Cereal disease update and risks for southern NSW crops in 2016

2015 cereal disease overview

The 2015 season presented many challenges for growers in SNSW and their advisers. The early sowing and wet winter created ideal conditions for early infection and development of diseases in crops. The dry spring helped to reduce further disease development of necrotrophs such as yellow leaf spot in wheat and scald in barley. For the rust pathogens these were mainly well controlled.

Root diseases were a common feature during 2015. Areas which suffered extended periods of water logging were prone to damage by Pythium and in some cases this was combined with evidence of root pruning caused by aluminium toxicity. The conditions were also ideal for infection of crown rot and take-all to occur during the winter months which when followed by the dry spring provided ideal conditions for crown rot to build up and cause losses across a wide area.

Wheat foliar diseases

Septoria tritici blotch (STB)

STB caused by Zymoseptoria tritici has not been common in southern NSW for more than a decade. However, in the high rainfall regions of Victoria, South Australia and Tasmania it has again become a significant disease. Its re-emergence in these regions poses a threat to southern NSW early sown high rainfall areas. This is because of changes detected in virulence to variety resistance and the discovery of fungicide resistance developing in the Australian population of this pathogen. STB has a long spore dispersal mechanism, this means that ascospores released from stubble during autumn in Victoria could be blown onto and infect emerging wheat crops in southern NSW.

Fungicide resistance alert - The triazole fungicide resistance status of Zymoseptoria tritici in Australia has been further confirmed in additional isolates collected from Victoria, South Australia and Tasmania during 2015. We have also confirmed the presence of isolates in NSW containing the resistance mutations in the Cyp51 gene. The levels of resistance detected thus far by our screening show that most fungicides will still be effective in the paddock. However, to avoid further losses in efficacy growers are encouraged to use multiple strategies against this disease. Further information and factsheets on managing STB and fungicide resistance are listed at the end of this paper.

The risk of widespread infection and subsequent crop losses is relatively low for the low to medium rainfall areas of southern NSW. However, the higher rainfall areas have weather conditions which are conducive for STB to become re-established. Growers are advised to be alert to symptoms in these areas and instigate an integrated approach that incorporates crop rotation/stubble management, variety selection and fungicides to provide effective control of STB.

It is critical that growers adopt strategies to reduce the selection rate of further mutations and hence extend the useful life of currently available fungicides. One strategy which is expected to slow the selection of more resistant strains is to mix or alternate different azoles. This is because not all azole fungicides are affected equally by mutations of the STB fungus. Products that combine azoles such as Tilt® Xtra (Propiconazole and cyproconazole) or Impact Topguard® (tebuconazole and flutriafol) which have a registration for STB could be used in this way. Equally, in crops where two fungicide applications are to occur (e.g. at Z31 and Z39) the same active should not be used at both applications. Growers MUST always follow label guidelines and ensure maximum residue limits are adhered to at all times.

In Australia, there are limited choices of fungicides with different modes of action for use on wheat. A number of products combine a strobilurin with an azole and these may provide some benefits in delaying or reducing the risk of resistance development. However, the strobilurins on their own are considered to be at high risk of developing resistance due to their single site mode of action. In some countries, resistance to strobilurins is so widespread in the STB population they are no longer recommended as effective, even in mixtures.

In addition to mixing or rotation of fungicides an integrated approach to disease control that includes crop rotation and avoidance of susceptible cultivars will reduce inoculum loads and therefore reduce the likelihood of resistance to fungicides developing.

Yellow leaf spot

Yellow leaf spot is the most commonly occurring foliar disease of wheat in southern NSW. This disease can cause concern for growers in the seedling stages. It is of particular concern when growing susceptible varieties sown into previously infected wheat stubble due to the apparent severity of symptoms early in the season. However, fungicide trials have been inconclusive about the benefit of spray timings prior to GS31. Our recent trials have shown that under high disease pressure small yield improvements can be gained with an early spray at the five leaf stage. However, larger gains in yield were observed by implementing spray timings of GS31 + GS39. Importantly this was only observed in the susceptible varieties. For varieties with resistance ratings of MR-MS there was no benefit of using two fungicide sprays even under high disease pressure. For detailed trial summaries see the NSW DPI southern NSW trials booklet 2016.

Barley foliar diseases

Scald

The incidence of barley scald was higher in southern NSW during 2015 than previous years. The dominance of Hindmarsh in southern NSW has led to changes in the virulence of scald isolates. Over the past two years in the NSW DPI Wagga scald nursery we have observed significant changes in the reactions of a number of varieties such as Hindmarsh, La Trobe and SY Rattler becoming more susceptible. In 2015 a number of naturally infected paddocks in southern NSW were surveyed and the isolates tested for virulence on 40 varieties. These tests have confirmed that virulence at the seedling stage exists in southern NSW for all available varieties. The pathogen that causes scald Rhynchosporium commune is genetically very diverse and capable of developing virulence rapidly to host resistance. Given our recent findings on the virulence diversity we recommend that all barley varieties be monitored for infection, sowing barley on barley should be avoided and if infection occurs consider fungicide applications to reduce potential yield losses. Dryland yield trials in 2015 demonstrated that in susceptible varieties yield losses of up to 50 per cent are possible. For detailed trial summaries see the NSW DPI southern NSW trials booklet 2016.

Root and crown diseases

Crown rot and take-all are common in the southern NSW farming system and their build up was favoured in 2015. Growers are advised to assess the risk of severe infection in 2016 by monitoring the build up of inoculum in paddocks with frequent winter cereal cropping rotations. The pre-sowing paddock surveys in 2015 for root diseases showed that 80 per cent of the 35 paddocks sampled had crown rot present while 95 per cent had take-all present. The level of inoculum in these paddocks ranged considerably from low to high risk. The post-harvest results will be presented and discussed at the 2016 Wagga Wagga GRDC Grains Research Update. Yield loss to crown rot was assessed in five barley and 12 bread wheat varieties. The losses in 2015 ranged from 1.5-5 per cent in the barley and 2-10 per cent in the wheat while the one durum line tested lost 20 per cent yield. For detailed trial summaries see the NSW DPI southern NSW trials booklet 2016.

Cereal viruses

Barley yellow dwarf virus and Cereal yellow dwarf virus were a feature of 2014 in southern NSW and were anticipated to be widespread in 2015 as well. The levels of virus infection appeared not to extend into the later sown crops in 2015 but were still present in the long season growing areas. We conducted three trials in conjunction with the NSW DPI Crop evaluation unit (Peter Matthews) and tested a range of varieties with and without upfront seed treatment to control aphids. Only one trial at Galong received significant incidence of virus infection and yield differences between the treatments for the varieties Scenario and SQP Revenue. In this single trial, for every one per cent increase in infected plants there was a yield loss of 0.6 per cent. This relationship is for low levels of infection up to 15 per cent. These yield losses were observed when there was no apparent dwarfing of plants, some yellowing was observed but not at high levels. The yield loss was avoided with the use of up front seed treatment including an insecticide. For detailed trial summaries see the NSW DPI southern NSW trials booklet 2016.