in is free of the disease and therefore needs to be protected against infection
- Conversely, always assume that the activity you are about to undertake has the potential to introduce the disease
- Clean equipment, machinery and footwear before entering the area
- When removing weeds, place them carefully into plastic bags for removal from the site
- Always work from the lower part of a slope to the upper part
- Always work in areas known to be free of the pathogen before working in infected areas
- Minimise activities where ever possible when the soil is very wet
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