Cereal diseases update for 2020 in South Australia

Take home messages

• With a very large area being sown to Spartacus and Compass we are seeing a heavy selection for increased virulence of net form net blotch (NFNB) on these varieties.
• The repeated use of fungicides on susceptible wheat and barley varieties has seen a loss of efficacy to NFNB in some demethylation inhibitors (DMIs) and Systiva^®, and to wheat powdery mildew in strobilurins.
• It is important to try and keep a step ahead of pathogens by rotating varieties and fungicides to disrupt selection in the populations.
• Rhizoctonia may have built up under the dry conditions of 2018 and 2019 and so could present significant problems in 2020.

Season summary

2019 saw another year with generally low levels of foliar diseases owing to low carryover of inoculum from 2018, relatively dry growing conditions in most areas and many growers using fungicides as protectants. Large areas of the state are now being sown to a very narrow range of wheat and barley varieties, such that the predominant varieties are determining which diseases are most prevalent.

Rusts were almost absent from 2019 South Australian (SA) crops. Just a smattering of barley leaf rust was observed in the most susceptible varieties in untreated National Variety Trials (NVT) plots on the Yorke Peninsula (YP) and in the South-East (SE). Stripe rust, although absent from SA this season, has changed in virulence once again with the new strain observed in Victoria and southern NSW. This new strain’s most notable feature is increased virulence on almost all durum varieties. It also has significantly increased virulence on DS Bennett, Emu Rock and Trojan.

In most parts of SA, Rhizoctonia has built up substantially over the last two seasons. This pathogen is favoured by the dry winter and spring conditions experienced in both 2018 and 2019, while a dry summer (such as the 2018/19) also ensures the soilborne inoculum carries through to the next season. Rhizoctonia is hosted by a broad range of plants, however cereals and grassy weeds are preferred hosts and will increase inoculum greatly. The run of bad seasons may put pressure on growers to plant repeated cereals, and sometimes also limit effective grassy weed control in pastures and break-crops, which is likely to favour Rhizoctonia.

There are two key messages for Rhizoctonia in 2020. While growers who have had Rhizoctonia problems are likely to recognise the ‘classic bare-patch’ symptoms, many crops which are badly affected do not express this symptom. This is usually due to an early sowing opportunity, as was the case in many parts of SA in 2019, and good agronomy. Crops establish well under these conditions, before Rhizoctonia becomes active. However, during the dry, cold winter, Rhizoctonia will build up and infect crop roots, often without obvious symptoms. Crop effects and even yield loss may have gone un-noticed or simply been attributed to the poor finish and/or frost. If you are unsure of whether Rhizoctonia has built up in your paddocks, request a PREDICTA^®B test from your agronomist.

The second message for 2020 is around management. Although not predicted at the time of writing, significant (>20mm), repeated summer rain (more than two to three events), can reduce soilborne inoculum carryover only if summer weeds are quickly controlled. If summer weeds are valued as stock feed, prioritise removing weeds from paddocks which will be sown to cereals, particularly barley, in 2020. Where possible, plant a break crop such as canola or a pulse; although still slightly affected by Rhizoctonia, these have all been shown to reduce inoculum overall. If a cereal must be sown into a high inoculum situation, wheat is likely to suffer less yield loss than barley; although both are susceptible and likely to increase inoculum for the following season. The single most important action to limit yield loss where a cereal is planted into high inoculum, is to prioritise planting these paddocks as early as possible while the soil is warm. Liquid streaming fungicides, particularly as split applications on and in-furrow, can also be effective, but generally only under higher yield potential situations.

Crops that establish well can still be affected by Rhizoctonia in mid-winter when root growth is slowed due to low soil temperatures (<10°C). In this situation, Rhizoctonia will infect the crown roots, causing reduced tiller number and height. Infection can then spread down the soil profile in spring resulting in reduced root mass to fill grain.

To reduce root infection, growers should consider using seed treatments or liquid streaming fungicide below the seed to help protect the seminal roots and consider liquid-streaming fungicide above the seed to reduce crown root infection. Roots are only protected inside the fungicide diffusion zone. Increased seeding rate can help compensate for loss of tillers.

The risk will be reduced if multiple rain events, each >20mm, fall during summer and early autumn, and weeds are controlled.

Crown rot was a big problem for cereal crops which had acceptable rainfall early in 2019 but had little rainfall during grain filling. Low rainfall at the start of the season in many areas meant that infection with crown rot has been lower than expected in those crops, and expression of crown rot has been limited. The low rainfall in 2019 also meant that breakdown of infested cereal residues will have been very slow, with inoculum levels after non-cereals higher than expected in 2019. It will be particularly important to know the crown rot risk (using the PREDICTA^®B service) prior to making the decision to sow very-susceptible cereal crops, such as durum wheat, in 2020.

Eyespot was less of a problem in most crops in 2019 due to low rainfall. There were some exceptions to this, where eyespot expression was much higher than expected given the low rainfall. Crops affected in this way seem to have had higher loads of infested stubble from previous crops. This suggests that the infested stubble has been wetted up by small rainfall events, producing a very humid environment at the base of the new crop, allowing higher than expected levels of spore production and infection.

Net form net blotch

The major concern coming out of 2019 is the rapid spread of resistance to fungicides observed in NFNB and wheat powdery mildew.

The large area sown to Spartacus and Compass in SA has seen virulence on these varieties increase in the past three years. This is particularly the case with Spartacus. Testing of specific samples collected from the YP by SARDI in 2019 has shown that a proportion of the pathogen population is now highly virulent on this variety. This is reflected in the lower rating provided in the 2020 Cereal variety disease guide.

The growing of barley in infected barley stubbles from the same variety, will have greatly sped up this natural evolutionary process. The use of fungicides may have helped protect barley crops up to this point. However, it is now apparent that the same evolutionary processes have led to the development of resistance to succinate dehydrogenase inhibitor (SDHI) products, including Systiva, and some DMI products including tebuconazole, which was the marker active ingredient (a.i.) in the tests. A limited survey conducted by Fran Lopez and SARDI across the YP suggests that the SDHI resistance is currently focussed on the mid to lower YP whilst the DMI resistance is likely to be much more widespread across SA.

There has been a suggestion in the literature that resistance to strobilurins is unlikely to develop in future in the net blotches and rusts, owing to the resistance mutation site being linked to a lethal gene. However, it is now apparent that this could be wishful thinking and that, given sufficiently high selection pressure, resistance to strobilurins could indeed occur, albeit at a slower rate.

Measures to reduce the new increased risks from NFNB must involve reducing inoculum levels by avoiding the sowing of susceptible barleys in high risk situations, disrupting pathogen selection processes by mixing up variety resistances in the landscape and rotating and mixing fungicides within crops.

The results of adult plant testing of NFNB isolates collected in SA in 2018 and 2019 respectively are shown in Tables 1 and 2. The results show how diverse the NFNB population is and, when compared across a number of years, how the pathogen shifts in response to the varieties being grown. For example, most isolates collected in the past two years have shown low virulence on Fleet and Maritime; varieties that had previously been very susceptible. Instead we have seen a gradual increase in virulence on Compass and Spartacus. RGT Planet is shown to be very susceptible in the SE and the Lower Eyre Peninsula (EP), but still moderately resistant in the mid and lower rainfall regions of the state. These results, along with others from previous years, have provided advanced warning of the potential susceptibility of new varieties and advanced breeding lines. The results also show how the older varieties; Clipper, Schooner, Sloop SA and Scope, have remained stable over a long period of time, indicating the durability of their resistance. The new variety Banks is seen to be mostly resistant but could become susceptible if grown widely as revealed by an isolate from Conmurra in 2018. Rosalind on the other hand shows promise as a potentially durable, resistant variety.

The isolates in Table 2 were all collected on the Lower YP. The first five were collected from paddocks where Spartacus was grown into Spartacus stubbles and where Systiva and foliar fungicides had been used. It is notable that these YP isolates are all highly virulent on Spartacus, and with the exception of the isolates from Urania and Pine Point, were generally less virulent on other varieties with the exception of Commander.

Table 1. Results of adult plant tests with NFNB isolates collected in 2018.

n.b. 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 NFNB isolates collected in 2019 on the Yorke Peninsula.

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

Powdery mildew in wheat

Powdery mildew disease has become a regular problem in the northern part of the YP, particularly around Bute. Close rotations with the very susceptible varieties, Scepter and Chief CL Plus, are largely responsible for this situation. Frequent use of fungicides to manage this disease as well as preventative sprays for rusts and Septoria have now resulted in the mildew population developing resistance to strobilurins and some DMI products. A limited survey conducted in the area by Fran Lopez from Curtin University revealed a high level of resistance to both strobilurins and tebuconazole in several paddocks.

This situation can only be managed by reducing the area sown to these, and related, susceptible varieties, avoiding sowing into stubbles infested with the mildew fruiting bodies and by using fungicides in a more strategic manner. Because the fungicides are also being used to manage other diseases, care will need to be taken in selecting varieties that are also not too susceptible to them. Or, if the varieties are susceptible, then only grow them over a limited area and select another, different variety to provide diversity across the landscape. It is landscape and temporal diversity in varieties and treatments that will provide the most sustainable way forward.

Acknowledgements

The research undertaken as part of this project 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.

Useful resources

Cereal Seed Treatments 2020 - SARDI factsheet

Cereal variety disease guide 2020 - to be released just prior to the Adelaide GRDC update.

Contact details

Hugh Wallwork,
Plant Research Centre, Hartley Grove, Urrbrae, South Australia 5064
0427 001568
hugh.wallwork@sa.gov.au