Phytophthora cinnamomi is a microscopic soilborne organism, invisible to the naked eye, which causes root rot of a wide variety of plant species including many native and introduced plants. Other species of Phytophthora may cause diseases on a wide range of plants but are generally less severe. The biology and control measures are very similar so this outline will concentrate only on Phytophthora cinnamomi.
Signs of Phytophthora infection
Infection often results in the death of the plant, with earlier symptoms including wilting, yellowing and retention of dried foliage and darkening of young feeder roots and occasionally the larger roots. Phytophthora cinnamomi requires moist soil conditions and warm temperatures to be active, but damage caused by the disease most often occurs in summer when plants are drought stressed. The plant is unable to adequately absorb enough water from the soil because its roots are damaged and consequently may die. Small swimming zoospores are released which attach to and infect roots, normally behind the root tip. All spores and structures of Phytophthora are microscopic and cannot be seen with the naked eye. There is no way of visually telling if the pathogen is present in the soil.
Phytophthora is easily spread
After infection Phytophthora grows through the root destroying the tissue which is then unable to absorb water and nutrients. Further zoospores are produced in sporangia, particularly when the soil is moist and warm, and are released into the soil. Consequently zoospore numbers can build up quite rapidly. Zoospores move in water and may infect neighbouring plants especially those down slope from a site of infection. These spores are easily transported in storm water, drainage water, contaminated soil and on tools, footwear and vehicles.
Phytophthora can survive in soil and dead plant tissue for a long time
A further two spore types may be produced, a chlamydospore and an oospore, which are survival structures produced when conditions become unfavourable such as when a food source is exhausted or in periods of low temperature or drought. These spores are capable of surviving for extended periods of time, and when conditions become favourable they germinate and renew the life cycle. This allows Phytophthora to survive in dead plant tissue and in the soil for extended periods.
Phytophthora Dieback kills plants and infection is permanent
Management objectives are:
- Keeping it out
- Limiting the spread and
- Reducing the impact, using
- Quarantine and
- Treatment or destruction of infected plants.
At present there is no one simple method for controlling Phytophthora cinnamomi. A combination of sanitation measures, good horticultural management, selective use of some fungicides and the addition of organic matter to soils can be used to retard the activity of Phytophthora.
Phytophthora prevention and control in the nursery
All plants should, wherever possible, be grown in soil mixes which have been correctly steam-air pasteurised (30 minutes at 60ºC). If it is not possible to pasteurise mixes make sure that the mix components are disease free. Ensure that the potting mix is not subsequently contaminated. eg. by water draining into soil bins in heavy rain or by careless handling with implements. Soil mixes should permit free drainage; a potting mix which allows air into 15% of air spaces after watering is recommended.
Plants brought into nurseries from outside sources should ideally be propagated by cuttings to prevent the importation of Phytophthora (and many other disease and insect pests), or quarantined.
All previously used pots and containers should be free of soil prior to use and sterilised by soaking in a solution of a disinfectant/detergent compound. It is essential to remove soil by washing prior to soaking in order to achieve maximum kill of the pathogen. It is also important to wash implements (cutting knives, secateurs etc.) regularly to remove any possibility of transferring the fungus from one plant to another.
Avoid bringing contaminated soil on boots and equipment into the nursery areas. Phytophthora cinnamomi can survive in very small quantities of soil for long periods of time so nursery sanitation is very important.
All plants should preferably be grown on raised wire-mesh bench at least 30 cm off the ground; this minimises water splash, which may possibly contain the fungal spores, from the ground onto the plants. If this cannot be achieved plants should be grown on free draining blue metal.
Keep the whole nursery area clean and free of dead plant material and refuse.
If a plant becomes infected, or is suspected of being infected, if possible it should be carefully examined (without contaminating other plants) for symptoms such as darkening of young rootlets. Infected and dead plants should be removed and disposed of carefully. Burning the infected plant or disposal in garbage are the most satisfactory methods of disposal. Infested potting soil should be carefully disposed.
Phytophthora prevention and control in the garden
Sanitation of tools, machinery and boots is probably the most effective means by which the spread of Phytophthora cinnamomi can be limited. Spades and other tools should always be washed free of soil before and between plantings. In addition, tools should be regularly drenched in a solution of detergent or disinfectant. A large drum containing this solution should be placed in a convenient place in the depot and tools should be regularly brought back, washed to remove soil and drenched. The more frequently this is done the better the control of any soilborne diseases, particularly Phytophthora, will be. In situations where you are planting a number of plants take a container of disinfectant with you and disinfect tools between replanting.
Boots and tyres are also an important means by which Phytophthora may be transported, as soil containing the fungus may cling to the boot or tyre. Wherever possible remove soil from boots and tyres and limit the movement of soil and the fungus. Vehicles should move towards known infected areas and be washed down after working in these areas before use in clean areas. It may not always be feasible to remove all soil, however limiting the movement of large amounts of soil by washing off with water will suffice in most situations. Sanitation procedures may seem time consuming and annoying, but prevention and limitation of a disease such as Phytophthora is the most effective means of disease control.
Regardless of whether the pathogen is present in a soil, it is important to add quantities of organic matter such as mulches, manures and composted material to the area (if this is appropriate to the plant species). These components increase the level of soil micro-organisms, such as fungi (eg. Trichoderma), actinomycetes and bacteria, which suppress the activity of Phytophthora and retard disease development. Mulches also minimise the contact between soil and footwear so that there is less potential for the transport of soil.
Maintain nutrient levels so that root growth is promoted, but however do not use inappropriate nutrient mixtures that may be deleterious to the plant (ie. take care with phosphate sensitive plants). If possible, plant in holes sufficiently large enough to promote rapid root growth, this combined with good nutrition will allow the plant to compensate for any root damage caused by pathogens.
Never use techniques such as post-hole diggers to prepare planting holes as these techniques result in poor drainage, thus enhancing disease development, and may aid in the spread of the pathogen. Ensure that drainage is adequate to prevent water logging, which promotes disease incidence and severity. All run-off water from known infected sites should be contained and directed to the storm water channels. Remember that water can very easily transport the swimming zoospores of Phytophthora cinnamomi.
Care of infected garden plants
Fungicides containing potassium phosphonate are registered for control of this disease in certain situations. Information on these fungicides can be obtained from your local nursery or on the websites of the manufacturers. It is however important to ensure that application occurs when the plant can be expected to be actively exporting from the leaves to the root system ie. in summer (once in early summer and then 4-5 weeks later), so that the chemical is transported to the roots where it is required. Plants should be sprayed for quite a wide area around the infected site.
If you have to move or replant material never move a plant from an infected site to an uninfected site. If the species is required in these circumstances, repropagate by cuttings. As in the case of initial plantings, the preparation of the site, the addition of organic matter and the attention to drainage are all essential when replanting material.
When removing plants it is essential to remove as much of the tissue, including roots, as possible. The pathogen may persist in dead tissue for many years. Dead roots and any pruned material should be disposed of carefully. Do not replant in the same plant hole; where possible plant away from the dead plant, preferably upslope as plants downslope from any site of infection will be at greatest risk from the disease. Remember that unassisted movement of Phytophthora up a slope is very slow, while downward movement may be quite rapid.