Shoot blight of spotted gums in Queensland

M H Ivory, G S Pegg and B N Brown

Summary

Ramularia shoot blight (RSB), caused by the fungus Quambalaria pitereka (formerly Ramularia pitereka and  Sporothrix pitereka), has been known in Queensland since 1964 affecting spotted gums and some other Corymbia species.

It affects new flushes of foliage causing spotting, necrosis and distortion of expanding leaves and young green stems, and can lead to dieback of the leader and upper lateral shoots. It is presently widely distributed in Southeast Queensland in natural forests and plantations and has been found in Corymbia species in central and north Queensland.

Symptoms

RSB typically affects the new shoots of young trees and is characterised by the association of pure white, shiny or powdery, fungal lesions with grossly distorted leaves and curled or twisted stems. First symptoms of RSB appear about five days after infection. They consist of tiny, pin-head-sized, white pustules, scattered on both sides of the leaves and young stems of affected plants. These symptoms occur mainly on immature foliage, especially at leaf margins, along the leaf veins and at the junction of the leaf lamina and petiole.

Fig. 1 Powdery lesions on leaf	Fig. 2 Powdery lesions on shoots

Within a few days these small, white pustules become necrotic, with their centres covered with tiny white tufts of fungus either above or beneath the cuticle of the leaf or young stem (Figs. 1 and 2). Each tuft then develops rapidly to bear numerous, white, minute spores of the fungus Quambalaria pitereka.  The pustules may also extend and coalesce, giving rise to large, irregular, white, shiny or powdery lesions.

Lesions on immature foliage give rise to leaf distortion as the infected leaves expand, whereas those which begin on fully-expanded foliage remain as small, discrete spots, however, mature leaves are rarely affected. Lesions can also form on young stems where they may cause the death of the shoot, or the curling of stemswhere the lesion only partially girdles the stem.

Severe infections, in which shoots are distorted or killed, lead directly to loss of the affected leader or side branch (Fig. 3). Affected trees subsequently produce further flushes of foliage, giving rise to the proliferation of apical and lateral shoots, which may become re-infected under suitable climatic conditions. Repeated infection can result in the growth of a stunted, bushy tree.

Fig. 3  Severe infection showing distortion and dieback of shoots, including leader.

Host trees affected

In Queensland, RSB mainly affects the spotted gums; Corymbia citriodora subsp. citriodora, C. citriodora subsp. variegata [syn. Corymbia variegata] C. maculata (spotted gum of southern NSW and Victoria) and C. henryi, in both nurseries and young plantations. Other host species include C. ficifolia and hybrid Corymbia such as C. torelliana X C. citriodora subsp. citriodora and C. torelliana X C. citriodora subsp. variegata.

In Queensland, RSB has been reported on a number of Corymbia species, mostly on young plants in tree nurseries, or in plantations up to three years of age. It has also been reported on coppice shoots of felled C. citriodora subsp. variegata and foliage of suppressed trees up to 17 years old. Corymbia torelliana and several Angophora species are also susceptible in the glasshouse, but, appear to be immune under field conditions.

Although Quambalaria pitereka and Ramularia have been reported from within capsules of three ironbarks (Eucalyptus crebra, E. drepanophylla and E. melanophloia) and poplar box ( E. populnea) and some of their hybrids in Southeast Queensland (published and Queensland Plant Pathology Herbarium  records) these records probably refer to another fungus (J.A. Simpson SFNSW personal communication). Quambalaria pitereka has also been reported from leaves of Corymbia ficifolia in Western Australia and from Eucalyptus nitens in Victoria.

Tree species affected:

Corymbia citriodora subsp. citriodora

Corymbia citriodora subsp. variegata

Corymbia eximia

Corymbia ficifolia

Corymbia henryi

Corymbia maculata

Corymbia papuana

Corymbia tessellaris

Corymbia torelliana

Corymbia torelliana X C. citriodora subsp. citriodora

Corymbia torelliana X C. citriodora subsp. variegata

Eucalyptus nitens

Geographic distribution

RSB, caused by the fungus Quambalaria pitereka, was first described on Corymbia variegata, C. eximia, C. ficifolia and Angophora costata in NSW (Walker & Bertus, 1971), with unconfirmed reports on the same host species from two Sydney nurseries dating back to 1955. The disease was confirmed in Queensland in January 1972, but herbarium records show that it was present in the greater Brisbane area in May 1964.

Southeast Queensland

Noted in the greater Brisbane area since May 1964 (Forestry Research records). Many reports since 1972 from east of the dividing range, from the NSW border in the south to Bundaberg in the north. It is probably endemic to most spotted gum forests in this area.

Central and North Queensland

Noted in a Cairns nursery in February 1977. Since then, it has been reported from nurseries in Ingham, Townsville and Walkamin and from Atherton. It has also been reported from plantings at Airlie Beach, Byfield, Davies Creek (Mareeba), Ingham and Mackay.

NSW

Many reports since 1970, with earlier unconfirmed reports dating back to 1955. Initially these were mostly from plant nurseries in the area around Sydney, it is now known to occur throughout coastal NSW (J.A. Simpson SFNSW personal communication). It is probably endemic to the native coastal forests of spotted gums in NSW (Old, 1990; Simpson, Stone & Eldridge, 1997), but does not appear to be significant there.

South Australia

Not reported here.

Northern Territory

Noted on small C. papuana trees in September 1984 from an unspecified location (Queensland Plant Pathology Herbarium records).

Tasmania

Not reported here.

Victoria

Recently recorded from Eucalyptus nitens (J.A. Simpson, SFNSW).

Western Australia

Reported from Corymbia ficifolia. A similar fungus has been reported from a canker disease of branches and trunks of the same host and on Corymbia calophylla.

Causal organism

Ramularia pitereka was described by Walker and Bertus (1971) and renamed Quambalaria pitereka by Simpson (2000).

Quambalaria pitereka is a mitosporic fungus, which has been found only in an asexual form with no known sexual phase. It sporulates profusely on all diseased leaf and stem tissues forming raised, white, shiny pustules under the cuticle. These soon erupt through the cuticle forming small, closely-packed, white tufts 0.1 mm in diameter on the affected surfaces.

Fig. 4.  Fungal hyphae growing on leaf surface. (SEM - Mark Self)

These tufts are composed of tightly-packed fungal threads on the tips of which numerous, minute (approx. 0.001 mm long), fungal spores are produced. These spores can spread to healthy tissues and initiate new infections under suitable conditions.

The fungus can be isolated easily from spores on to PDA or Malt agar, where it grows slowly forming white to grey/white, raised colonies covered with an abundant, dry mass of spores, or slimy yeast-like colonies. Older colonies become cream/pink in colour and often develop fine, radial folds. Growth is favoured by temperatures between 20-25 ºC.

Disease development

Spores of Q. pitereka germinate very rapidly on leaf surfaces, with at least 50% germination taking place in five hours. Tiny lesions subsequently develop on immature leaves and stems within five days. These may develop into large, sporulating lesions by day 10. The disease cycle can, therefore, be completed within 10 days under favourable conditions.

In Queensland, the fungus is active throughout most of the year following short periods of heavy rain, giving rise to new infections whenever these periods coincide with new flushes of immature foliage. Subsequent development of fungus lesions on infected shoots is favoured by dry conditions.

Plantations of spotted gums may develop symptoms of RSB within three to six months of planting if the disease is introduced on the nursery stock plants, or if disease inoculum is available close to the plantation. These infections may then build up rapidly so that almost every tree is affected within one year. At this stage many of the trees will be severely affected with repeated dieback of the leading shoots. Few such trees will grow through the disease to produce trees of acceptable stem form and growth. Plantations which remain free of disease in the first year, are unlikely to be affected so badly in subsequent years as trees appear to become tolerant of infection with increasing age. They are thus much more likely to produce adequate numbers of acceptable trees.

Plantations of Corymbia citriodora subsp. variegata Woondum provenance produce far fewer highly susceptible trees than other tested provenances, such as that from Presho. The former provenance is, therefore, capable of producing acceptable plantations with a greater proportion of highly tolerant trees.

Disease control

1. Infection should be avoided during years one and two in spotted gum plantations by using healthy stock-plants from a clean nursery, or by using plants grown on site. These could be sprayed with fungicide in the nursery just prior to outplanting (Jones and Elliot, 1986).
2. Spotted gum plantations should be established well away from existing plantings of other Corymbia species, particularly those with young trees present, which may be infected with Q. pitereka. A distance of at least 200 m is probably required, particularly in the direction of prevailing winds.
3. Control in plantations using fungicides is unlikely to be effective unless done frequently to ensure that all new flushes of foliage are protected. Presently no materials are registered for this purpose in plantations. However, three sprays of copper oxychloride at weekly intervals, were recommended for nursery plants by Jones and Elliot (1986).
4. Alternative chemicals for control of RSB are currently under study by Forestry Research pathologists.
5. Roguing of all infected trees at the first sign of the disease may significantly delay the progress of the disease. Fungicide sprays applied to the adjacent trees at the same time may also be beneficial.
6. Silvicultural controls, such as pruning, thinning, or spacing, are unlikely to have significant effects in plantations as RSB normally affects the apical leaders and is easily dispersed over the distances likely to be involved.
7. A few trees in most affected C. citriodora subsp.variegata plantations appear to be highly field tolerant to RSB, with the remainder mostly highly susceptible. Additionally, some provenances, such as that from Woondum, appear to produce much higher proportions of such highly field tolerant trees. Propagation and testing of provenances against a range of Q. pitereka strains is presently being given high priority.

References

Alcorn J.L. (1976). Host index of plant diseases in Queensland Supplement 1. DPI, Brisbane.

Jones D.L. and Elliot W.R. (1986). Pests, Diseases and Ailments of Australian Plants. Lothian Gardening Guide, Lothian Publishing Co., Sydney, pp 253 - 254.

Old K.M. (1990). Chapter 13. in: K.W. Cremer (Editor). Trees for Rural Australia. Inkata Press pp 210 - 216.

Simpson J.A. (2000). Quambalaria a new genus of eucalypt pathogens. Australian Mycologist 19 (2), 57-62.

Simpson J., Stone C. and Eldridge R. (1997). Eucalypt plantation biological agents - crop loss study. Forest and Timber Protection Section, NSW State Forests R & D.

Walker J. and Bertus A.L. (1971). Shoot blight of Eucalyptus spp. caused by an undescribed species of Ramularia. Proceedings of the Linnean Society of New South Wales 96 (2), 108-115.

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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File No: FOR0049
Last reviewed February 2005