A large-scale survey in late 1998 revealed widespread infection of canola with two viruses, beet western yellows and cauliflower mosaic, but not turnip mosaic.
Beet western yellows was found in 73 per cent of canola crops with incidence of infection of up to 65 per cent of plants. Worst-affected districts were Mt Barker-South Stirlings, Dowerin, New Norcia, Goomalling, Toodyay-York and Katanning.
Cauliflower mosaic virus was found in 30 per cent of canola crops with incidence of infection of up to 17 per cent of plants. Mingenew, Meckering and Carnamah were the district crops with highest infection levels. The two viruses were detected occurring together in 24 canola crops.
Both viruses are spread from infected to healthy plants by aphids. Wild radish was found infected with both viruses, and is a likely source from which aphids spread them to canola crops. Another likely infection source is volunteer canola, in which infection was also found.
Beet western yellows virus causes an insidious disease that is difficult to see in infected canola plants in the paddock. Mild plant dwarfing, leaf distortion and reddening or pallor of lower leaves are its symptoms when they appear. These mild symptoms are easily confused with those of nutritional disorders. European work has found seed yield losses of 10-15 per cent associated with this viral disease in canola. It also reduces oil content.
Cauliflower mosaic virus causes a more visible disease that is seen readily in infected plants in the paddock. It causes leaf symptoms of mottle, vein netting, ringspots, leaf distortion and shortened petioles. Pods may be twisted and distorted. Infected plants are dwarfed, specially if infected early. European work found that the impact of infection on seed yield is severe, up to 80 per cent yield loss with early infection of plants.
Relation to rainfall
Judging by what occurs with virus diseases in other broadacre crops in WA, losses due to these two viral diseases in canola crops are likely to be greater in years when summer and early autumn rainfall is high.
Under such conditions wild radish and other weeds proliferate before crops are sown. Aphids and viruses can then build up in them early. The aphids then spread the viruses to the crops early, resulting in increased spread of the virus infection later on.
From: Canola News issue 64, published by the Canola Association of Australia Inc. Edited version.