Managing red leather leaf disease and preventing weather damage in oats

Take home messages

• Red leather leaf (RLL) is a common foliar disease that causes yield losses in milling and hay oats in the medium and high rainfall zones of Victoria.
• Avoiding planting of very susceptible rated varieties significantly reduces losses.
• Foliar fungicide applied at mid tillering (Z25) and early stem elongation (Z31) provided the best suppression of RLL.
• Foliar fungicide application also provided suppression of fungal saprophytes and reduced hay discolouration post hay cutting.

Introduction

Red leather leaf (RLL) is a common seed and stubble-borne foliar disease of oats in south-eastern Australia. It is favoured by cool, wet conditions. Growers should avoid sowing oats into paddocks with oat stubble from previous years as this is the main source of infection. RLL can also be seed-borne, so it is important to monitor all crops and use seed from clean sources. The majority of varieties are susceptible to RLL, especially the commonly grown varieties such as Yallara, Mitika and Wintaroo. Some varieties such as Bannister, Kowari and Forester have better resistance, and therefore, their use reduces the risk of yield loss from disease.

Surveys (funded by AgriFutures Australia) during 2018 and 2019 determined that RLL was present in approximately 90% of oat crops in the medium and high rainfall zones of Victoria. Severity was greater with higher winter rainfall. Red leather leaf was less common and had low severity in the low rainfall Mallee environment and was unlikely to have caused a yield loss.

No fungicides are registered for control of RLL in oats, however products are registered for other diseases in oats and can provide suppression of RLL. Some of these products can also provide prolonged green leaf retention and suppression of saprophytic fungi that colonise hay post cutting. It is important to always follow the label instructions for the other diseases including the relevant withholding periods for harvest and cutting for stock food.

Experiments were conducted to determine the grain and hay yield losses due to RLL in different varieties and different environments; to determine the best timing for foliar fungicides for the suppression of RLL and suppression of saprophytic fungi post hay cutting.

Grain yield loss

Experiments conducted at Horsham, Victoria in 2019 demonstrated that RLL caused up to 13% (0.5t/ha) grain yield loss in susceptible varieties (Table 1). Yallara was the worst affected, while Mitika and Williams also recorded reductions in grain yield. Growers should grow moderately susceptible (MS) or better rated varieties to reduce losses from RLL. Most of the RLL infection occurred on the flag minus two leaves and lower in the canopy. Greater losses are possible during wet weather conditions that favour greater infection on the top two leaves.

Table 1. Red leather leaf severity and grain yield of six milling oat varieties in response to disease and fungicide treatments near Horsham during 2019.

General analysis of variance for a completely randomised design was used for analysis.*= statistically significant at P < 0.05; ns= not statistically significant. Maximum Disease - Spread 1kg infected stubble, no fungicides; Minimum Disease = no stubble infection, propiconazole at Z25, Z31 and Z39. #rating = moderately resistant (MR), moderately susceptible (MS), moderately resistant – moderately susceptible (MRMS), moderately susceptible – susceptible (MSS), susceptible (S), susceptible – very susceptible (SVS), very susceptible (VS), taken from the Victorian Cereal Disease Guide.

Hay yield loss

At Horsham, RLL severity varied significantly between varieties (Table 2). The worst affected were the susceptible rated varieties Mulgara, Yallara and Wintaroo. Stem thickness and plant height were also significantly reduced. There was no significant effect on hay yield (data not shown), this was most likely due to the dry spring conditions limiting infection later in the season.

Table 2. Impact of Red leather leaf on hay characteristics of six hay oat varieties in response to disease and fungicide treatments at Horsham during 2019.

General analysis of variance for a completely randomised design was used for analysis. ** = statistically significant at P < 0.001; *= statistically significant at P < 0.05; ns= not statistically significant. Maximum Disease – Spread 1kg infected stubble, no fungicides; Minimum Disease = no stubble infection, propiconazole at Z25, Z31 and Z39.

Table 3. Red leather leaf severity and hay yield of six hay varieties in response to disease and fungicide treatments at Inverleigh.

At Inverleigh, RLL infection was more severe compared to Horsham (Table 3) due to wetter seasonal conditions during winter and spring (Table 3). This resulted in significant hay yield loss of 0.5 – 1.0 t/ha in the susceptible rated varieties. Moderately susceptible or better rated varieties did not record significant reductions in hay yield. These findings demonstrate the benefits of avoiding growing oat varieties which are rated susceptible to RLL that to reduce risk of loss.

Fungicides

There are no fungicides registered for control of RLL, however there are products registered for other diseases in oats. We found that fungicides suppress RLL but do not provide complete control. Foliar fungicide applications at Z25 and Z31 were most effective as they coincide with early disease development, while application at Z39 can provide benefits during seasons with wet springs.

Hay discoloration

One experiment was conducted near Horsham to investigate the efficacy of repeat applications of propiconazole compared to propiconazole followed by (fb.) pyraclostrobin plus epoxiconazole on the suppression of saprophytic colonisation and mould growth on cut hay. Seasonal conditions were dry and the first observation of saprophyte growth was 40 days after hay cutting. Application of propiconazole and propiconazole fb. pyraclostrobin plus epoxiconazole significantly reduced saprophytic fungal growth and mould colonies (Table 4).

Table 4. Visual estimate of percentage colonisation by saprophytes and mould colony units on oaten hay 40 days after cutting.

To avoid problems with maximum residue limits, growers are requested to adhere recommended withholding periods following application of respective fungicides.

Useful resources

Cereal disease guide

Field Crop Diseases Victoria website: https://extensionaus.com.au/FieldCropDiseasesVic/

National Variety Trial (NVT) website

Victorian crop sowing guide

Acknowledgements

This research was made possible with investment from the Grains Research and Development Corporation (GRDC), the Victorian Government and AgriFutures Australia. Thanks to Agriculture Victoria’s field crop pathology team. Thanks to the Foundation for Arable Research for field trials at Inverleigh.

Contact details

Mark McLean
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 4344 3111
mark.s.mclean@agriculture.vic.gov.au
@msmclean777

Hari Dadu
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 43443111
Hari.dadu@ecodev.vic.gov.au
@lmharidadu