Ground Cover Supplement : GC Supplement - Pastures
GROUND COVER PASTURES 10 ANNUAL LEGUMES IMPROVED DISEASE TOLERANCE will ensure medics remain the legume of first choice for low- and medium-rainfall alkaline farming systems well into the future. Annual medics have served farmers since the 1960s, but like any plant introduction (medics all origi- nate from overseas) the honeymoon period eventually ends and diseases and pests catch up with them. The first major adversary of the medic was the aphid invasion of the late 1970s, but this was successfully repelled with the breeding and development of tolerant varieties such as HeraldA and Paraggio. But in recent times, more insidious forces have been at play, with the phrase ‘medic decline syndrome’ being used to describe medic pastures performing below expectation. Root lesion nematode (Pratylenchus neglectus) is now known to play a significant role in this syndrome. Indications are that the nematode is reducing medic production by 25 per cent, in about 25 per cent of paddocks. The South Australian Research and Development Institute (SARDI), in partnership with the GRDC and the South Australian Grains Industry Trust, has set about developing medic cultivars that are tolerant to the nema- tode. Also of critical concern has been the question: do medics play any role at all in building up the nematode populations in farming systems? The development of a nematode-tolerant version of the strand medic cultivar HeraldA is well advanced. HeraldA is reasonably intolerant and can suffer heavy root damage when high numbers of the nematode are present (see Figure 1). However, the cultivar has now been crossed with a tolerant line, selected after intensive screening of SARDI’s medic genebank. The fifth gener- ation progenies continue to show excellent tolerance to the nematode in growth-room assessments, and material from these populations will be available for field screen- ing in 2007. A similar approach is being used to develop a tolerant version of the barrel medic cultivar CaliphA. Until now the role of medics in the build-up of root lesion nematode in the farming system has been unclear. Fewer nematodes mean less root damage and yield loss for intolerant crops and pastures. Four field trials have clearly shown that medic pastures behave similarly to the most resistant cereals, and limit the multiplication of these nematodes. For example, in two recent trials on Eyre Peninsula (Figure 2) the susceptible Machete wheat multiplied the number of nematodes in the soil nearly four times, the resistant Tahara triticale and medics by 0.6 to 1.3 times. This is good news and means that the current varieties of medic can play an important role in limiting nematode number in cropping rotations, even though the medics themselves may suffer some root damage and yield loss. Subtle differences have also been measured between medic cultivars, and in this regard CaliphA medic has consistently resulted in some of the lowest nematode multiplication measured, and will be used as a platform in future breeding efforts. On another front, we have seen the emergence of powdery mildew as a relatively new threat to medics. It has been increasing in incidence over the past five years and has already been observed on medic seedlings this year. It is typically seen in spring as a white powder on the upper leaf surface. Severe cases can cause leaf loss. Commercial medic cultivars are intolerant and this has provided the impetus to search for better lines. After screening nearly 400 medic lines in the greenhouse, sev- eral that resist attack by the disease have been selected and crossed with commercial cultivars. Second-gen- eration progenies are available and will be tested in the field this year and further selections made. In the longer term, it is planned to combine the improved disease tolerance traits with the other valuable traits, including sulfonylurea herbicide tolerance, that have been developed by the SARDI Pasture Group. Ross Ballard is group leader, rhizobiology and pasture pathology and Barbara Morgan a scientist in pasture pathology, SARDI. GRDC Research Code DAS00042 More information: Ross Ballard, 08 8303 9388, email@example.com FIGURE 2 COMPARISON OF NEMATODE MULTIPLICATION UNDER THE MEDIC CULTIVARS HERALD AND TOREADOR ; AND THE CEREALS TAHARA (RESISTANT CONTROL) AND MACHETE (SUSCEPTIBLE CONTROL) IN 2004 MINNIPA, SOUTH AUSTRALIA SMOKEY BAY, SOUTH AUSTRALIA Machete-Wheat Herald -Strand medic Toreador -Disc medic Tahara-Triticale 0 1 Rate of nemotode multiplication 3 4 2 Machete-Wheat Herald -Strand medic Toreador -Disc medic Tahara-Triticale 0 1 Rate of nemotode multiplication 3 5 4 2 A A A A A A Researchers close in on developing disease-tolerant medics PROJECT: PATHOLOGY SUPPORT FOR ANNUAL PASTURE LEGUMES. ROSS BALLARD AND BARBARA MORGAN FIGURE 1 Root lesion nematode can be found in high numbers in medic roots (above), and in intolerant cultivars can cause significant damage, including lesioning and constriction of the tap root and the removal of most lateral roots (below).
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