Australasian Plant Conservation (formerly Danthonia)

Originally published in Danthonia 8(4), March 2000

Mundulla Yellows: A growing concern

David C. Paton¹ & Joanne Cutten^2
1 Dept of Environmental Biology, University of Adelaide, Adelaide SA 5005
² Biodiversity Branch, DEHAA, GPO Box 1047, Adelaide, SA 5001

The eucalypt leaves on the left of the photo show the interveinal chlorosis typical of Mundulla Yellows. Photo: David Paton

In the late 1970s Geoff Cotton, an observant beekeeper, noticed unusual yellowing of the foliage on a few mature red gums (Eucalyptus camaldulensis) at Buckingham (near Mundulla) in the south-east of South Australia. By 1990 some of the trees showing the yellow foliage were dead and adjacent trees were showing the symptoms. Other eucalypts in the region were also showing symptoms, particularly along roads in and around the towns of Keith and Bordertown.

A 1992-93 survey of the health of hundreds of eucalypts in the Keith-Bordertown area (Paton & Eldridge 1994; Paton et al. 1999) showed that trees showing symptoms of Mundulla Yellows (patches of yellow foliage on one or more branches or sub-branches with inter-veinal chlorosis of the leaves) were largely found along roadways or waterways. A survey of sections of roadside vegetation between 1994 and 1999 showed that of 477 eucalypts examined, none recovered over the five-year period. While some were about the same, most had deteriorated and a reasonable proportion had died (Table 1).

The conclusion from these figures is that once a plant shows symptoms of Mundulla Yellows its condition deteriorates and death is inevitable, although it may take 10 or more years. It is likely that all of these trees will be dead within 10-20 years. Furthermore, the only eucalypt seedling to establish along these roadsides over the last five years also contracted Mundulla Yellows, severely dampening any thoughts of ever being able to re-tree these areas. A range of possible treatments was tried, involving injecting infected trees with either phosphorous acid, tetracycline, rogor or aquasol nutrient solution. No obvious recovery was found, although the ability to record a response was limited because the local council pruned many of the test trees. It was also found that the numbers and types of birds using the trees declined as the trees lost vigour (Paton et al. 1999).

Table 1. Changes in the percent of trees showing symptoms of Mundulla Yellows and fate of five species of eucalypt examined along three 1-km sections of roadside in the Keith-Bordertown area in 1994 and 1999.

By 1999 all of the species of native shrubs found along the sections of road had also suffered from Mundulla Yellows, including species of Allocasuarina, Xanthorrhoea, Melaleuca, Bursaria, Dianella and Acacia, and some had contracted the symptoms and died over the five year period (notably Acacia, Xanthorrhoea, Dianella,and Allocasuarina). For these sections of road, then, there will soon be no eucalypts and no native understorey shrubs left alive, and the landscape and aesthetics of the area will change dramatically.

This loss of trees and shrubs is no longer restricted to just a few locations in the south-east of South Australia. Recent reconnaissance surveys reveal that plants expressing the symptoms are now widespread in SA, not to mention being reported from Western Australia, New South Wales, Tasmania, Victoria and the Northern Territory as well. If left unchecked, Mundulla Yellows has the potential to devastate the native vegetation of SA, significantly reduce biodiversity, contribute to problems of increased dryland salinisation and lead to substantial reductions in agricultural productivity, including threatening agro-forestry, since commonly planted eucalypts, like E. globulus, are susceptible to Mundulla Yellows. Mundulla Yellows will also affect amenity plantings in urban areas and detract from tourism. Furthermore, Mundulla Yellows attacks trees and shrubs of all ages including seedlings and saplings in revegetation programs and so threatens much of the work initiated under Natural Heritage Trust revegetation programs.

At present the likely causal agent appears to be a phytoplasma (an organism that lives inside plant cells), possibly transmitted by sap-sucking insects. Phytoplasmas have caused similar yellowing symptoms and mortality in other plants including native cabbage trees in New Zealand, and they have recently been isolated from eucalypt tissue showing the symptoms of Mundulla Yellows, but considerably more work is required to demonstrate that a phytoplasma is the causal agent. Funds have recently been allocated by Environment Australia for plant pathologists like Professor John Randles to determine if phytoplasmas or some other biotic agent(s) are involved and to develop a diagnostic test for Mundulla Yellows. Until the causal agent(s) and method of dispersal are identified and appropriate remedial action found, untold and irreparable damage to the Australian environment will occur.

References

Paton, D.C. & Eldridge, S. 1994. Maintenance of Mature Trees in Agricultural Areas in the Upper South East of South Australia. STB report. ANCA, Canberra.

Paton, D.C., Prescott, A.M, Davies, R. J-P. and Heard, L.M. (1999). The distribution, status and threats to temperate woodlands in South Australia. In R.J. Hobbs & C.J.Yates (eds). Temperate Eucalypt Woodlands in Australia. Biology, Conservation, Management and Restoration. Surrey Beatty & Sons, Chipping Norton.  

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