Beware of rust with return to 'normal' seasons by Eammon Conaghan

Critical period for containing stripe rust until new resistant varieties are released.

CROP INTENSIFICATION, new rust strains and some favourable (wetter) seasons in the past 10 years have increased the frequency of rust diseases in WA crops.

In 1992, an eastern strain of leaf rust ventured west, where it proved itself a troublemaker, with several major outbreaks cutting yield by up to 37 per cent. But wise variety selection, including CarnamahPBR logo and CalingiriPBR logo, helped contain the problem.

However, on leaf rust's 10th anniversary, stripe rust crashed the party and WA's leading varieties are now faltering under the stress. But according to Department of Agriculture pathologist Robert Loughman, reinforcements are on the way.

"The Cereal Rust Control Program at Cobbitty, NSW, tested and designated the WA pathotype as 134 E16 A+, which doesn't occur in eastern Australia, and so local breeders have a new challenge within Australia told the recent Perth Agribusiness Crop Updates.

"The origin of this strain, which halved yields in areas where drought didn't restrict it, remains unclear, with candidate regions of origin including America, west Asia and east Africa."

WA's new stripe rust delicately out-manoeuvred some of the pre-emptive breeding by the Department and so foiled most genetic barriers in existing varieties like CalingiriPBR logo and WestoniaPBR logo .

Strip rust defences

Most, that is, except CammPBR logo which, with its defiant Yr17 gene, offers the greatest current resistance. But the outlook is promising, with CammPBR logo and partially-resistant varieties such as WyalkatchemPBR logo set to play a role in intermediate management strategies, while breeders generate new stripe rust defences.

"Breeding lines resistant to WA stripe rust are already present in the Crop Variety Testing program and shape as good prospects," Dr Loughman said.

"The best strategy is to incorporate multiple minor genetic resistances in future new varieties rather than rely on one major resistance gene. Screening early generations in disease nurseries should optimise the accumulation of those genes."

Meanwhile hopes of containing the disease's development in WA face a critical period up to the release of highly-resistant varieties.

"While the industry adjusts to stripe rust in the next five years, high levels of disease will put available resistance genes under pressure. Large rust populations that develop in epidemics increase the likelihood of rust mutation for increased virulence," Dr Loughman warned.

"Highly susceptible varieties rapidly develop the disease and produce vast quantities of spores which promote epidemics. These varieties must be removed from all areas of the wheatbelt as soon as possible."

Avoid varieties susceptible to stripe rust

Crop variety testing at Gnowangerup identified the most susceptible varieties to be WestoniaD , H45PBR logo , Bonnie Rock, CascadesPBR logo, Cadoux, Amery and BrooktonPBR logo .

Avoiding them will put less disease pressure on more resistant cultivars and help them perform better. It will also allow those resistant varieties to be grown with lower fungicide applications and moderate costs.

Fungicide management

Fungicide applied as seed dressings or in-furrow with fertiliser would mitigate disease risk and help control yield losses.

After early detection of stripe rust on a CalingiriPBR logo crop at early booting stage, different fungicides were assessed for their impact at four and seven weeks.

Untreated plots yielded 2.4 t/ha, while all fungicide treatments increased yield to around 3 t/ha. Folicular 430EW at 290 mL/ha saw yields climb to 3.3 t/ha. The best results were achieved at full rates.

Dr Loughman's message was clear. New breeding strategies, combined with promising genetic material, should topple WA's latest rust threat, but in the meantime growers must exercise great caution with variety selection and fungicide strategies.

Program 3 Contact: Dr Robert Loughman 08 9368 3691 email rloughman@agric.wa.gov.au