Two new pathotypes of wheat stripe rust were detected by staff at the University of Sydney in 2021, but fortunately neither is expected to pose any threat beyond that posed by existing pathotypes.
Stripe rust pathotypes 198 (first detected in 2018), 239 (detected 2017) and an older strain (134) are the main stripe rust pathotypes likely to affect 2022 eastern states crops.
Professor Robert Park and Mumta Chhetri (University of Sydney) and Steven Simpfendorfer (NSW DPI Tamworth), leading stripe rust (and other rusts) pathologists warn that stipe rust potentially can cause yield losses of over 50 percent in more susceptible varieties.
Resistance rating have been revised for 2022, largely because of increased incidence of the newer pathotypes.
Park, Chhetri and Simpfendorfer stress the importance of checking latest official variety ratings. This will assist in devising control strategies involving fungicide use. New ratings are detailed on the GRDC web site (nvt.grdc.com.au) as well as in the NSW DPI 2022 edition of the Winter Crop Variety Sowing Guide.
A milder cropping season in 2021 resulted in some slower activation of some adult plant resistance (APR) genes. Hence crops of some varieties with these resistances exhibited more stripe rust than would normally be expected.
All varieties, unless rated resistant (R), are susceptible as seedlings and move towards increasing resistance as they develop and their APR genes become active. Growth stage at which APR is activated differs between wheat varieties and relates to their resistance rating. An MR (moderately resistant) rated variety would generally have APR active by early stem elongation (GS 30-32) and MR-MS (MS equals moderately susceptible) by flag leaf emergence (GS37-39).
MS rated varieties have APR activated by awn peep to start of flowering (GS49-60) and MS-S by flowering to mid-milk (GS61-75). Varieties rated S or worse have relatively weak levels of resistance, generally of limited value in disease management.
Professor Park, Dr Chhetri and Dr Simpfendorfer detail how fungicide treatments depend on aspects that include whether or not the crop is sown with appropriate up-front fungicides, how the season is progressing, and what is the varieties disease resistance rating.
For example, in MS varieties a two-fungicide input strategy normally provides effective stripe rust management, with flutriafol on starter fertiliser or in-crop fungicide application at GS30-31 the first input, followed by a second fungicide application at GS39.
This strategy relies on extended control of in-furrow flutriafol (normally to GS37-39) or approximately three-weeks leaf protection from a foliar fungicide applied at GS30-31.
With a two-spray strategy the GS30-31 application provides three weeks protection to flag leaf and lower leaves to limit stripe rust development.
A second application at full flag emergence (GS39) provides a further three weeks protection of the top three leaves, until the heads emerge APR becomes active in MS varieties.
Generally, varieties with MR-MS or MR ratings only require one fungicide treatment, if any treatment at all.
Higher levels of nitrogen nutrition can play a role in fungicide decision making. High soil nitrogen can delay crop maturity and expression of APR genes. High soil nitrogen can also provide in-crop environments more conducive to stripe rust infection (thicker canopy and leaf nitrate food source for the pathogen).
In milder seasons like 2021, gaps between key growth stages became extended as crop development slows resulting in longer periods where leaves are exposed to stripe rust infection using traditional two-fungicide input strategy. In milder seasons, more susceptible varieties potentially require a third fungicide input to provide full overlap of protection across susceptible growth stages.
For a detailed analysis on managing stripe rust see the GRDC document "Wheat stripe rust epidemic in 2021 - learnings for 2022" authored by Robert Park and Mumta Chhetri and Steven Simpfendorfer.
Next week: Fleabane not just a crop weed problem. Control strategy for pastures.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.