Ballard, Ross A.

Humphries, A.

Kobelt, E.

McKay, Alexander D.

McKay, Suzanne Faye

Adelaide Research &amp; Scholarship

FUNGI CAUSING ROOT DISEASE OF SEEDLING LUCERNE IN THE MURRAYMALLEE REGION OF SOUTH AUSTRALIAS.F. McKay (formerly Colmagro), E. Kobelt, A. McKay, A. Humphries and R.A. BallardThe South Australian Research and Development Institute (SARDI)GPO Box 397, Adelaide, South Australia, 5001INTRODUCTIONLucerne (Medicago sativa) is a deep-rooted perennial fodder plant that is used to reduce groundwater recharge in dryland cropping systems. Financial subsidies are offered to encourage farmers to plant lucerne in the Murray Mallee Region of South Australia (SA). However, root diseases are believed to be affecting successful plant establishment (1). Root diseases have not previously been considered a significant problem in the drier regions, but have long been recognised as a problem for lucerne grown under irrigation. Pathogens implicated in root diseases of irrigated lucerne in Australia include Acrocalymma medicaganis, Colletotrichum trifolii, Fusarium spp., Phytophthora medicaganis, Rhizoctoniaspp. and Sclerotium sp. (2, 3 and 4). The aim of the research outlined here was to identify the principal pathogens of seedling lucerne grown under dryland conditions in the Murray Mallee.MATERIALS AND METHODSUp to 100 plants were collected from each of twenty sites, newly planted to lucerne, in the Murray Mallee in 2005 and 2006. Roots were scored for disease severity (0=no disease; 5=dead roots) and examined microscopically for the presence of fungal structures and disease symptoms.  Roots, of up to 10 plants per site, were washed, excess water removed and 1-2 mm pieces of diseased roots were placed onto tap water agar and media selective for Pythium, Fusarium and Rhizoctonia. Pure cultures were established from hyphal tips, identified to at least genus level using morphological characteristics and grouped according to cultural characteristics. The numbers of Pratylenchus neglectus were quantified in the dried root samples using real-time PCR by the Root Disease Testing Service at SARDI.  Pathogenicity testing. Isolates were first screened for pathogenicity to lucerne seedlings using an in-vitro Petri-dish test. Those isolates resulting in high disease scores were subsequently used in pathogenicity tests carried out in pots in controlled growth conditions. Inoculum for in-planta pot tests was grown on sterilised millet seed mixed with course sand. Soil, collected from the Murray Mallee, was steam sterilised and mixed with inoculum (0.5% w/w) from each isolate. Five lucerne seeds were sown in pots containing 400 g of inoculated soil and maintained at 20oC (15oC for Rhizoctonia) for 4 weeks, and assessed for plant survival and scored for root disease severity (0-5 scale). Shoot and root dry weight (DW) were also measured and data analysed using ANOVA. RESULTSA total of 591 fungal isolates was established from the lucerne roots. Of known pathogens to lucerne roots, 21 isolates of Rhizoctonia (including binucleate species), 14 Pythium, 17 Phoma, six Sclerotinia and over 200 Fusarium spp. were obtained. Other fungi included Alternaria, Epicoccum, Bipolaris, Papulospora, Cylindrocarpon, and 170 unidentified fungi. P. neglectuswere detected in the dry roots from every sample site, and ranged from 10 to 24 800 nematodes/g dry root (median = 1223 nematodes/g root). Between four and 87 isolates were obtained per site and up to nine genera isolated per plant. Isolates of Fusarium, particularly F. oxysporum, were ubiquitous, whereas Rhizoctonia was recovered from 13 sites, Pythium from seven and Phoma from four. Disease scores ranged from 1.5 to 4.5 and there was no obvious link between the relative numbers of P. neglectus, or frequency of isolation of Rhizoctonia, Pythium or Fusarium spp. at each site.Pathogenicity tests indicate that isolates of Rhizoctonia, Fusarium, F. oxysporum, Pythium and several unidentified fungi are pathogenic. Virulence varied among isolates. Several, e.g. F. oxysporum, caused significant (P< 0.05) seedling death. Others, e.g. R. solaniAg8, caused extensive rotting and lesions on roots, decreased shoot and root DW but resulted in little seedling mortality.DISCUSSIONResults indicate that the cause of root disease(s) of seedling lucerne in the Murray Mallee is likely to be a complex of Rhizoctonia, Pythium and Fusarium spp. High frequency of P. neglectus indicates it might be contributing to disease. However, the exact nature of the interaction and relative importance of each organism is yet to be elucidated. The remaining isolates in the collection will undergo pathogenicity tests and isolates confirmed as pathogenic will be identified to species level. Further work is needed to quantify yield losses associated with lucerne root diseases and to understand how paddock management might be used to influence pathogen levels and disease severity.ACKNOWLEDGMENTSThis project is funded by the Centre for Natural Research Management. We thank the Murray Mallee LAP group, and cooperating farmers for their assistance.REFERENCES1. Colmagro, S.F., Ballard, R.A., Kobelt, E., Humphries, A. and McKay, A., (2006). Root diseases affect newly established dryland lucerne in the Murray Mallee region of South Australia. 4th Australasian Soilborne Diseases Symposium, Queenstown, New Zealand, Sept 2006.2. Irwin J.A.G. (1977) Factors contributing to poor lucerne persistence in southern Queensland. Australian Journal of Experimental Agriculture and Animal Husbandry 17, 998-1003.3. Marclay M.D. (1970) Fusarium oxysporum as a cause of lucerne decline in Western Australia. Plant Disease Reporter 54, 1061-1063.4. Irwin J.A.G., Mackie J.M., Marney T.S., Musial J.M., Roberts S. (2004) Incidence of Stagonospora meliloti and Acrocalymma medicaginis in lucerne crowns and roots in eastern Australia, their comparative aggressiveness to lucerne and inheritance reaction to S. meliloti in lucerne. Australasian Plant Pathology 33, 61-67.

Fungi causing root disease of seedling lucerne in the Murray Mallee region of South Australia

https://digital.library.adelaide.edu.au/dspace/bitstream/2440/49319/1/McKay_2007a.pdf

Fungi causing root disease of seedling lucerne in the Murray Mallee region of South Australia

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Fungi causing root disease of seedling lucerne in the Murray Mallee region of South Australia

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Adelaide Research &amp; Scholarship

Adelaide Research & Scholarship