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Source: GRDC's The Crop Doctor

Autumn is here and growers should consider if rainfall is helping establish a ‘green bridge’, or ‘ramp’ of grasses and volunteer cereals, hosting pests and diseases that could transfer to crops in adjacent paddocks.

A recent GRDC-supported study on the epidemiology of Wheat Streak Mosaic Virus (WSMV) demonstrates how this can occur.

In August 2007, a WSMV outbreak was detected in the Koorda district of WA.

According to DAFWA entomologist, Geoff Strickland, the preceding summer was wet, with more than 100 millimetres of rain in January/February, leading to growth of grasses and volunteer wheat in which the virus vector, the Wheat Curl Mite (WCM) built up populations and spread WSMV.

WSMV incidence and WCM populations in the wheat crop declined rapidly with increasing distance from the virus source in an adjacent pasture with WSMV-infected volunteer wheat. The wheat crop was probably not downwind from the WCM source and this probably limited WCM spread, he said.

Within the wheat crop margin, WSMV reached 40 per cent and WCM numbers eventually reached 4800 mites per wheat ear, Mr Strickland explained.

A similar scenario in Merredin in 2006 saw high temperatures and sufficient
pre-season rainfall generate a substantial ‘green ramp’ of grasses and volunteer cereals before sowing. Almost 200 millimetres was recorded from January to April.

Where the ‘ramp’ was inadequately controlled there was widespread WSMV infection across the whole crop paddock, rather than just in the paddock margin, as in 2007. Strong westerly winds also appear to have helped spread the WCM.

Wheat is subject to infection by three predominant rust species: leaf rust, stem rust and stripe rust. These are not seed or stubble borne, but require living host plants to survive. The major green bridge risk for cereal rusts is associated with cereal regrowth.

Rainfall or high relative humidity, creating extended periods of leaf wetness, is essential for rust infection and autumn weather often provides these conditions.

Autumn green cereal regrowth can provide a head-start for rust diseases, increasing the risk of early stripe rust infection in crops. In highly conducive circumstances, stem and leaf rust can be found on wheat and barley volunteers. Visible infection on regrowth strongly indicates an elevated rust risk for the coming season.

GRDC-supported DAFWA Plant Pathologist, Geoff Thomas said maximising the pre-plant chemical fallow with early weed control is important to reduce rusts and pest activity before cropping.

He indicated that early weed control, rather than delayed sowing, should be used to maximise the pre-plant chemical fallow period. Pre-plant chemical fallow is most important when autumn rains promote prolific pre-season cereal growth, as is the case this year.

Growers are reminded that weeds are more readily controlled when small. Grazing and burning also provide good weed control, in addition to chemical control, if compatible with other farming practices.

For leaf diseases, particularly rusts, early control of autumn cereal regrowth can delay commencement of disease, reducing disease impact and therefore the expense of subsequent control measures.

Regular updates of plant disease risk, including maps outlining regional rust risk, are available from the Plant Disease Forecast 2008 page on the DAFWA website www.agric.wa.gov.au/cropdisease

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