Following favourable conditions across eastern Australia during 2020, NSW Department of Primary Industries plant pathologist Dr Andrew Milgate is encouraging careful attention to pre-sowing paddock hygiene to prevent the carryover of disease into 2021 crops.

Dr Milgate says this means removing volunteers from paddocks and reducing stubble loads to prevent the survival of pathogens over summer and autumn.

“Allowing cereal volunteers from 2020 to persist uncontrolled in paddocks will enable rust pathogens to carry over into 2021 early sown crops,” Dr Milgate says.

“Other diseases like Septoria tritici blotch and Yellow leaf spot survive on stubble, so it is important to manage stubble loads to reduce early disease pressure, improve trafficability and reduce nitrogen tie-up.”

Variety selection

From a pathologist’s point of view, Dr Milgate says any variety selection decision needs to factor in the variety’s available disease package.

“Not all diseases are a threat in all environments, so think about your biggest disease constraints in an average year and work backwards from there,” he says.

“Also consider how the farming system is set up, whether your system is likely to expose a crop to particular diseases and the management options available.”

When it comes to variety selection, Dr Milgate says to look for the highest-yielding variety with the best genetic disease package available.

Varieties that come on to the market are tested for their level of disease resistance or susceptibility through the National Variety Trials (NVT).

In southern New South Wales, for example, Dr Milgate says yield-limiting diseases in average to above-average seasons include Septoria tritici blotch and Yellow leaf spot. For these diseases, he encourages the selection of varieties that are moderately susceptible (MS) or better.

“Having a minimum level of resistance such as MS across a wide area means overall disease pressure is kept at a manageable level,” he says.

“In seasons with average to above-average rainfall, the disease will likely move to the upper canopy and cause yield loss, so having some adult plant resistance to these diseases will mean multiple fungicides are less likely to be required to protect yield.”

For other diseases such as Stem rust, Dr Milgate says to avoid susceptible varieties because of the widespread risk it poses to the industry.

Check disease status

During 2020, Dr Milgate says DS Bennett, previously rated as moderately resistant for Stripe rust, showed it was now more susceptible to the disease, particularly in early sowing situations.

He says this change was first detected late in 2019 through the NVT nurseries. The change was published on the NVT website 2020 disease ratings and in the NSW DPI Winter Crop Sowing Guide 2020.

“When a lot of susceptible crops are infected with Stripe rust, as was the case in 2020 across eastern Australia, it increases the pathogen’s effective population size and this raises the chance of a mutation occurring which changes the virulence of the pathogen on additional resistance genes,” he says.

“We are always monitoring the rust population for changes in virulence, through the national rust survey coordinated by the Australian Cereal Rust Control Program supported by GRDC.”

Risk management

If 2021 is an average to above-average season, which is favourable for disease development, Dr Milgate says, necrotrophic diseases such as Septoria tritici blotch, Yellow leaf spot in wheat and Scald and Spot form of net blotch in barley will pose a higher risk because of the build-up of inoculum during 2020.

“Keeping records of disease development in paddocks through the rotation over time as well as using tools such as Predicta^® B DNA testing can help inform likely inoculum levels,” he says.

“Also, if we have a wet harvest or summer rainfall, this increases the likelihood that volunteers will survive, which creates an environment for increased survival of rust pathogens.”

In 2020, Dr Milgate received samples and reports of Stem rust in oats grown in southern and central NSW. If susceptible varieties are infected early, he says, this disease can cause 100 per cent yield loss.

“Stem rust of oats can also infect wild oats, so there is potential for it to survive and build a base to jump on to the next crop of oats in 2021, which could be difficult to control with fungicides and cause significant losses,” he says. “It is critical to control volunteer oats and wild oats.”

Dr Milgate says if conditions are dry during 2021, Crown rot is likely to be an issue because infections of this disease have still occurred during the season, but the symptoms have been less obvious.

“It causes white heads and basal stem browning. Longer straw length provides the opportunity for Crown rot growth to extend up the stem,” he says.

“Crop pathogens are resilient, which makes summer hygiene through the control of volunteers and stubble reduction all the more important in helping to reduce disease infections during 2021.”

More information: Andrew Milgate, 02 6938 1900, andrew.milgate@dpi.nsw.gov.au