INCREASED rhizoctonia bare patches in cereals have been observed around WA this season, with plant pathologists providing potential reasons why, suggested management options and recent trial data at an industry forum.
At a recent Department of Primary Industries and Regional Development (DPIRD) Grains Industry Day at Optus Stadium research scientist Daniel Hberli said seasonal conditions were most likely at fault.
According to Dr Hberli, the main reason for the higher levels of rhizoctonia inoculum was late rain and the subsequent later emergence of crops in a lot of regions this year.
"Plants established in cool soils which allowed Rhizoctonia solani to get the front foot - the pathogen likes cool soils so it started attacking the roots that were developing much slower," Dr Hberli said.
"The second reason was that growers were likely sowing into a high inoculum load, most of the paddocks probably had some cereals in there previously and with cereals, Rhizoctonia solani inoculum increases very quickly.
"We also had a very dry summer in 2019/20 and the pathogen inoculum can survive well in dry conditions, so the paddocks started with a high base of inoculum and that continued to rise after another cereal was sown."
However, if those paddocks had canola in 2019, the pathogen inoculum would have dropped considerably and less disease would have been evident in those paddocks.
Dr Hberli said the management of rhizoctonia required an integrated approach to reduce inoculum and control infection and impact on yield.
He said sowing early, if the season allowed, meant the plant could benefit from warmer soils, allowing the roots to get ahead of the pathogen.
Disturbance below the seed at sowing promotes rapid root growth away from the rhizoctonia and disrupts hyphal networks, with the ideal depth being at least 10 centimetres below the seed.
Fungicides applied through in-furrow liquid banding can provide useful suppression, while adequate nutrition is also key and it is that which DPIRD ran trials on in 2020.
The trial was run at the Muresk Institute and was part of a Grains Research and Development project, with a secondary trial running at Pinery in South Australia.
The main reason for the trial was to establish if the rhizoctonia disease changed with the application of nitrogen to the crop.
A total of five rates of nitrogen were applied, with 140 units the highest and a six unit starter application the lowest.
"While the yield data is not yet available, there was a clear biomass response with a significant increase in shoot weights in response to the higher nitrogen applications," Dr Hberli said.
"The spear tip proportions did not change with the nitrogen applied which is good news as it means growers can apply nitrogen and get a response in yield in plants that are able to outgrow the pathogen."
A similar project had also been run at Wongan Hills to measure the effect of nitrogen on crown rot, however that had vastly different results.
"In the plots with no crown rot, there was a really good yield response to the higher nitrogen applications, however when crown rot was added into the mix, it did start to impact the nitrogen response," Dr Hberli said.
"With the low levels of inoculum, there was a reduced impact of the nitrogen and that reduction was worse with the high levels of inoculum.
"The message is if there are paddocks with high crown rot inoculum, and it's a season that is likely to finish dry, then being conservative with the amount of nitrogen application is advisable in order to not drive excessive biomass that can compromise yield by crown rot."
Crown rot blocks the plant's ability to get water to the developing grain heads and when there are more tillers to support the limited access to water results in heads with shrivelled or no grain.
In that trial, Scepter (which is susceptible to crown rot) yielded better than Emu Rock (which is moderately susceptible to susceptible) at high crown rot inoculum levels.
In a different trial run by DPIRD at Yerecoin and Darkan in 2019 and 2020, the impact of deep soil amelioration practises on reducing the infection of root diseases and nematode pests was investigated.
The soil constraints at the Darkan site included high gravel content, moderate water repellency, compaction, high levels of rhizoctonia root rot (R. solani) and medium levels of the two types of root lesion nematodes species most commonly found (P. quasitereoides and P. neglectus).
The site was ameliorated in May 2019 with the susceptible barley variety La Trobe sown afterwards.
There were three types of amelioration to 40 centimetres depth - deep mixing, deep ripping and inversion renovations completed and the results compared to a nil treatment.
DPIRD research scientist Sarah Collins is the trial leader and said in the 2019 results from the Darkan site, soil borne disease and nematodes mostly reside in soils at depths from zero to 10cm
"By the end of the first season post renovation soil inversion significantly reduced root lesion nematodes (RLN) in the topsoil," Dr Collins said.
"While for the rhizoctonia, both the soil inversion and deep mixing amelioration treatments were successful at reducing the load in that top 10cm after renovation compared to the nil treatment.
"As we expected all amelioration methods pushed RLN deeper into the soil profile than they naturally residem, but for rhizoctonia, it was the deep mixing and deep ripping that made a significant change."
2020 was the second season post renovation, with wheat sown at the Darkan site.
At the beginning of season, testing indicated that for topsoil, soil inversion was once again successful for both of the soil borne disease and nematode constraints, while deep mixing also caused a reduction.
"2020 was an interesting season with a late start - in spring we looked to see if the pathogen levels at the beginning of the season conferred to the disease during a season like this," Dr Collins said.
"Plants from every plot were collected to be assessed for root health - soil inversion had better root health than the nil treatments including an increase in the shoot and root weights which fitted with our early season testing results."
So far 2020 results reflect what was found in 2019, with soil inversion being a potential method to reduce the disease impacts from RLN and rhizoctonia in a highly impacted paddock.
