Crop rotation disease link

The West Australian

WA grain production is shifting, with more land sown to wheat and canola and less to legumes and pastures, which has resulted in a corresponding increase in root diseases.

These findings from the long-running Focus Paddock project will be profiled at the 2015 WA Agribusiness Crop Updates in Perth, hosted by the Department of Agriculture and Food WA and the Grains Research and Development Corporation.

The data from 184 farm paddocks from across the Wheatbelt provides insight into the State's grains industry and emerging challenges.

Department research officer Martin Harries said annual paddock surveys from 2010 to 2014 showed the vast majority of paddocks were in a suitable condition for wheat production when sown in sequence with break crops and pastures.

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However, the survey highlighted that such a sequence, or crop rotation, had created an environment conducive for an increase in root diseases.

"Our analysis shows the incidence and severity of paddocks with root diseases increased steadily over the study period for most pathogens," he said.

"While less than 6 per cent of paddocks had pathogens present at levels where there could be high yield losses, visual ratings indicated that the majority of paddocks contained some plants with root disease, mostly rhizoctonia and root lesion nematode.

"While less than 50 per cent of plants sampled showed symptoms, this is an emerging constraint for growers, especially those in the southern agricultural region, to address, particularly in seasons conducive to disease."

While the increase in canola plantings, as the break crop of choice, does suppress some root diseases like rhizoctonia and crown rot, root lesion nematodes are not necessarily controlled.

The risk of sclerotinia in broad-leafed crops also increases when sowing canola in tight rotations.

Mr Harries said about 12 per cent of land was sown to canola across the northern (NAR), central (CAR) and southern agricultural regions (SAR), while wheat continued to dominate land use.

"Most years, about 60 per cent of land was sown to wheat within the monitored paddocks, with 68 per cent in the NAR, 58 per cent in the CAR and 49 per cent in the SAR," he said.

"Crop diversity was limited, with wheat, canola, barley and lupins accounting for 84 per cent of land use.

"Pastures made up 12 per cent of overall land use, with most sown in the SAR (21 per cent), followed by the CAR (12 per cent) and the NAR (7 per cent)."

This diversity is reflected in the regional differences in crop sequences, with wheat-on-wheat sown mainly in the NAR (61 per cent), then the CAR (46 per cent) and the SAR (30 per cent).

The survey also highlighted WA growers are committed to good weed control, with low weed burdens recorded.

"Grass weed density increased under wheat crops, even from low starting levels. However, when canola was sown the weeds declined significantly," Mr Harries said.

"The SAR paddocks had more grass and broadleaf weeds at high densities, which is expected to be due to higher use of pastures in the region."

Soil nitrogen was high across the focus paddocks, with an average value of 53 to 108kg of nitrogen in the soil profile pre-sowing per hectare over all sampling years.

The survey also confirmed soil acidity continues to be a major challenge for grain growers, with 61 per cent of paddocks sampled from 2010-14 recording top soil with a pH of less than 5.5.

"Eighty-eight per cent of SAR paddocks had acidic soils, pH of less than 5.5 at 0-10cm, compared with 68 per cent in the CAR and 48 per cent in the NAR," Mr Harries said.

"However, the frequency of acid soil below 10cm, pH less than 4.8 was lower in the SAR (4 per cent), compared with the CAR (23 per cent) and the NAR (43 per cent).

Agribusiness Crop Updates will be at Crown Perth on February 24 and 25.

Visit giwa.org.au/2015-crop-updates or call the Grain Industry Association of WA on 6262 2128.

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