Australian grain growers know all too well that there's nothing like years of drought to test resilience, fortitude and adaptability.
But in recent years, one organism has proven particularly adept at surviving the low rainfall conditions - the fungal disease Ascochyta blight.
What's out there
Experts are warning of a potential widespread outbreak of Ascochyta blight in chickpea crops across New South Wales and Queensland if forecasts of a wet winter materialise - due to higher than expected levels of the disease inoculum in some paddocks.
Ascochyta inoculum for 2020 crops may have originated as far back as 2017 or 2016, with drought slowing the breakdown of plant residues and, therefore, inoculum not necessarily decreasing as expected.
In a recently released GRDC podcast, NSW Department of Primary Industries (DPI) Senior Plant Pathologist, Dr Kevin Moore, urges growers to closely monitor any crops planted into chickpea residue.
"Testing has shown that even though Ascochyta inoculum hasn't increased over the past three seasons, it hasn't decreased much either because the dry conditions have slowed the breakdown of plant tissues," Dr Moore says.
"Disease inoculum for 2020 chickpea crops may be more than three years old, so if growers can see intact chickpea stubble, they should assume that Ascochyta is viable.
"NSW DPI has also released a detailed fact sheet outlining management recommendations relating to paddock and variety selection, disease risk assessment, seed testing, crop monitoring and fungicide strategies.
The Ascochyta pathogen survives and spreads in infected seed, stubble and on volunteer chickpea plants.
Under ideal conditions, it can reproduce in as little as five to seven days and is readily spread by:
- raindrop splash;
- surface water flows;
- machinery; and
Dr Moore says it is critical for growers to determine the risk level of individual paddocks and implement appropriate management strategies.
High risk paddocks for Ascochyta blight are those:
- where there's viable Ascochyta inoculum;
- that have been planted to chickpeas in the past three years;
- seed that is of unknown Ascochyta status; or
- seed that hasn't been properly treated with a registered seed dressing.
Dr Moore recommends growers in high risk situations should apply a preventative fungicide before the post-emergent rain event.
"Those who are planning to plant into a high risk paddock should ensure they are growing a variety with the highest level of Ascochyta resistance suitable to the area, test seed for germination and vigour and treat all seed with a properly-applied, registered fungicide," he says.
In comparison, Dr Moore says low risk paddocks are those where:
- no infected chickpea residue has entered the paddock - via a dirty header which has previously harvested chickpeas, wind or surface water movement;
- no chickpeas have been grown for three years (after 2016);
- chickpeas weren't grown in adjacent paddocks in 2019;
- seed has been tested for pathogens with nil detected; and
- seed has been properly treated with fungicide dressing.
"Growers planting into low risk paddocks should regularly monitor the crop throughout the season, especially 10-14 days after rain, and only apply foliar fungicide if Ascochyta is detected," he says.
Disease prevention is important during the reproductive stage, as the disease on pods causes seed abortion, seed infection and seed defects - and may render the seed unsuitable for planting the following season.
Preventative fungicides include active ingredients, such as chlorothalonil and mancozeb, which provide excellent protection when applied before rain with good coverage and high water volumes as per label recommendations.
While post-infection or salvage applications should not be considered part of a grower's standard Ascochyta management program, Dr Moore says there are two new chemical options with preventative and curative activity which could be useful in situations where a spray is missed.
"This new chemistry, which includes products such as Aviator® Xpro® (prothioconazole + bixafen) and Veritas® (tebuconazole + azoxystrobin), are registered for use on chickpeas - but with very stringent conditions," he says.
"These fungicides are much more expensive than chlorothalonil and no more effective when applied as preventatives.
"However, they may be beneficial if a grower misses a spray, providing they're applied within 48 hours after the rain starts.
Additional information is available in Dr Moore's 2020 GRDC Update paper.
GRDC Research Code: DAN00213
More Information: Anne Brook, NSW DPI, 0477 358 305, firstname.lastname@example.org