Pulse disease research update

Take home messages

• Selecting a more resistant variety will reduce grain yield losses caused by disease.
• The faba bean, PBA Amberley will require fungicide application to prevent grain yield losses.
• Fungicide strategies incorporating newer fungicide actives are providing equal or better disease control than older actives in faba bean and chickpea.
• Applying fungicides to prevent ascochyta blight in chickpea when grain yields are above 0.5t/ha resulted in an economic advantage in field experiments.
• Inter-row sowing into standing cereal stubble, compared to slashed stubble, is almost as effective at reducing chickpea grain yield losses due to ascochyta blight, as choosing a moderately susceptible variety compared to a susceptible variety.
• Numerous pathogens have been detected in pulse roots, and many Fusarium spp. and Phoma/Didymella spp. have been shown to cause root disease.

Background

Management of pulse diseases is essential to on-farm profitability, and this was especially the case during 2020. Consistent winter rainfall and foggy conditions provided conducive conditions for disease development in Victoria. In field experiments, 35% loss in grain yield was measured in faba bean at Lake Bolac (high rainfall) and 92% in the susceptible chickpea variety PBA Striker at Curyo (low rainfall), highlighting the importance of controlling pulse diseases in a range of environments.

The following report will provide an update on experiments investigating;

1. Effect of faba bean plant densities on disease severity,
2. Disease management options in chickpea and faba bean varieties utilising varietal resistance and fungicides,
3. The interaction of stubble height on chickpea ascochyta blight management, and
4. Soil-borne diseases in pulse crops.

Faba bean disease management

Sowing rate

Chocolate spot in faba bean is a large threat to yield, particularly in high rainfall areas. Chocolate spot disease development can occur at most growing season temperatures, but disease development is quickest when canopy humidity is high (greater than 70%) and temperatures are warm (15-25°C). These environmental conditions can differ between crops, depending on the prevailing weather but also the canopy density.

Field experiments at Lake Bolac and Lake Linlithgow showed that chocolate spot severity increased in faba bean plots as target plant densities increased from 5 to 45 plants per square metre, with similar results observed at both sites (Figure 1). The variety PBA BendocA (rated moderately susceptible (MS)) had greater disease severity in all plant densities, compared to PBA Amberley (provisionally rated moderately resistant (MR)). Additionally, PBA Amberley had over 60% disease severity in high density (45 plants/m²) plots, equivalent to PBA Bendoc at a low density of 5-15 plant/m². This highlights that the ideal conditions for disease will still put pressure on the moderate resistance in PBA Amberley and result in grain yield losses. The chocolate spot resistance rating of faba bean is reviewed annually, so it is important to always check for up to date disease ratings.

Standing stubble can reduce ascochyta blight

In the last several years it has been observed that when planting chickpea into cereal stubble rows, ascochyta blight spread was confined to the row. Adjacent rows often remained disease free.

The effect of inter-row sowing chickpea into standing (30cm tall) versus slashed cereal stubble, with two chickpea varieties (PBA Striker (S) and Genesis 090 (MS)), and a complete disease control (no disease/full fungicide) or untreated (diseased/no fungicide) plots was investigated. To inoculate the disease plots, six ascochyta infested stubble pieces (10cm length) were pinned in the geometric centre of plots. The ascochyta blight severity and grain yield were measured.

Inter-row sowing into standing cereal stubble resulted in 31% yield loss due to ascochyta blight compared to 55% in slashed stubble (Table 9). In this experiment, the magnitude of the yield increase due to standing stubble was almost as much as changing from a susceptible variety to a moderately susceptible variety (Table 10).

These results highlight the importance of not only selecting more resistant varieties but combining this with good agronomy practices.

Table 9. Grain yield of chickpea inter-row sown into standing versus slashed cereal stubble with and without disease at Dooen during 2020. There was a significant interaction (P=0.001) between stubble and fungicide. Different letters indicate a significant difference in a pairwise analysis between fungicide and stubble.

Table 10. Grain yield of two chickpea varieties with and without disease at Dooen during 2020. There was a significant interaction (P<0.001) between variety and fungicide. Different letters indicate a significant difference in a pairwise analysis between fungicide and stubble.

Soil-borne disease

Growers and agronomists have reported patches of poor performing crops and, in some situations, crop failures. Preliminary investigations into these poor performing areas identified that soil-borne disease may be the cause. In response to these concerns, a national soil-borne disease project investigated the presence of soil-borne pathogens (pathogens cause disease) on pulse crops.

Detecting pathogens in pulse roots

Throughout Australia, root samples of poor performing pulse crops were sent to local pathologists for visual assessment and then to SARDI for molecular identification of pathogens present. In 2020, 533 samples were processed, root health was scored and photographed, and molecular techniques were used to determine the pathogens present in pulse crops.

The most common pathogens detected using qPCR were Pythium spp., Pratylenchus spp. (root lesion nematodes), Rhizoctonia solani AG8, and Phoma pinodella. Pythium and Pratylenchus spp. are known to have broad host ranges. R. solani AG8 prefers cereals but will infect a broad range of plant types. Phoma pinodella along with Didymella pinodes causes blackspot of field pea, but it has a much broader host range. These pathogens are very often present together in the one root system (‘disease complex’).

There were also infrequent detections of Aphanomyces and Phytophthora genera. These genera have been reported to cause severe and widespread yield losses in pulses in Europe and North America. During 2020, Aphanomyces euteiches was detected in six faba/broad bean samples from South Australia (SA) and New South Wales (NSW) and one lentil sample from Victoria; the collecting agronomists reported significant yield loss in many of these paddocks.

Phytophthora medicaginis, a known problem in northern NSW, was detected in 26 (25 chickpea, 1 faba bean) samples from northern NSW; P. megasperma was detected in 33 samples (multiple crop types) across Australia, and P. drechsleri (tentative identification), was detected in 14 samples, mostly lupins from Western Australia (WA); this species was also detected in SA, Victoria and southern NSW. SARDI is currently undertaking work to confirm the identity of this species.

Figure 2. Frequency of detection over threshold levels of pathogens using molecular tools in pulse samples received nationally during 2020.

Confirming the cause of disease

To confirm the role of specific organisms in causing root diseases in pulses, the pathogens were isolated from the diseased pulse samples and then inoculated onto soil, in which various pulses were grown. Pathogens which re-infect the seedlings are termed pathogenic. This study aimed to identify and confirm pathogenicity of isolates that are likely to be the main causes of symptoms observed in growers’ paddocks.

Fusarium avenaceum isolates were highly pathogenic on all crops tested, with just one strain appearing non-pathogenic (Table 11). Fusarium oxysporum and F. redolens isolates’ pathogenicity varied between crops, but all isolates of both species were highly pathogenic on chickpea. Fusarium tricinctum isolates were highly pathogenic on chickpea and moderately pathogenic on faba bean, lentil and lupin. Phoma. pinodella isolates were highly pathogenic on chickpea, field pea and lentil, but less so on faba bean and lupin (Table 12).

Further work will investigate yield losses and control options to ensure growers have management options to reduce grain yield losses due to soil-borne diseases.

Table 11. Frequency of detection of Fusarium spp. in 2019 survey samples and pathogenicity (ability of the pathogen to infect plant) of representative isolates collected from 2018-2019 crop samples. Pathogenicity on each crop is indicated as “-“ non-pathogenic, “+” weakly pathogenic, “++” moderately pathogenic or “+++” highly pathogenic.

Table 12. Frequency of detection of Phoma spp. in 2019 survey samples and pathogenicity (ability of the pathogen to infect plant) of P. pinodella isolates collected from samples 2018-2019 toward common pulse crops in a controlled environment assay. Pathogenicity is indicated as either non-pathogenic “-“, weakly pathogenic “+”, moderately pathogenic “++” or highly pathogenic “+++”.

Conclusion

Without a disease management plan that incorporates varietal resistance, paddock rotations, good agronomy practices and fungicides, grain yield losses of greater than 90% can be experienced. The results from these projects show the importance of varietal resistance in both chickpea and faba bean to minimise grain yield losses. The importance of fungicides, both type and timing of application play a critical role in disease control. However, the importance of good agronomy cannot be understated with ensuring that disease management plans incorporate planting densities and orientation of stubble (standing versus slashed). Inter-row sowing chickpea into standing cereal stubble was shown to be almost as effective as changing resistance classification of the variety from S to MS as compared to sowing into slashed stubble.

Investigations into soil-borne diseases in pulses are still continuing, with both Fusarium spp. and Phoma/Didymella spp. identified as pathogens of significance. Following these investigations, control methods will be investigated.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. This work is supported by both the GRDC and the Victorian Government (Agriculture Victoria) through the GRDC projects; DJP1097-001RTX, DAV00150, DJP1907-002RMX, DJP1907-004RTX.

Appreciation is given to the Agriculture Victoria Southern Pulse Agronomy, Field Crops Pathology, Southern Farming Systems and SARDI research teams and the growers and agronomists who have assisted with these experiments.

Useful resources

Pulse disease guide

Victorian Crop sowing guide

Crop Protection Products including Minor Use Permits, can be viewed at the Australian Pesticides and Veterinary Medicines Authority (APVMA) website

Contact details

Joshua Fanning
110 Natimuk Road, Horsham Vic 3400
03 5344 3335
Joshua.fanning@agriculture.vic.gov.au
@FanningJosh_