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Australian Lucerne Yellows Disease:
Testing and extension of disease management strategiesby GM Gurr, MA Getachew, MJ Fletcher, A Mitchell, A Nikandrow and LJ Pilkington
August 2007
RIRDC Publication No 07/120 RIRDC Project No US-131A
Who is the report targeted at?
This report is targeted at lucerne seed growers, seed production agronomists and consultants who may be involved in ALuY disease management. The report (as well as an associated paper1) is also targeted towards other researchers who are likely to be able to build further on knowledge of this disease.
Background
This project built on previous
RIRDC-funded work that established the impact of ALuY disease and generated
information on its cause and possible disease management approaches.
Aims/Objectives
This project aimed to generate
additional information on the ALuY disease, its cause and means of spread
and to test possible disease management methods that can be communicated
to growers.
Methods used
PCR assays were used to
test for the DNA of phytoplasmas in lucerne plants collected from stands
in NSW and SA. Insects suspected to be the vectors of the phytoplasmas
were also tested using PCR.
Field surveys in NSW and SA identified plants that harboured potential vector species and these plants were also tested for presence of the phytoplasmas using PCR. Seeds, and the seedlings grown from them, were likewise assayed for phytoplasma using PCR. Field experiments evaluated the efficacy of pesticidal treatment of field margin vegetation and of barriers on the perimeter of lucerne crops for preventing immigration of leafhoppers.
Results/Key findings
Polymerase chain reaction
(PCR) assays established an association between ALuY symptoms and two genetically
distinct phytoplasmas: Candidatus Phytoplasma aurantifolia and Candidatus
Phytoplasma australiense, which were detected in 31% and 24% of ALuY
symptomatic, field-collected plants, respectively. Mixed infection by these
phytoplasmas was not found in ALuY symptomatic plants except in one sample
collected from Griffith, suggesting that these phytoplasmas could independently
cause ALuY disease. Because phytoplasmas cannot be cultured and inoculated
into plants, causality has not been proven and it is not possible to rule
out the possibility that another pathogen is involved.
PCR also detected Candidatus Phytoplasma aurantifolia in field collected, whole body samples of the leafhopper Orosius orientalis (Matsumura) (=Orosius argentatus (Evans)) (Deltocephalinae: Opsiini) and the plants yanga bush (Maeriana brevifolia (R. Br.) P.G. Wilson), nettleleaf goosefoot (Chenopodium murale L.), night shade (Solanum nigrum L.), nitre goosefoot (Chenopodium nitrariacium (F. Muell) F. Muell ex Benth, ruby salt bush (Enchylaena tomentose R. Br.), Trifolium pratense L. (red clover) and cotton bush (Maeriana microphylla (Moq.) P.G. Wilson). Several plant species in the Chenopodiaceae (M. brevifolia, E. tomentose, M. microphylla, C. nitrariacium and C. murale) were found to be breeding and winter survival hosts of O. orientalis. Large numbers of O. orientalis adults and nymphs were sampled from M. brevifolia, E. tomentose, M. microphylla and C. nitrariacium in summer, autumn, winter and spring and from C. murale in summer. The results suggest that, if Ca. Phytoplasma aurantifolia is confirmed to be the etiological agent responsible for ALuY disease, targeted management of chenopod weeds in and around new lucerne fields could reduce ALuY disease incidence. Where there are legislative restrictions on the control of native plant species that host the phytoplasma, growers will need to consider siting new lucerne stands remotely. The leafhoppers O. orientalis and Batracomorphus angustatus (Osborn) were successfully cultured and maintained on C. murale L. and T. pratense L. plants, respectively, under controlled conditions. Such information is useful for future disease studies.
Seeds harvested in March 2005 from a seed lucerne crop (cv. CW 5558) that had high ALuY disease incidence during the 2004/2005 crop season were used in DNA studies. DNA of Candidatus Phytoplasma australiense was amplified from lucerne seeds and seedlings grown in vector free environments suggesting that seed transmission of this phytoplasma is possible in lucerne. Candidatus Phytoplasma australiense was detected in 7/40 and 23/40 seed samples from the ‘first’ and the ‘second’ grade seeds, respectively.
Vector management field experiments showed that insecticide or herbicide treatment of a 2-10m wide and 20m long strip of field margin vegetation or 60cm high insect exclusion fence on one boundary of a new lucerne crop field were not effective in restricting the migration of suspected ALuY vector leafhoppers into the crop or to reducing ALuY disease incidence in the crop.
Future ALuY disease studies may consider the use of resistant/tolerant varieties as part of an integrated disease management (IDM) strategy for ALuY disease. If Candidatus Phytoplasma australiense is confirmed to be the causal agent for ALuY disease and, if seed transmission of this phytoplasma is confirmed, phytosanitary arrangements at both the local and larger scale also could be part of an integrated disease management strategy for ALuY disease.
Implications for
relevant stakeholders for industry:
ALuY disease is a challenging
problem for growers and researchers alike because the pathogen cannot be
cultured and inoculated onto test plants. Detection of the phytoplasma
requires electron microscopy or PCR techniques as used in this project.
The scientific community needs to develop increasingly powerful methods
for the study of phytoplasma diseases which will aid future ALuY disease
researchers aiming to definitively resolve causality.
For growers, consultants and seed production technicians aiming to manage ALuY disease more effectively, the results contained in this report provide significant new information that will allow new disease management options to be implemented. These include avoidance of non-crop plants that may harbour the pathogen, control of the likely leafhopper vector, inspection of seed crops for ALuY disease symptoms and more careful choice of seed source to avoid use of seeds that may carry phytoplasmas associated with ALuY disease. This objective may be supported by PCR testing of seed batches and scope exists to develop and offer this as a fee-for-service activity.
An illustrated extension
pamphlet has been drafted (see Appendix) and, subject to Corporation approval,
this can be mailed to growers. It may also be placed on the Charles Sturt
University and/or Corporation web site with a link to the present document
for fuller information.
1 Getachew, M.A., Mitchell,
A., Gurr, G.M., Fletcher, M.J., Pilkington, L.J., and Nikandrow, A. 2007.
First report of a "Candidatus
phytoplasma australiense"-related strain in lucerne (Medicago sativa) in
Australia. Plant Disease 91(1): 111
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