Plant Diseases

Plant Diseases in the south-west of Western Australia - a summary.

by Dr Ray Wills

November 1995

School of Earth and Environment, The University of Western Australia.

Introduction

This paper presents the three main plant diseases in south-west Australia - Phytophthora, Armillaria, and cankers - and discusses the threat these fungi pose for conservation values in the State.

Organisms that cause plant diseases are called pathogens. They include microorganisms such as fungi, bacteria and viruses, some species of protozoas and nematodes. Pathogenic organisms are usually a normal component of the soil population and naturally exist in relatively low numbers. Some pathogenic species only cause disease in one species of plant, but others can cause disease on plants that are not closely related to each other.

To enter a plant root the bacteria or fungi must first be present in the rhizosphere of the plant. It then uses molecular signals to recognise whether the plant root is susceptible to entry or not. The pathogen attaches to the root surface possibly by the use of hair-like structures and enters the root. Some pathogens enter through areas that have been damaged by animals and some fungi and bacteria produce enzymes that dissolve the chemical compounds that make up the cell wall. Once the pathogen is in the plant cells, the plant may try to prevent its spread by producing chemical or physical barriers. These procedures may confine the pathogen to a portion of the root.

Dieback Disease

The disease known variously as "jarrah dieback", "wildflower dieback", or "dieback disease" has long been recognised as a serious threat to the conservation of both flora and fauna in Western Australia, especially in the infertile Northern and Southern Sandplain regions of the state where the greatest diversity of plants are found. The disease is caused by a soil-borne pathogen (technically classified as a water mould, not a fungus), Phytophthora cinnamomi. The unwitting movement of pathogen-infested mud by vehicles travelling from infested into healthy areas is by far the most important means of dispersal of the organism, although movement of the pathogen in mud carried along trails by both bushwalkers and animals can also be important.

Impact of dieback on a stand of jarrah at the Ronson Formation near Dwellingup, Western Australia.

The scientific name "Phytophthora" (pronounced "fy-tof-thor-a" - derived from Greek "phyton" meaning "plant" and "phthora" meaning "destruction"). Many plant species are killed by this water mould, and important families of plants from Western Australia contain a large number of susceptible species. In 1980, Phytophthora cinnamomi had been isolated from 967 plant species world-wide, with nearly half of these records from Australia. Other species of Phytophthora also pose a threat and are also capable of killing a large range of native plants. Although it is not known how many species of plants might be susceptible, it is estimated as many as 2000 of the 9000 species in the south west may be susceptible to dieback disease (Wills 1993).

Which plants are affected?

Dieback disease generally affects woody shrubs. Grasses, sedges, rushes, orchids etc. have rarely been recorded as hosts and have usually been regarded as resistant to infection. However, the dieback wate mould is exceedingly invasive and is capable of infecting the roots of many of the so-called 'resistant' species so that even these species will act as hosts. For example, the primary symptom of infection - death and discoloration of tissue in the root - has been observed in some grass species, but rapid root regeneration by the plants allowed them to survive. Thus, in the case of dieback disease, the term 'resistant' is applied to many species which show no immediately apparent secondary symptoms of the disease (yellowing of the leaves, canopy dieback, and/or death of the plant), although they may suffer a decrease in vigour. Only a few species have so far been found which can completely inhibit fungal growth from the point of entry, and only these can be regarded as truly resistant. The water mould invades the conducting elements of the plant and causes the failure of root conductance, and so plants which are killed die largely as a result of drought.

A study of plant communities in the Stirling Range National Park assessed 330 species for susceptibility to P. cinnamomi. 36% were recorded as having at least some individuals in a population killed by the water mould, with 28% of these highly sensitive to the pathogen (more than 80% of plants in a population killed). 47% of woody perennials surveyed were susceptible. Species' susceptibility generally affected groups of related plants with the result that some families had large numbers of susceptible species while others were apparently unaffected by the pathogen.

LANDSAT image of the Stirling Range National Park, Western Australia.

The four most important woody plant families in Western Australia are the Myrtaceae (eucalypts, paperbarks, bottlebrushes etc. making up 737 described species), the Papilionaceae (pea-flowered plants with 639 described species), the Proteaceae (e.g. Banksia, Grevillea, Dryandra, Hakea etc. making up 536 described species), and the Mimosaceae (wattles, 422 described species).

Notably, 85% of species from the Proteaceae assessed for susceptibility to P. cinnamomi were at least partially affected by the water mould. My research while at the Department of Conservation and Land Management (CALM) suggested that as many as 2000 of the 9000 native plant species in south-west Australia may be susceptible to Phytophthora cinnamomi (Wills 1993).

Why does the water mould cause so much damage?

Variation in annual rainfall has a significant influence on populations of Phytophthora cinnamomi due to its affect on soil moisture. Any rainfall pattern promoting moist soil conditions over summer will allow rapid growth of the water mould within the host and thus may cause significant disease expression. However, the most severe impact of the pathogen may well occur after the combination of autumn and summer rains. Autumn rains would contribute to the early recharge of soil moisture after summer and allow an increase in population size of the water mould before growth becomes limited by winter temperatures. As temperatures begin to rise in spring, populations of the water mould that had built up in autumn would continue to expand as long as moisture is available. Substantial rainfall in summer would then create ideal conditions for the growth and sporulation of P. cinnamomi.

Substantial plant deaths in the have been reported every few years in the south west - early in 1974, 1976, 1978, 1982, 1986 and 1990. In all cases, deaths were preceded by substantial falls of rain in the previous year, especially in autumn and summer.

Phytophthora cinnamomi has infested many of those habitats which provided a suitable food-base and year-round moist conditions for the water mould. However, areas with seasonally dry soils and dominated by species susceptible to the pathogen are also being invaded in years when above average rainfall produces high soil moisture conditions. Variability of summer rainfall is particularly important since, every few years, heavy summer rains occur, generating free soil water in the warmest months. These circumstances not only provide ideal conditions for the growth of this tropical organism but also favour the initiation and growth of fine roots which are an important avenue of invasion for the pathogen.

Armillaria

Another serious pathogenic fungus in Western Australia is Armillaria luteobubalina. Like the dieback fungus, this fungus can also attack a wide range of plant species, and is also active in many parts of the south-west. As yet no substantial surveys have been undertaken to assess the distribution or impact of Armillaria luteobubalina. Research so far shows that many areas are at risk including wandoo woodlands, karri forest and coastal shrubland communities.

Fruiting bodies of Armillaria luteobubalina.

 

Cankers

Aerial canker diseases caused by a group of fungi including Diplodena are an increasing problem in Western Australia. Several aerially-dispersed, canker-causing fungi have been found in diseased plants from many plant communities over the past decade by CALM researchers. The cankers, including the fungi Botryosphaeria ribis and Diplodina sp., have caused extensive damage to large stands of vegetation on the south coast of WA, particularly since February 1991. It appears likely that unusual climatic conditions along the south coast of WA, with 6 months of serious to severe rainfall deficiency up until May this year, and a heat-wave lasting four days and reaching 47deg. C, have contributed to the rapid growth of the cankers observed in native plant communities since that time. Initial studies reveal that these fungi also have a broad host range, with 46% of species assessed from a range of families were damaged by canker fungi. Again, many Proteaceae were affected, with 82 % of species damaged, and often killed by the fungus. While canker fungi are not a major problem in the Jarrah Forest, they are found throughout the south west and have the potential to cause very serious damage.

Impact of Plant Disease

Plants which provide the main elements of an ecosystem are called 'keystone species' - that is, their presence in an area is critical to the survival of many of the plants and animals that live there, too. In large areas of the south-west, the Proteaceae are the most abundant plant group, acting as keystone species and so providing the fundamental elements of many plant communities. While species of Proteaceae are abundant at healthy sites, they are much less important at sites which had long been infested by Phytophthora cinnamomi. Other species which display low levels of susceptibility to the disease, such as sedge and rush species, are more abundant at sites with a long history of infestation than at healthy sites.

If a keystone species disappears, it is inevitable that many of the species that rely on it will also disappear even though these species are not directly affected by the disease. Plants which must grow in the shade of a dense canopy may disappear as the susceptible plants are killed. Animals may suffer from a loss of refuges since the more open vegetation may offer less protection from predators.

A white-cheeked honey-eater rests on a Banksia coccinea killed by canker disease near Cheyne Beach, east of Albany, Western Australia.

Notably, many species of Proteaceae (e.g. Banksia, Grevillea, Dryandra) have large flowers that are pollinated by nectar-eating birds or mammals. The destruction of large numbers of proteaceous species by wildflower dieback may cause animal populations to dwindle as the nectar sources on which these animals rely are eliminated.

Management

Currently, the most practical management technique for the control of P. cinnamomi in native plant communities is foliar application of the fungicide phosphonate. Field trials in various areas in the south-west on plant communities already infested with Phytophthora cinnamomi have shown that one application of phosphonate gives excellent control of the disease over several years.

While canker fungi are not a major problem in south-west Australia, they are distributed throughout this region and have the potential to cause very serious damage. Fire appears to be the most practical management tool for the regeneration of native plant communities after infestation by canker.

Further reading

Shearer B. L. & Bailey R. (1989) Tree Killer: the fight against jarrah dieback. Landscope 5 (1), 38-44.

Shearer B. L. & Tippett J. T. (1989) Jarrah Dieback: the dynamics and management of Phytophthora cinnamomi in the Jarrah (Eucalyptus marginata) Forest of Western Australia. Research Bulletin No. 3. Department of Conservation and Land Management, Como, Western Australia.

Shearer B. L., Wills R. & Stukely, M. (1991) Wildflower Killers. Landscope 7 (1), 28-34.

Wills R. T. (1993) The ecological impact of Phytophthora cinnamomi in the Stirling Range National Park, Western Australia. Australian Journal of Ecology 18 (2) 145-159.

Wills R. T. & Kinnear J. (1993) Threats to the biota of the Stirling Range. In: Mountains of Mystery - A Natural History of the Stirling Range. (eds eds C Thomson, G P Hall & G. R. Friend) pp 135-141 Department of Conservation and Land Management: Como, Western Australia.

Wills R. T. & Keighery G. J. (1994) Ecological impact of plant disease on plant communities. Journal of the Royal Society of Western Australia 77 (4), 127-131.

Wills R. T. & Robinson C. J. (1994) Threats to flora-based industries in Western Australia from plant disease. Journal of the Royal Society of Western Australia 77 (4), 159-162.

Cowling W. A. & Wills R. T. (1994) Plant Diseases In Ecosystems: threats and impacts in south-western Australia - Foreword. Journal of the Royal Society of Western Australia 77 (4), 97.

Or see my other publications

 

For more information on dieback, check out Giles Hardy's Murdoch University web site,

and the Dieback website at http://www.dieback.org.au/

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Created November 9, 1995

Last updated: November 30, 2013

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