NSW DPI plant pathologists Dr Steve Simpfendorfer and Brad Baxter, in a podcast hosted by Penny Heuston (DPI Trangie), emphasised that while stripe rust challenged wheat and triticale varieties in 2022, resistance status of varieties largely performed as expected, with a few exceptions due to a mutation change.
They reported there has been no breakdown in gene resistance effectiveness, rather, the ability of newer pathotypes to acquire virulence on particular genes, which led to changes in a variety's resistance rating.
The relative distribution of stripe rust pathotypes across Australia can change the way an individual variety acts to stripe rust between regions and between seasons.
Simpfendorfer and Baxter pointed out that stripe rust protection via resistance genes is complex involving many aspects. Most varieties have more than one resistance gene.
Some genes provide protection from seedling to maturity (major gene resistance), and some varieties only have adult plant resistant genes (minor gene resistance).
Adult plant resistance genes, in the absence of effective seedling resistance ones (all of plant life), commonly are not effective on stripe rust until stem elongation or later (growth stage 30 to 39 or more).
Adult plant resistance expression depends on a variety's resistance rating and environmental conditions, with cooler temperatures delaying the expression of some genes.
Varieties fully reliant on adult plant resistance can be moderately or heavily infected by stripe rust until protection from adult genes kicks in. In a year with early infection, early fungicide control can be important.
Steven Simpfendorfer and Brad Baxter pointed out that sowing with appropriate fungicides (e.g. flutriafol) added to starter fertilisers provides seedling stripe rust control.
However, early sowing of stripe rust susceptible winter wheats, such as Bennett, can commonly see fungicide effectiveness run out of varieties well before the crop has ended its winter habit period.
The complex population dynamics of stripe rust is shown by the change in the dominance of the two most common pathotypes, 239 and 198.
These, first detected in 2017 and 2018, originated from overseas. Some Australian varieties are resistant to 198 but susceptible to 239 and vice versa.
Limited 239 frequency and distribution in 2020, but a build-up in 2021 and 2022 meant that some varieties initially displaying resistance, appeared susceptible in 2021 and 2022.
This seemed to make varieties such as Catapult, Coolah, Flanker, Rockstar, and Vixen appear more susceptible.
There were some reductions (more than one resistance level) to ratings of varieties for the 2022 season, including Astute (triticale), Boree, Catapult, Coolah, Coota, Devil, Oryx, Rockstar, Sheriff CL Plus, Sting, Valiant CL Plus, Vixen and Yitpi.
Resistance rating changes are likely for the 2023 season.
Steven Simpfendorfer and Brad Baxter urge you to check the rating of varieties for 2023 sowing.
As the 2022 season has shown, it is often unreliable to depend solely on fungicide control.
Crops were often too wet for ground rig application, or planes were unavailable.
A 10-day delay in spraying at a critical growth stage on a susceptible variety in 2022 resulted in an estimated 40 per cent yield loss for some growers.
Both researchers emphasise the vital role farmers and agronomists play in helping to monitor rust pathotypes by sending rust-affected leaf samples to the Australian Cereal Rust Survey conducted by the University of Sydney, with GRDC funding at Cobbitty (sydney.edu.au/agriculture/our-research/plant-production.html).
Comprehensive tables detailing leaf, stem and stripe rust genes within Australian varieties are regularly updated and made available via the University of Sydney Cereal Rust Plant Breeding program led by Professor Robert Park.
The search for new resistance genes is ongoing for breeding to improve rust resistance. Many sources are used for finding new genes, including wild types of distant wheat relatives, plus screening and cooperating with other world breeders.
As plant pathologists, Simpfendorfer and Baxter are involved in the complex of cereal diseases and stress the importance of managing them all.
For further details, brad.baxter@dpi.nsw.gov.au or Steven Simpfendorfer steven.simpfendorfer@dpi.nsw.gov.au.
The Podcast is available on the normal streaming platforms under the handle NSW DPI Agronomy.
Next week: Not-so-new pasture legumes performing well.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.
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