In a new finding relevant to barley growers in higher-rainfall zones, research has demonstrated the effectiveness of using a fungicide as foliar spray at ear emergence to flowering stage to control seed infection by the fungal disease Loose smut.

Previously, the only proven control method for Loose smut had been the use of seed dressings prior to sowing - a method that attempts to kill the pathogen contained in the seed at the start of the season.

The new research trials demonstrated the effectiveness of tackling Loose smut in-season to reduce development of seed infection, at the time when airborne spores are being dispersed and have the potential to infect flowering plants.Fungicide as foliar spray at ear emergence to flowering stage has been found in trials to help control loose smut in barley.

Exploring in-season options

Researchers have stressed that, while the findings have exciting implications, tebuconazole products - such as those used in the trials - are no longer recommended for stand-alone use for managing leaf diseases of barley, given implications for fungicide resistance.

This is due to the development in recent years of genotypes highly resistant to tebuconazole in diseases of barley.

But the use of tebuconazole in-season may be acceptable if it is part of a spray program where consideration is given to the mix and rotation of modes of action, and to the presence of fungicide resistance in the area.

Growers should also be mindful of restrictions relating to withholding periods relating to in-season fungicide application.

Potential costs

Loose smut is estimated to cost the industry millions of dollars in yield losses every year, with evidence that - in some susceptible varieties - yield loss is equivalent to the percentage of infected plants.

WA Department of Primary Industries and Regional Development (DPIRD) research scientist Kithsiri Jayasena, who has been managing the research, says the rule of thumb is that a one per cent infection rate usually equates to a one per cent yield loss.

"With heavily infected crops, this then translates into a significant yield loss at harvest time," he says.

Dr Jayasena says seed dressing efficacy is often variable and seeds are asymptomatic of the disease.

"It is impossible to tell if your seed has the pathogen just by looking at the seed.

"One of the only ways to be sure your seed has very low levels of disease is to have it tested in a laboratory or to purchase certified seed."

Fungicide as foliar spray at ear emergence to flowering stage has been found in trials to help control loose smut in barley. Photo: Natalie Lee

Barley varieties with a Dash parentage, including Hindmarsh (PBR), La Trobe (PBR), Spartacus CL (PBR) and Rosalind (PBR), have all been shown to be more susceptible to the disease.

So, Dr Jayasena says planting less susceptible varieties is another proven method of reducing the impact of Loose smut on yields.

Know the opponent

Dr Jayasena says understanding the behaviour of the pathogen is critical to developing effective and alternative control methods.

Measuring the airborne spore count at the trial site and in surrounding regions - to determine the period of time when Loose smut is airborne - was an important part of the research project.

"The critical time with this disease has been at ear emergence and flowering time, when the infected plants produce smutted heads consisting of several million spores encased in a fragile membrane," Dr Jayasena says.

"It is really only at this stage that symptoms become obvious, and growers will be able to see the infection when the leaf sheath begins opening-up and the boot is semi-open."

The rule of thumb is that a one per cent infection rate usually equates to a one per cent yield loss.

If the plant carries infection from the seed, the fungus will sit where the seeds are forming. Spores will be released at the heading stage - being dispersed by wind to nearby flowering heads.

The spores germinate in the florets, spreading to the embryo and surrounding tissue, surviving dormant until the seed is sown.

"This is why we decided to find a way to tackle this pathogen at its peak - during the ear emergence to flowering stage - because this is when the damage to the crop is done and when the disease spreads," Dr Jayasena says.

The research

Trials on the property of Mark and Robyn Slattery, at Kojaneerup South, in 2017 and 2018 tested the efficacy of an in-season foliar fungicide with the active ingredient tebuconazole (Orius® 430SC) for Loose smut control.

Tebuconazole is registered for use in WA as a seed dressing (for example, Raxil T) for Loose smut control, and for in-season control of other fungal diseases of barley - such as Powdery mildew and Scald.

Untreated certified Hindmarsh (PBR) seed was grown in replicated trials and surrounded by a Hindmarsh (PBR) crop with a known high level of seed infection.

Foliar fungicide treatments were applied to the Hindmarsh (PBR) trial to control infection arising from spores coming from the surrounding infected crop.

In 2017, the embryo infection (determined by laboratory testing) on the unprotected crop was 4.3 per cent. In 2018, the infection was one per cent.

Dr Jayasena says that fungicide sprays during ear emergence reduced the Loose smut embryo infection by 84 per cent in both 2017 and 2018, and 87 per cent during flowering stage in 2018, compared to the unprotected treatments.

This showed this method of control showed promise as an option to fight the damaging pathogen.

Mark says while smut in his barley crops may have posed a problem in the past, he is now extremely vigilant in managing his crops - particularly those earmarked for seed. Barley intended for seed will always go into his cleanest paddocks.

"We treat our seed every year, and I would think most growers in this region would also do the same," he says.

"But seed dressings do not always give 100 per cent protection. Once you see the smutted heads every four or five metres in the crop, you really need to get new seed in for the next crop."

Mark says experience has taught him that once even one per cent of the crop is infected, the battle is lost with that particular seed source.

"Seed dressings will not stop it at that level and once you see it in the crop, even at that small rate of one per cent, you are going to lose the battle eventually," he says.

Mark has now moved away from the more susceptible varieties, such as Hindmarsh (PBR) and La Trobe (PBR), and this year will plant Flinders (PBR) and RGT Planet (PBR).

He is also looking at the new shorter-season malting variety Maximus CL (PBR) for future seasons to replace the more-susceptible, shorter-season feed variety Rosalind (PBR).

For Mark and Robyn, and other growers in higher-rainfall regions, Dr Jayasena's findings give them hope that alternative management options will become available that will help them fight Loose smut.

"It will allow us to launch another attack when the disease is at its most vulnerable," Mark says.

"It will also give us another layer of security for our seed paddocks to really make sure the seed going into next year has that double protection shield."

More information: Dr Kithsiri Jayasena, kithsiri.jayasena@agric.wa.gov.au