Beware drought impact on 2019 crop diseases and pests

Beware of drought's bite when planning agronomy management this season


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Narrabri grower Ian Gourley in mid September in his wheat crop, one of many which was deemed unharvestable in last year's New South Wales drought. PHOTO Shanna Whan

Narrabri grower Ian Gourley in mid September in his wheat crop, one of many which was deemed unharvestable in last year's New South Wales drought. PHOTO Shanna Whan

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Dry 2018 growing season disrupts above and below ground processes impacting agronomy.

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Growers have been advised to take special care this year when selecting both the winter crop and variety to plant after the failed or low-yielding crops caused by last years drought.

NSW Department of Primary Industries (DPI) plant pathologist Dr Steven Simpfendorfer says yellow spot in wheat and net blotch in barley are among the foliar diseases that may be lurking in cereal stubble.

One of the key processes in a rotation is for stubble from the previous crop to get wet during the break crops cycle, and that didnt happen for a lot of growers last year, Dr Simpfendorfer says.

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The bigger issue with chickpea crops in a very dry season like we had last year is that they are not able to perform their function as a break crop for those fungal pathogens that could still be in 2017 cereal stubble.

Just because you havent had a crop doesnt mean the fungus has gone away, and growers should be using their longer-term memory to think back to what was there two years ago, and testing to help them assess disease risk.

One of the key processes in a rotation is for stubble from the previous crop to get wet during the break crops cycle, and that didnt happen for a lot of growers last year - NSW Department of Primary Industries plant pathologist Dr Steven Simpfendorfer

Crown rot risk

Dr Simpfendorfer says crown rot should not be forgotten as a risk for the 2019 crop.

If the crop failed last year and was baled, a large proportion of the potential inoculum would have been carted out of the paddock as hay," he says.

"But the infected crown still remains, so a bigger issue for growers may be planting over a failed crop."

Dr Simpfendorfer says it might be worth using PREDICTA® B testing to determine crown rot inoculum risk so a suitable crop and variety can be selected.

Sowing on time in 2019 is crucial, because later sowing will increase chances of moisture stress during flowering, which exacerbates yield loss from crown rot, he says.

If growers cant sow on time, they might want to avoid durum and go for a cereal with more tolerance to crown rot, such as a bread wheat with resistance or tolerance to crown rot, or barley.

Sowing on time in 2019 is crucial, because later sowing will increase chances of moisture stress during flowering, which exacerbates yield loss from crown rot - NSW Department of Primary Industries plant pathologist Dr Steven Simpfendorfer

If a paddock was in a low crown rot cycle, a failed wheat crop wont have blown levels of this disease through the roof in terms of risk.

Nematode monitoring advised

University of Southern Queensland Centre for Crop Health (CCH) researcher Dr Kirsty Owen says root lesion nematodes (RLN) Pratylenchus thornei and P. neglectus can survive in dry conditions in extended fallows.

She says poor growth of a susceptible crop due to dry seasonal conditions is no guarantee that nematode populations are low.

In the dry winter-cropping season of 2017, P. thornei populations increased as much as sevenfold after growing the faba bean cultivar Cairo, she says.

While RLN populations decrease quickly in hot and dry topsoil, they survive in cooler conditions below 15 centimetres and will infest the next susceptible crop.

When testing for nematodes using PREDICTA® B, collect soil samples from the surface down to 30cm soil depth to get a better picture of the population densities of the nematodes, particularly where there has been a long fallow, Dr Owen says.

If summer crops have been planted, remember that mungbeans are susceptible to P. thornei but sorghum is moderately resistant to P. thornei - University of Southern Queensland Centre for Crop Health researcher Dr Kirsty Owen

She says RLN each have a unique and broad host range, and some cultivars of wheat allow more reproduction of one nematode species than the other.

Growers are advised to check National Variety Trials results carefully and choose varieties with tolerance and some resistance to RLN.

If summer crops have been planted, remember that mungbeans are susceptible to P. thornei but sorghum is moderately resistant to P. thornei, Dr Owen says.

Manage for low AMF

Arbuscular mycorrhizal fungi (AMF) colonise the roots of a wide range of crops and greatly improve the uptake of phosphorus and zinc from soil and fertiliser sources.

AMF depend on living plant roots to grow and multiply. During periods of weed-free fallow, CCH researcher Professor John Thompson says their numbers decline in the soil.

Professor Thompson says population densities of AMF in the soil before sowing in 2019 are likely to be low if there has been an enforced long fallow through lack of planting rain in 2018, or if a crop was sown but grew poorly.

On the other hand, if a summer crop was grown successfully in 2018-19 and there is opportunity to double-crop leading into winter 2019, population densities of AMF will be high, Professor Thompson says.

Levels of AMF can be quantified using a PREDICTA® B soil test down to a depth of 15cm.

If a summer crop was grown successfully in 2018-19 and there is opportunity to double-crop leading into winter 2019, population densities of AMF will be high - CCH researcher Professor John Thompson

If population densities are really low, then avoid planting crop species such as faba beans and linseed, as these are highly dependent on AMF for growth, he says.

Usually, barley and wheat are good options as they are less dependent on AMF, although in soil of low phosphorus status under low-growing-season rainfall conditions, wheat too can be dependent on AMF for nutrition.

Chickpeas are also mycorrhizal dependent and the chickpea growth phase helps build AMF populations.

Legumes such as faba beans and chickpeas benefit doubly from AMF because the phosphorus they supply to the roots ensures effective nitrogen fixation by rhizobia bacteria in root nodules.

GRDC Project Codes: DAN00175, DAV00128.

More information: Dr Steven Simpfendorfer, 0439 581 672, steven.simpfendorfer@dpi.nsw.gov.au; Dr Kirsty Owen, 07 4631 1239, kirsty.owen@usq.edu.au; Professor John Thompson, 07 4631 1148, john.thompson@usq.edu.au

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