Ascochyta risk remains in 2019

Ascochyta blight risk remains for pulse crops despite dry season


Weeds, Pests, Diseases
South Australian Research and Development Institute (SARDI) research scientist, Sara Blake, has warned that Ascochyta blight inoculum will still be present in stubble this season. PHOTO Alistair Lawson

South Australian Research and Development Institute (SARDI) research scientist, Sara Blake, has warned that Ascochyta blight inoculum will still be present in stubble this season. PHOTO Alistair Lawson

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Researchers call for southern region growers to remain vigilant against Ascochyta blight.

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Despite dry conditions in 2018, researchers are calling on pulse growers across the southern region to remain vigilant about Ascochyta blight.

Ascochyta blight inoculum will still be present in stubble, even in areas where below-average rainfall was received in 2018, according to South Australian Research and Development Institute (SARDI) research scientist Sara Blake.

Adding to researchers' calls for vigilance is the fact that lentil variety PBA Hurricane XT (PBR) is now rated moderately resistant to moderately susceptible (MRMS) for foliar Ascochyta blight in South Australia due to shifts in pathogen population, which is likely as a result of intensive planting of the variety.

Ms Blake says the Ascochyta lentis pathogen population is naturally variable and aggressive forms can be selected for over time in intensive cropping systems.

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"This has been seen with the intensive cropping of PBA Hurricane XT (PBR) on the Yorke Peninsula and in the Lower North in SA where crops have developed moderately susceptible leaf lesions over the period of 2015 to 2018," she said.

"Anecdotal evidence suggests that in 2018, PBA Hurricane XT (PBR) was the dominant cultivar representing up to 90 per cent of the lentils cropped on the Yorke Peninsula in SA.

"The downgrading of the foliar rating of PBA Hurricane XT (PBR) to MRMS in SA reveals that the longevity of this source of resistance is threatened by the current cropping practices in these regions."

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The newly released lentil cultivar PBA Hallmark XT (PBR) is also rated MRMS to foliar Ascochyta blight and appears to have the same source of resistance as PBA Hurricane XT (PBR).

"Therefore, the risk of Ascochyta blight developing in PBA Hallmark XT [Plant Breeders Rights] is increased if this cultivar is sown into, or near, paddocks that have recently grown PBA Hurricane XT (PBR)," she said.

"Both cultivars may require podding sprays if disease is present during the growing season.

"Growers are urged to diversify their cultivar and crop rotations and monitor crops regularly for symptoms of disease."

PBA Hurricane XT (PBR) has retained its moderately resistant (MR) rating against Ascochyta blight in Victoria due to the fact that the same shift in pathogen population towards more aggressive isolates observed in SA has not been observed interstate.

Growers in Victoria are less reliant on a single cultivar than their counterparts in SA and have longer rotations between their lentil crops, along with climatic factors such as cooler and drier conditions.

"Ascochyta lentis requires cool weather and humidity for infection to occur where frequent rainfall events and a high inoculum load in the form of infested lentil stubble can promote the likelihood of disease epidemics," she said.

"Therefore in Victoria, the lower inoculum load coupled with suboptimal climatic conditions for the disease to develop means that disease pressure for development of Ascochyta blight in lentil crops is lower than that seen in SA."

Growers are asked to consider cultivar rotation wherever possible in their farming system. - SARDI research scientist Sara Blake

Researchers test isolates of Ascochyta lentis in controlled environment conditions to help determine resistance ratings and if any changes to these ratings occur.

One notable finding from this testing in 2018 was that the shift in the pathogen population has also seen a decrease in the number of Ascochyta lentis isolates capable of infecting the variety Nipper (PBR), which once made up 30 per cent of lentil cropping in SA.

Nipper (PBR) is rated MRMS to foliar Ascochyta blight and is no longer as widely grown as it was in 2011, when it lost effective resistance to Ascochyta blight just four years after being commercialised.

"Less than 40 per cent of the isolates collected in 2017 and tested in 2018 were capable of infecting Nipper (PBR), whereas two years ago all the isolates tested were capable of infecting the cultivar," Ms Blake said.

"This suggests that cultivar rotation has a strong effect on aggressiveness in the pathogen population structure by selecting virulent isolates to that cultivar.

"Growers are asked to consider cultivar rotation wherever possible in their farming system."

Chickpeas

Ms Blake says all chickpea cultivars in the southern region remain rated moderately susceptible (MS) or susceptible to Ascochyta blight.

"Chickpea growers should carefully consider their risk of developing Ascochyta blight and ability to effectively control the disease prior to choosing to grow the crop in southern Australia," she said.

"Despite the dry season in 2018, there were still numerous reports of Ascochyta blight in chickpea crops, likely due to the aggressiveness of this pathogen."

"All chickpea varieties will require a thiram-based seed dressing prior to sowing. Susceptible varieties will need regular monitoring and may require regular fungicide sprays every two to three weeks throughout the growing season ahead of rain events."

Regular fungicide applications are vital for the health of chickpea crops, says SARDI's Sara Blake. PHOTO Alistair Lawson

Regular fungicide applications are vital for the health of chickpea crops, says SARDI's Sara Blake. PHOTO Alistair Lawson

"Moderately susceptible cultivars will generally require three to four strategic sprays ahead of rain events which will offer two to three weeks protection, starting six to eight weeks post sowing."

Additionally, pods of all commercial chickpea cultivars are susceptible to ascochyta blight and will require fungicide sprays ahead of rain fronts to protect pods from seed staining and seed abortion, Ms Blake says.

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Research conducted in 2018 by SARDI, a division of Primary Industries and Regions SA, in a strategic partnership with GRDC demonstrated how important it is for regular and timely fungicide applications on chickpea crops.

"Fortnightly sprays of chlorothalonil resulted in no disease symptoms in any of the plots," Ms Blake says.

"Strategic sprays of either chlorothalonil or Aviator XPro(r) significantly reduced disease severity below the nil treatments in all cases except for two advanced breeding lines."

"Disease severity in strategically sprayed plots was equivalent between chlorothalonil and Aviator XPro(r) plots per variety, except for Howzat where Aviator XPro(r) treated plots had significantly less disease compared to the chlorothalonil treated plots."

"This may indicate that in extreme epidemics, Aviator XPro(r) has improved efficacy over chlorothalonil however further testing is required to validate these results."

To best manage Ascochyta blight in chickpea, growers are urged to implement integrated disease management practices which includes a three to four-year break between crops of the same type, rotating cultivars and avoiding sowing in paddocks in close proximity to the previous years' crops.

GRDC Research Codes: CUR00023, DAS00139

More information: Sara Blake, 08 8429 2248, sara.blake@sa.gov.au

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