Ornamental Tree and Shrub disease problems

Mr John S. Brereton (BSc Dip Ag Sci Grad Dip Ed Admin)

Tree and shrub die back caused by Phytophthora cinnamomi

Many Australians have now heard of the “dieback” disease which has devastated some of our valuable heath land and forested areas as well as causing significant horticultural production problems in some areas. In some cases the impact of the presence of this disease can be extreme with very large losses of susceptible species. For example thousands of native plants died in the Canberra Botanic Gardens at Black Hill between 1969 and 1972 when an outbreak of Phytophthora cinnamomi occurred there. In the Brisbane ranges in Victoria the understorey of the dry sclerophyll woodland areas has been decimated with the losses of shallow rooted species such as Xanthorrhoea australis. Many dead stumps of this highly susceptible plant can be seen on walking through this area. Other significant areas affected by this disease include parts of the Grampians, Wilson’s Promontory area in Victoria and Jarrah forest areas in Western Australia. In addition to the immediate problems caused by Phytophthora cinnamomi longer term ecological problems often occur as native flora changes over time accompanied by plant and animal habitat change and possibly water table changes.

Phytophthora cinnamomi is one of more than 17 significant fungal species and is recognised as the most widely distributed species in Australia with an extensive plant host range including both native and exotic plants. Particular interest in this disease problem has occurred because of problems in Australia with commercial crops such as avocadoes, pineapples, proteas and other proteaceous plants.

Phytophthora cinnamomi is a microscopic, soil inhabiting, root rotting fungal organism causing dieback in native and exotic vegetation. In Victoria commercially significant trees affected by the organism include Eucalyptus obliqua, E sieberi and E. mullerana.
Early attempts in Australia to diagnose this problem of dieback in native vegetation confused the symptoms of this disease with drought stress. Growers of trees and shrubs should be aware that when plant roots become stressed or diseased for a variety of reasons the early symptoms are often expressed as dieback in the crown particularly when the trees or shrubs are drought stressed. Initially this will commence with wilting and gradual dessication of foliage.

It is for this reason that in the absence of a pathological examination that this disease problem can be confused with drought problems.

Methods of infection

Phytophthora is a root invading pathogen that develops rapidly in the presence of susceptible roots and warm moist soil. The disease causing organism will attack fine feeder roots and destroy them so that water uptake is diminished. Symptoms of the disease normally appear when the plant becomes stressed such as under hot dry conditions. As infection takes place the root cells collapse in advance of the invading fungal organism. The extent of injury and speed with which the host plant dies depends on whether the Phytophthora has a root pruning affect or is spreading rapidly within root tissue.

The disease causing organism will be transmitted rapidly in water, along drainage channels and natural water courses. The fungus will spread via soil, infected roots and fungal-growth itself into the soil. Warm, wet to water logged soils are ideal for the development of the disease

The most important methods of dissemination are through movement of contaminated soil, water and plant material particularly infected roots. Other methods of distribution include vehicles, footwear, roadside grading and trenching equipment. Persons engaged in the selection and planting of trees and shrubs should source plants carefully to avoid the introduction of this problem into a new growing sites.

Species susceptibility

Susceptibility of trees and shrubs both exotic and native varies. As far as Eucalypts are concerned the subspecies monocalyptus is particularly susceptible. Under story trees and shrubs in open areas are often shallow rooted and highly susceptible. The fungal organism is actually attracted to the roots of many native plants including a wide range heathland species, hakeas, proteas, Banksia spp, proteaceus plants generally and Xanthorrhoea spp.

Soil conditions that are conducive

Detrimental conditions include soils that are shallow and poorly drained or subject to intermittent waterlogging; often duplex-soils where the topsoil has poor infiltration characteristics. With increasing topsoil depth and increasing levels of organic matter susceptibility may decrease.

Suppressive soils

Suppressive soils are characterised by high levels of organic matter, nitrogen in the ammoniacal form and high cation exchange properties, Such conditions may be found on red basaltic soils. Significant improvement in soil conditions to make them more suppressive to the disease causing organism can be achieved through the addition of organic matter such as composted pine bark. The basis for suppressive soils is related to the presence of competitive and antagonistic micro-organisms in the soil, which act against the Phytophthora sp.

Strategies for control in nurseries include:

Careful sourcing of plant material and growing media,
Heat treatment of potting mixes,
High levels of hygiene,
Testing of water supplies, water disinfestation via filtration, UV light treatment, chlorination, ozone treatment of contaminated water supplies,
Avoidance of over watering,
Isolation when outbreaks occur,
Use of composted growing media,
Appropriate use of eradicant fungicides and disinfection of equipment with quaternary ammonia compounds.

Strategies for open spaces include:

Avoiding movement of infected plants, soil and water into clean areas,
Monitoring runoff water,
Improving drainage conditions,
Public education to avoid moving of infected plant material or soil,
Planting more tolerant species or dilution of plant population which consist of highly susceptible species,
Maintenance of high organic matter levels where possible,
Monitoring of soil and water for the presence of the disease causing organism

Where outbreaks are suspected to have occurred accurate diagnosis is important, persons should contact a plant health services laboratory for testing and confirmation of the presence of this organism. Portable field test kits are also available. This is a disease problem to avoid.