Disease survey in three-year-old lucerne pastures by Dominie Wright & Roger Jones, Department of Agriculture, Western Australia

The research and strategies reported in this issue's Farmer Trials are edited versions of material that growers and researchers in WA recently published in the book, Farming Systems to Manage Salinity — Research Results 2002. The on-farm trials are part of the GRDC-supported and ongoing 'Salinity management with plants' projects. The book will be updated annually to encompass new strategies and is available from Richard O'Donnell, at the WA Department of Agriculture.

THE BENEFITS from sowing lucerne in pastures in the WA wheatbelt need to be balanced against potential problems. For example, they provide a 'green bridge' for legume pests and diseases to survive over summer. Such a continuous 'green bridge' is absent with annual pastures, as these die from lack of moisture over summer.

'Green bridge' danger: check your seed source

At the 2001 Crop Updates in WA, we reported the results of a survey of commercial seed stocks of lucerne being sown in the WA wheatbelt. Seed stocks were found to be commonly infected with alfalfa mosaic virus and occasionally also with cucumber mosaic virus. The consequence of sowing such untested, unknowingly infected lucerne seed stocks is that they introduce seed-borne virus infection within pastures.

This practice initiates epidemics of alfalfa mosaic virus in lucerne pastures in which the virus is spread by aphids. Epidemics not only induce losses in lucerne production but also can spread to cause serious epidemics of alfalfa mosaic virus in nearby pulse crops and annual pastures.

Luteoviruses

Widespread sowing of lucerne can also lead to damaging epidemics of luteoviruses in pulses, like those that occur where lucerne pastures are abundant in NSW. In eastern Australia, luteovirus diseases of legumes are caused by a complex of three different viruses, beet western yellows, subterranean clover red-leaf and bean leaf roll. They are spread by aphids, are not seed-borne and require a 'green bridge' of herbaceous hosts to survive the dry summer period.

Lucerne is the key reservoir host for them, especially for the most damaging one to pulses, bean leaf roll virus. In NSW, epidemics of these luteovirus diseases are a major cause of growers' giving up sowing pulses, especially in the Liverpool Plains area of northern NSW.

Three-year survey in WA, methods

Three-year-old lucerne pastures were surveyed during the spring growing season (September-November 2001) on 30 properties throughout the wheatbelt. The following numbers of pastures were sampled in each Agzone: Agzone 1 (North West) 4, Agzone 2 (Central) 12, Agzone 3 (South West) 7, Agzone 4 (North East and Central) 4 and Agzone 5 (Lakes/Mallee) 3.

For fungal disease evaluation, 20 individual plants selected at random were sampled per pasture. These were tested for fungal pathogens causing leaf and root diseases. In addition, 100 tops of plants were randomly collected, for assessment of viral diseases. Approximately eight different lucerne varieties were sampled during the survey. The varieties included: SaladoPBR logo, SceptrePBR logo, GenesisPBR logo, AquariusPBR logo, Trifecta, EurekaPBR logo, L69PBR logo and FlairdalePBR logo and some unidentified varietal mixes.

Results of pasture survey

Infection with alfalfa mosaic virus was found within 29 of the 30 lucerne pastures sampled. Incidences within individual pastures were high with 50-98 per cent of plants infected in 20 of them, and only four having less than 10 per cent infection. Lucerne varieties found infected included AquariusPBR logoEurekaPBR logo, FlairdalePBR logo, GenesisPBR logo, L69PBR logo, SaladoPBR logo, SceptrePBR logo, Trifecta and various varietal mixtures. Table 1 shows the numbers of pastures infected within each Agzone.

Two other seed-borne viruses, cucumber mosaic and bean yellow

Tests for luteoviruses revealed infection in 11 of the 31 pastures sampled, with a range of infection incidences within them of 1-5 per cent. In five of the infected pastures, the luteovirus(es) present were not identified further. In the six others, one or more of the three luteoviruses were detected, beet western yellows, bean leaf roll and subterranean clover red-leaf. One pasture had infection with both bean leaf roll and subterranean clover red-leaf, three had beet western yellows alone, and one each had bean leaf roll or subterranean clover red-leaf alone. Lucerne varieties infected with luteoviruses included SaladoPBR logo, SceptrePBR logo and various varietal mixtures. Bean leaf roll had not been found previously in WA. Table 1 shows the numbers of pastures found infected within each Agzone.

Leaf diseases

Eight leaf diseases were detected across the pastures sampled during the survey. Phoma leaf spot (Phoma medicaginis), Stagonospora leaf spot (Stagonospora meliloti) and Stempylium leaf spot (Stemphylium sp.) were the most common. All lucerne varieties collected in the survey were susceptible to Stemphylium leaf spot and Phoma leaf spot, and the majority of them were also susceptible to burn (Leptosphaerulina trifolii), Stagonospora leaf spot, and rust (Uromyces striatus). Table 2 shows the number of positive infected pastures in each Agzone. In most cases the pastures sampled had more than one leaf disease present.

Root diseases

Four pathogens that can cause root disease were commonly detected in the lucerne pastures sampled during the survey (Table 3). Often more than one root pathogen was infecting individual pastures. The pathogen Rhizoctonia sp. was detected only in the roots. The type of Rhizoctonia found was binucleate which is not the same as the R. solani types that cause root diseases in cereal and pulse crops.

Charcoal rot (Macrophomina phaseolina) was detected in all varieties and in all Agzones. This is a common pathogen for legumes and is usually present in older legume crops. It tends to be more prevalent when seasonal conditions are dry than when they are wet. Fusarium spp. were detected across all varieties and Agzones. This is a common soil pathogen and causes both root rot and crown rot in dry conditions. The pathogens Sclerotinia sp. and Pythium sp. were both detected in some pastures. These pathogens are generally associated with root rot when conditions are cool and wet. These diseases were found in all eight varieties sampled.

What does this mean?

  • Growing lucerne has major implications for the productivity of crops and nearby annual pastures.
  • • The detection of luteoviruses, including one new to WA (bean leaf roll), in lucerne pastures means that a new set of problems will occur in legume crops grown in proximity to lucerne.
  • The widespread infection with alfalfa mosaic virus in lucerne pastures and seed stock suggests that the virus will cause much greater problems in pulse crops than it has in the past.
  • Since the majority of the seed from the eastern states being sown in WA is contaminated with alfalfa mosaic, a local lucerne seed industry needs to be developed to produce healthy seed.
  • Fungal leaf diseases, e. g. Phoma and Stagonospora leaf spots, will spread to medic, clover and pulses such as peas.
  • Root diseases, e. g. Charcoal rot, will build up and infect the following legume crop.
  • More information is required to determine the level of impact these diseases have on lucerne production under WA conditions.
  • Similarly more information is needed on their impact on crops and annual pastures.
  • Research on control measures is needed.

We would like to thank Peter Hill and Allan Sweetman for collecting the samples and Brenda Coutts, Danae Harman, Nichole Burges, Sandy Mack and Johnathon Warden for help with processing the samples in the laboratory

Table 1: Numbers of lucerne pastures found infected with viruses within each Agzone of the WA wheatbelt.
Virus NameAgzone 1 (4 pastures)Agzone 2 (12 pastures)Agzone 3 (7 pastures)Agzone 4 (4 pastures)Agzone 5 (3 pastures)
Alfalfa mosaic411743
Cucumber mosaic00000
Bean yellow mosaic00000
Beet western yellows00210
Bean leaf roll10010
Subterranean clover red-leaf11000
Luteovirus +ve (unidentified)13100
Table 2: Leaf diseases detected in lucerne pastures surveyed throughout the WA wheatbelt. Figures give the number of positive pastures, and the numbers in brackets indicate the minimum and maximum number of positive subsamples.
Common NameAgzone 1 (4 pastures)Agzone 2 (12 pastures)Agzone 3 (7 pastures)Agzone 4 (4 pastures)Agzone 5 (3 pastures)
Anthracnose3(1-4)2(1)1(2)
Burn2(1-2)5(1-3)2 (2-3)1(2)
Downy mildew1(1)1(1)
Phoma leaf spot2(2-5)10 (1-20)6(1-15)2 (12-13)2 (6-9)
Common leaf spot1(1)
Rust1(11)4 (1-18)1(15)
Stagonospora leaf spot1(5)3(1-2)2 (2-5)3(1-9)1(4)
Stemphylium leaf spot3 (2-18)11(1-19)7 (2-16)3 (3-20)3 (4-13)
Table 3: Number of times each disease was detected in roots and crowns of pastures sampled.
Common NamePlant partAgzone 1 (4 pastures)Agzone 2 (12 pastures)Agzone 3 {7 pastures)Agzone 4 (4 pastures) Agzone 5 (3 pastures)
Fusarium crown rotCrown49733
Fusarium root rotRoot411723
Charcoal rotCrown29121
Charcoal rotRoot27411
Pythium root rotRoot411733
Rhizoctonia root rotRoot47521
ScleroticaCrown1
SclerotiniaRoot1