Cereal Pathology Update 2021

Take home messages

• Resistance to Systiva® and tebuconazole in the net form net blotch (NFNB) pathogen population was readily found on the West Coast of the Eyre Peninsula and is present in much of South Australia (SA).
• Increased virulence on barley in the NFNB pathogen population has been identified on the Northern Yorke Peninsula and in isolates tested from across Western Australia (WA).
• Resistance to Systiva has been identified in the spot form net blotch (SFNB) pathogen population in WA. Growing barley on barley was again part of the problem.
• Powdery mildew in wheat was a problem across wide areas of SA in 2020 and fungicide resistance is a significant threat limiting our ability to manage it.
• Crown rot was severe in 2020 and inoculum levels will be high, going into 2021. An effective seed treatment is on the horizon.

Season summary

A reasonable start to the season with good early rains gave warning that foliar diseases could have been a problem in 2020. This was followed by a dry winter that delayed the development of most diseases and, in particular, ensured that Septoria, which requires more consistent rain periods, would not be a serious problem to most growers outside of the South-East. These early wet and then very dry conditions greatly favoured the development of crown rot in winter and many crops were seriously affected in 2020. Inoculum levels will be very high in stubbles from these crops for the 2021 season, so care should be taken in choosing what to grow in these paddocks in 2021.

The disease that resulted in the greatest number of enquiries to SARDI’s cereal pathology team was powdery mildew in wheat. This has become a regular problem on the upper Yorke Peninsula, but in 2020 the disease was found to be a concern across a much wider area. The cooler, more humid conditions through spring favoured spread and development of the disease through the canopy and also onto the heads and many crops were given a spray to try to manage the problem. Apart from the favourable spring conditions, this disease has also been building up in recent years because of the widespread cultivation of very susceptible varieties that trace their pedigree back to Wyalkatchem. Amongst these varieties are Scepter and Chief CL Plus. Mace was the exception amongst these varieties in that it is rated only moderately susceptible to susceptible (MSS) to powdery mildew and is therefore significantly better than the susceptible to very susceptible (SVS) ratings of the newer varieties that have replaced it. Stubbles from these infected crops will be hosting the survival fruiting bodies of the fungus so there will be a substantial carryover of inoculum across the state going into the new season.

Unfortunately, the pathogen causing powdery mildew in wheat is another that has developed resistance to some fungicides. In this case resistance to the strobilurins and some of the older demethylation inhibitors (DMI), or azole, fungicides is common on the upper Yorke Peninsula. It is highly likely that this resistance is now much more widespread. This will mean that where inoculum levels are likely to be high, growers should avoid the most susceptible varieties and try and manage any subsequent infection with a mixture of fungicides applied early to keep inoculum levels down.

Stripe rust made a reappearance and this season a mixture of the older, Scepter virulent strain and a new strain from the east was seen which has reduced virulence on Scepter and related varieties but increased virulence on Trojan, DS Bennett and many of the durums. Leaf and stem rust were again absent from wheat crops apart from some leaf rust in the South-East. Barley leaf rust was more common and was assessed in National Variety Trial (NVT) plots at Brentwood and Roseworthy.

Net form net blotch

Testing of samples of NFNB on adult plants of many barley varieties reveals their ‘pathotype’, that is their pattern of virulence which distinguishes them from each other. Repeating this testing each year reveals the selection that the pathogen population is under and this largely reflects the varieties that are grown in the areas the samples are collected from. This is not entirely the case because some varieties are released which have almost no resistance and other varieties have resistance which is more durable than others. Added to this is the somewhat random nature of mutations that determine which virulences arise, and therefore, can be selected for.

Table 1 shows the 2020 isolates tested so far, and indicates that Commander, Compass, Fathom and Spartacus CL are all susceptible (S) or very susceptible (VS) at multiple sites. In the case of Compass and Spartacus CL this situation has evolved since their release and widespread cultivation. Fleet and Maritime, which were very susceptible in the past, now appear to be quite resistant at all sites bar one. This shows that in the absence of that variety being grown, the pathogen can quite rapidly lose virulence for that variety. One isolate collected from Maritime in an NVT trial at Bute showed however, high virulence on both these varieties as well as on the older varieties; Clipper, Schooner and Sloop SA that had previously shown a high level of durable resistance to this disease. These data show how variable this pathogen is and how important it is to keep the level of disease in crops as low as possible. The more the disease is allowed to develop the more the variation and virulence will develop in the future.

Amongst the newer varieties, Banks, Bottler, Traveler and Kiwi appear to have the best resistance. RGT Planet continues to show moderate resistance (MR) in much of SA, although in the South-East this variety is VS and in previous years virulence on RGT Planet has been recorded on the Lower Eyre Peninsula. Other varieties with notable resistance are Rosalind, Scope and Vlamingh.

In 2020 for the first-time similar tests were conducted with NFNB isolates collected from other states. In Western Australian isolates, despite lower levels of reported disease, the level of virulence was surprisingly high and more so than in South Australian isolates (Table 2)

These pathotype tests have revealed that the most useful resistance was to be found in the varieties Banks, Fleet, Kiwi, Maritime, Scope and Vlamingh. However, it is also known that, apart from Scope and Kiwi, all these varieties have shown high susceptibility to at least one, and often many, isolates in tests carried out in previous years. Scope appears to have the most durable resistance of all varieties tested, although it has rated S to one isolate in SA (Bute 2020) and one from NSW (Moree 2003).

The information from these tests can be used to help select varieties that have effective or different resistance patterns to NFNB in each region. Keeping a mix of varieties in an area helps to control the disease in a manner similar to mixing and rotating chemical control measures.

Table 1. Results of adult plant tests with net form net blotch isolates collected from SA in 2020.

1= resistant (R), 3 = moderately resistant (MR), 5 = moderately susceptible (MS), 7 = susceptible (S), 9 = very susceptible (VS)

Table 2. Results of adult plant tests with net form net blotch isolates collected from WA in recent years by Curtin University.

1= resistant (R), 3 = moderately resistant (MR), 5 = moderately susceptible (MS), 7 = susceptible (S), 9 = very susceptible (VS)

Fungicide resistance testing of the SA isolates by Fran Lopez of Curtin University has revealed that resistance to Systiva and tebuconazole is now very common along the stretch of country from Streaky Bay down to Elliston on the west coast of the Eyre Peninsula as well as across the Yorke Peninsula. Resistance to Systiva has also been confirmed from Lock and Kybybolite in 2019 and at Avon in 2020. Where resistance has been detected, growers are advised to avoid using Systiva for the following year at least and are advised to use alternative fungicides for early season control, where required. For further advice and up to date information on fungicide resistance, go the Australian Fungicide Resistance Extension Network (AFREN) website.

Spot form net blotch

In 2020 Curtin University researchers detected resistance to Systiva in the spot form net blotch (SFNB) pathogen population in WA. The resistance was detected in three paddocks near Cunderdin that had grown barley in successive seasons with Systiva applied to the seed each time. This is precisely the scenario where resistance to NFNB was first detected in SA in 2019. Growing barley on barley is not a good idea but if a grower does so despite this advice, then it is advisable to ensure that different chemical treatments are rotated, more resistant varieties are used, and any control is applied early to keep inoculum levels low.

Acknowledgements

The studies undertaken to enable this information to be provided is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would therefore like to thank them for their continued support. The NFNB isolates were also kindly contributed by Simon Ellwood from Curtin University.

Useful resources

Cereal Variety Disease Guide 2020

Cereal Seed Treatments 2021

Grower Advice

Contact details

Dr. Tara Garrard,
Research Scientist Cereal Pathology
SARDI
08 8429 2247
tara.garrard@sa.gov.au
@TaraGarrard

Varieties displaying this symbol  beside them are protected under the Plant Breeders Rights Act 1994