L.J. Pilkington1*, K.S. Gibb2, G.M. Gurr1, M.J. Fletcher3, A. Nikandrow3, E. Elliott4, R. Van de Ven3 and D.M.Y. Read1
1 University of Sydney Orange, P.O. Box 883, Orange, New South Wales 2800, Australia
2 Faculty of Science, Northern Territory University, Darwin, Northern Territory, 0909, Australia
3 Orange Agricultural Institute, NSW Agriculture, Forest Road, Orange, New South Wales, 2800, Australia
4 New South Wales Agriculture, P. O. Box 369, Forbes, New South Wales, 2871, Australia
Accepted: 23 Oct 2001
Lucerne (Medicago sativa) or alfalfa is a pasture legume of world-wide significance. The production of lucerne seed is an important sector of Australia's pasture seed industry but is being impacted by the disease Australian lucerne yellows (ALY) .
Symptoms associated with ALY include a discolouration of leaves ranging from bright yellow to red (Fig. 1). Discolouration is consistently evident across the entire foliage of the plant . Further symptoms include a characteristic brown discolouration immediately under the periderm of the taproot (Fig. 2). Phytoplasmas are known to cause similar yellowing symptoms in other crops such as grapevine yellows (Padovan et al., 1995).
Using transmission electron microscopy (TEM), phytoplasmas were found in phloem of leaf midrib and stems prepared from infected plants and were absent from similar tissue of healthy (symptomless) plants. The structures visualised in ALY affected plant tissue by TEM were consistent in size, shape and structure with phytoplasmas that are associated with other plant diseases (Fig. 3).
Fifty individual symptomatic lucerne plants were tested using polymerase chain reaction techniques with approximately 40% of individuals testing positive for the presence of a phytoplasma. Fifteen non-symptomatic plants were tested using identical techniques with no positive results. A phytoplasma was shown to be associated with Australian lucerne yellows although it may not be the exclusive causal agent. Analysis of ALY 16S rDNA revealed this strain to be closely related phylogenetically to Australian tomato big bud phytoplasma.
We thank the Rural Industries Research and Development Corporation for providing funding for this work, Mr Don Gowanlock (University of Queensland) for his aid in interpretation of the electron micrographs, Dr Deborah Hailstones (NSW Agriculture) and Lucy Tran-Nguyen (Northern Territory University) for their aid in molecular biology techniques.
Gowanlock DH, Ogle HJ, Gibb KS, 1998. Phytoplasma associated with virescence in an epiphytic orchid in Australia. Australasian Plant Pathology 27, 265-8.
Padovan AC, Gibb KS, Bertaccini A, Vibio M, Bonfiglioli RE, Magarey PA, Sears BB, 1995. Molecular detection of the Australian grapevine yellows phytoplasma and comparison with grapevine yellows phytoplasmas from Italy. Australian Journal of Grape & Wine Research 1, 25 - 31.
Pilkington L, Gurr GM, Fletcher MJ, Nikandrow A, Elliott E, 1999. Occurrence and severity of lucerne yellows disease in Australian lucerne seed crops. Australasian Plant Pathology 28, 235 - 39.
Stovold GE, 1983. Diseases of lucerne. Agfacts publication no. P2.AB.1, 2nd Edition. NSW Agriculture, Australia.
©2001 The Authors