News reports on COVID-19 over the past 18 months have, at times, made reference to the field of epidemiology – the study of the distribution and causes of diseases in populations. Epidemiology is also vital in plant disease control – for example, delaying epidemic onset and slowing disease progress (‘flattening the curve’) are critical in dampening the epidemic potential of a plant pathogen.

Destroying plants that establish during the non-cropping summer and early autumn months (the ‘green bridge’) in Australia plays an important role in delaying epidemic onset of many cereal diseases, especially those caused by the fungal rust pathogens that can only live and reproduce on a living plant.

When considering how rust pathogens survive and build up, and the composition of the green bridge, often only the cereal species they attack are considered. However, long-term research on cereal rust pathogens in Australia has shown that some grasses, both weedy and native, can serve as ‘ancillary’ hosts, and that these non-cereal grasses play a significant role in the epidemiology of many of these pathogens.

Importance of native grass

In the north-eastern grain growing region, research dating back to the 1950s showed the importance of the native grass Anthosachne (formerly Agropyron) in harbouring the stem rust pathogens that infect wheat and cereal rye. The stripe rust pathogen infects a variety of weedy grasses; it has occasionally been found to infect prairie grass (Bromus unioloides) and phalaris (P. minor and P. paradoxa) and is very common on wild barley grass (Hordeum glaucum and H. leporinum). Research on stripe rust on barley grass has shown clearly that these grasses play an important role in the within-season build-up of this pathogen.

Of all non-cereal grass species occurring in Australia that are vulnerable to cereal rust infection, wild oats (Avena barbata, A. ludoviciana, A. fatua) create the biggest problems for rust control. Both oat crown rust (Puccinia coronata f. sp. avenae) and oat stem rust (P. graminis f. sp. avenae) are common on wild oats wherever they occur in Australia. Both pathogens were common in wild oats along roadsides in southern NSW earlier this year, pointing to a higher risk of these diseases developing in oat crops in 2021.

Apart from enabling disease build-up, the oat rust pathogen populations maintained on wild oats are a source of tremendous pathogen variability, making the development of genetically resistant oat cultivars very challenging. Mutations in rust pathogens are random events that are estimated to occur at the rate of about one per 10 billion spores. Consequently, more rust means more mutants, and more mutants threaten the in-built genetic resistance of cereal cultivars.

When inspecting crops for rust diseases, it is well worth looking at nearby weedy grasses such as barley grass and wild oats for signs of rust. Should anything suspicious be found, please send freshly collected samples in paper only to the Australian Cereal Rust Survey, at University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan, NSW, 2567.

More information: Robert Park, 02 9351 8806, robert.park@sydney.edu.au