Managing cucumber mosaic virus in lupins
"We seem to be in the CMV capital of Australia," was how one Dongara grower described the ravages of cucumber mosaic virus on his last crop.
"This year the crops were late establishing. They were OK until September when the aphids arrived — then the virus went berserk," he said. "I have been affected to the extent of 50 per cent in some of my paddocks."
He said growers are using tested seed and try to sow into wet soil, but cannot control the seasons. This year CMV infestation was described as "horrendous" following dry weather and late rains which did not allow early soil cover. There was good growth but no seed, and bunching of plants characteristic of the virus.
Mosaic virus is a serious problem for lupin production in a number of states. 1992 estimates from the Northern Agricultural Area of Western Australia indicate yield losses to the virus equivalent to 45,000 hectares of lupins. At a 50 per cent loss rate growers could be out of pocket $3.7 million.
The WA Department of Agriculture, with GRDC funding, has been working on practical management strategies to reduce cucumber mosaic virus (CMV) infection through the control of aphid vectors, and agronomic and cultural methods.
Two chemical treatments were tested in 1991: Nitofol, an aphicide reported to be effective against insecticide-resistant Green Peach Aphids (GPA), and a mixture of Pirimor and Ekatin, two commonly used aphicides. The results confirmed hopes that aphicides reduce CMV spread.
Agronomist Annette Bwye said that determining the full effects of seasonal conditions and picking the best treatment will require further trials.
CMV seed infection
Seed infection levels of 1 per cent or greater can lead to unacceptable yield losses. Mrs Bwye said that the late arrival of aphids in 1991 interfered with a trial to explore the effects of lower levels of seed infection.
As in previous years, farmers are being encouraged to test their lupin seed for CMV, and to use seed with low levels of CMV.
"Farmers are spending $ 1000s for seed tests. It's really the only defence we have against it at the moment," said mid-western lupin grower John Hutchinson. More than 1600 samples were CMV-tested by the Department of . Agriculture's seed testing laboratory in 1991-92 following the development of; CMV/germination/seed weight/ test package. This was developed by the WA Department of Agriculture and sponsored by Co-operative Bulk Handling and the WA Grain Pool. CMV is tested for first, and if this is low, germination is assessed.
High seeding rates generally provide an advantage. Dense populations of seedlings reduce the survival of infected seedlings, thereby reducing CMV spread. However, later sowing may reduce this effect by delaying the time at which the infected seedlings become shaded by the canopy, making them less accessible to aphids.
"The main way to reduce disease is good crop density and tested seed," agreed Eradu lupin grower Warwick Speechly.
Cultivation methods can make big difference
However, recent work is showing that certain practices can also work to the growers, advantage against CMV. Deep banding of phosphate fertiliser reduced the spread of CMV, while direct drilling with wide rows had the opposite effect. Retaining stubble with wide row spacing halved virus spread in recent trials.
Mrs Bwye said trials are ongoing to examine the effects of these agronomic practices on CMV spread.