With CSIRO’s high-rainfall zone (HRZ) wheat-breeding team successfully incorporating wheat streak mosaic virus (WSMV) resistance into elite wheat germplasm, high-performing varieties with resistance can be expected within the next few years.
[Photo (left) by Robert Freebairn: Susan Kleven, a winter wheat breeder, check lines in the glasshouse with dual resistance to WSMV and BYDV.]
The team released Mackellar
in 2002 – Australia’s, and possibly the world’s, first variety with resistance to the often devastating disease barley yellow dwarf virus (BYDV). Now, the team has been able to combine WSMV resistance with BYDV resistance, and resistance to stem leaf and stripe rust.
However, what makes the breeding progress especially noteworthy is that much of the program’s genetic material was destroyed in 2003 when WSMV was discovered in glasshouses, as well as in other breeding centres across Australia. It was later found that the disease had probably been in Australia for many years prior to this discovery (not originating from CSIRO) and was widespread across the nation’s wheat areas.
[Photo (left) by Robert Freebairn: CSIRO’s Dr Phil Larkin, who leads a team searching for BYDV and WSMV.]
WSMV is a particular concern for early sown dual-purpose wheat crops, although it can also adversely affect main season sown wheats.
Plants infected with WSMV show yellowing and stunting. Individual leaves are mottled between veins with light green-yellow streaks (discontinuous yellow stripes, streaks and dashes parallel to the vein), and can be necrotic. The impact of the disease can be as high as 100 per cent loss of grain yield and 50 per cent reduced grazing.
Since 2005 some dual-purpose winter wheat crops have suffered grazing and grain yield losses of up to 100 per cent, although more commonly the disease impact is sporadic.
The national grazing and grain industry has the potential to grow more than one million hectares of dual-purpose wheat annually, but for dual-purpose varieties to regain a prominent role in grain and graze systems, resistance to WSMV, combined with other important disease resistances, is an important industry goal.
On average, when dual-purpose winter wheats are sown early they produce more grain after grazing than other cereals, with perhaps the exception of the latest triticale varieties Tobruk and Endeavour (which yield similarly in some areas).
Breeders Susan Kleven and Dr Garry Rosewarne are part of a small CSIRO wheat-breeding program targeting the HRZ with both longer-season spring wheats and dual-purpose winter wheats. Team member Dr Phil Larkin leads a group whose research focuses on finding suitable sources of resistance to both WSMV and BYDV.
In addition to developing varieties that can realise and protect the very high yield potential of the HRZ, the team intends to improve grain quality. Dr Greg Rebetzke and Dr Richard Richards focus on novel genetic traits that deliver more efficient use of the available moisture.
CSIRO has also teamed up with the New Zealand Institute for Crop and Food Research in an alliance called AUSGRAINZ. Apart from gaining access to additional valuable international germplasm, an advantage of the alliance is access to highly effective New Zealand plant disease nurseries for evaluation.
The GRDC is funding the winter wheat breeding program, including the work to incorporate resistance to the various viruses.
WSMV discovery
General despondency about finding WSMV-resistant germplasm gripped the wheat industry (triticale, oat, and barley varieties are mostly highly tolerant of WSMV) when it was first identified in Australia. However, Dr Larkin has confirmed three strong sources of resistance, made available through research colleagues in Canada and the US, and each is effective against the Australian strains of WSMV.
He plans to pyramid these genes to strengthen their protection and safeguard against breakdown. However, Dr Larkin says even single gene resistance should provide good protection if WSMV suddenly became very widespread and damaging to the industry.
In the interim before WSMV-resistant varieties will be released, AUSGRAINZ is proceeding with the release of CS95102.1 (parentage Brennan x Madsen, a Washington State, US, soft red winter wheat) with its likely commercial availability in 2010.
The variety has been extensively tested for more than three years and has yielded 20 per cent more grain than other winter wheats such as Mackellar, Rudd and Tennant. Grazing yield has been at least as good.
It possesses moderate resistance to stem rust and resistance to leaf and stripe rust with a robust combination of genes suggesting durable resistance.
Agronomic characteristics such as straw strength, non-bearded, weather tolerant red grain, and feed grain quality are similar to other CSIRO-developed winter wheats. Maturity is between Mackellar and Tennant suiting environments such as the NSW tablelands and upper slopes, and higher-rainfall areas in south-west Victoria, south-east SA and Tasmania.
CS98048, a BYDV-resistant line, is also a likely candidate for release from the CSIRO program in 2010 or 2011. In field testing it has indicated yield superior to Mackellar, similar maturity, and good agronomic type. Maturity is similar to Mackellar and Rudd indicating suitability over a wide range of environments.
Magnesium and salt supplements for grazing wheats
CSIRO chief research scientist Dr Hugh Dove has extensively researched sheep and cattle nutrition on grazing winter wheats. His work, supported by the GRDC, has shown that relative to mineral requirements for lamb growth, winter wheat forage is usually adequate for calcium, marginal for magnesium, deficient in sodium and has excessive concentrations of potassium. Oats and barley have high concentrations of potassium, but were adequate for magnesium, calcium and sodium.
His studies have confirmed live-weight responses to magnesium, though the extent depended on factors such as stocking rate. In two trials there were marked responses to sodium supplementation. Dr Dove says that although this is in part a response to sodium, it also appears to be because sodium supplementation increases magnesium absorption within the rumen of animals grazing winter wheat with low sodium but very high potassium content (a high potassium/sodium ratio reduces magnesium absorption in the rumen).
Dr Dove says it would be sensible and cheap to provide livestock grazing winter wheat with a magnesium/sodium supplement consisting of 1:1 Causmag: salt.
WSMV grazing studies
Dr Dove combined forces with the WSMV team at CSIRO, with GRDC support, and also tested if WSMV could be spread by sheep, as suggested by some anecdotal observations. The first aspect examined was whether sheep could pass the disease from infected to clean wheat via their saliva. Housed sheep ate infected wheat and then were fed clean wheat. None of the clean wheat became infected and in addition, extremely sensitive testing also failed to detect the virus in sheep saliva.
Landholders and research workers have repeatedly shown that dual-purpose winter wheats, if sown early and managed to a high standard, are capable of providing 50 to 120 days of productive grazing followed by grain yields as high as 7.5 tonnes a hectare. In better years and safer rainfall areas grain recovery after grazing is often similar to crops grown only for grain.
However, the potential for greater adoption of dual-purpose winter wheats and their ability to contribute to increased farm income depends on reliable disease-resistant varieties.
The development of high-yielding WSMV-resistant varieties combined with resistance to other important diseases such as the three rusts and BYDV will have an impact on upwards of one million hectares of cropping per annum.
Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.
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