Wheat Stripe Rust Recap from 2021

While for most producers the 2021 season was extremely productive, a large influx of rust in cereals was observed across the Central West region. The amount of stripe rust in wheat was particularly high, with a handful of varieties being significantly more affected than in previous seasons, such as Flanker, Coolah, Condo and Wedgetail. In this article I will aim to provide some insight as to why this has occurred and what lessons we can draw from this year which can be applied in future seasons.

Cause of stripe rust in wheat

Stripe rust in wheat is caused by the fungus, Puccinia striiformis f.sp. tritici. It is a leaf disease which requires living plants, as it robs its host of nutrients to reproduce. As the disease spreads, through the release of air-borne spores and infection of leaves, it reduces the photosynthetic area of infected plants. This can cause significant yield reductions, particularly when the ‘money leaves’ (flag leaf, flag leaf –1 and flag leaf –2) are impacted. Several factors have been proposed to explain the high rust load throughout our region. I would like to suggest at least three, knowledge of which I hope will benefit both advisors and producers.

Pathotypes

Firstly, we should recognise the ever-changing nature of the beast. As we are all aware, COVID-19 has appeared in slightly different forms, identified as different variants. Stripe rust pathotypes act in a similar fashion. There are several pathotypes of stripe rust which the inbred resistance of a wheat variety may or may not be able to adequately defend against. In recent years, relatively new pathotypes ‘239’ (239 E237 A- 17+ 33+) and ‘198’ (198 E16 A+ J+ T+ 17+) have caused issues in some varieties. DS Bennett was a prime example of this in 2020 where significant levels of stripe rust infections occurred in this variety as the ‘198’ pathotype became widespread in NSW. This caused the stripe rust rating to change from Resistant (R) to Susceptible (S) in two seasons. Likewise, the increased distribution of the ‘239’ pathotype has meant that varieties such as Coolah and even Gregory, were seen with higher incidences of stripe rust in 2021. Additionally, wheat varieties which are bred in other parts of the country, like WA, can be exposed to new stripe rust pathotypes when grown elsewhere. Therefore, varieties such as Vixen or RockStar may require more protection against stripe rust when grown in eastern parts of Australia. This is why it is important to keep up to date with changing resistance ratings in all the varieties which are part of your cropping program. These can be accessed in the NSW DPI Crop Variety Guide here.

Climate

Secondly, climate plays a critical role in the likelihood of rust development. Several producers this season remarked how the Adult Plant Resistance (APR) appeared to be less effective in 2021 in reducing stripe rust infections. This is despite some of these varieties being considered quite good against stripe rust on paper. What is likely being seen, is the impact of cooler than average temperatures. These temperatures influence the development of plants and by doing so inadvertently delay the expression of APR in wheat. On average, varieties with greater resistance express APR earlier than varieties with poorer levels of resistance. However, cooler temperatures can push expression back and can lead to resistant varieties having infections, which seem to take much longer for the APR to kick in and control infections. Nutrition also can delay the expression of APR. High nitrogen rates not only benefit plant growth but also boost the growth of the fungus responsible for stripe rust.  The takeaway is not to reduce nitrogen use, but rather be mindful of its interaction with stripe rust in a favourable season and adjust management strategies accordingly.

Lack of 'hot finish'

Thirdly, building on the last point, a ‘hot finish’ can often act as a fungicide spray in western environments. Typically, wheat is sprayed at Z32 (2-nodes) and Z39 (flag leaf emergence), with the second spray being the more important. 2021 had a ‘soft finish’ with lower-than-average temperatures during Spring, which significantly improved yield accumulation west of the Newell highway. However, these lower temperatures also provided ideal conditions for stripe rust cycling and meant the impact of earlier infections was still being felt later in the season. This is demonstrated in the number of infections found in wheat heads in 2021. Due to the number of spores in the crop canopy, heads were infected during flowering and expressed as stripe rust spores accumulating on the inside of the glumes. Fortunately, this year, this was nothing to be too concerned about as there was plenty of moisture and plant resources to support both grain fill and the pathogen in many regions. The key here is to control stripe rust in the canopy before it can infect heads. This is opposed to considering a very late ‘head wash’ fungicide option once head infections are noticed during grain fill, which poses a greater risk to producers. Since it may lead to a failure to comply with Maximum Residue Limits (MRLs) on delivered grain.

Lessons from 2021

The lessons we can take from 2021 include, the importance of minimising ‘green bridges’ for stripe rust. As the fungus requires a living host, being vigilant to remove volunteer wheat is critical. Likewise, pay close attention to early-sown varieties, especially with low resistance levels, which may act as the staging ground for rust build up to spread across to paddocks of main-season and later sown varieties. Also, it is important to remember that fungicides should rarely be used prescriptively. Frequent crop checks and proper identification of pathogens are crucial for making a good decision as to when a spray is necessary. NSW DPI Plant Pathologist, Dr Steven Simpfendorfer and his team, are a great resource to consult and send samples to, if advisors or producers require further confirmation on the identification of stripe rust. For example, various varieties can express yellow tipping on the leaf edges. Often thought of as rust, this has caused some producers to be concerned and to consider a fungicide application. However, this can simply be a plant response to frost and not have anything to do with rust. To ensure the longevity of products, producers are advised to be very strategic with fungicide use.

For more information on stripe rust, please contact your local LLS Ag Advisor. A more comprehensive GRDC article on Wheat Stripe Rust in 2021 can also be found here. For future assistance in identifying stripe rust and other cereal diseases producers can send a clear photo to NSW DPI Plant Pathologist Dr Steven Simpfendorfer on 0439581672 or contact your local LLS Cropping Officer Tim Bartimote on 0409838914.