Management of diseases of canola - 2012

| Date: 11 Apr 2012


Kurt Lindbeck1, Stephen Marcroft2, Angela Van de Wouw3 and Vicki Elliott2
1Department of Primary Industries, Wagga Wagga; 2Marcroft Grains Pathology P/L, Horsham; 3School of Botany, The University of Melbourne.

Introduction

The large areas sown to canola in 2011 and the predicted increases in the area sown to canola in 2012 in southern NSW and northern Victoria will increase blackleg severity and increase the likelihood of yield losses. Growers and advisors should be aware that control of disease is an integral component of canola production and has to be considered at all points of crop management. The other important disease of canola, sclerotinia stem rot, also has the potential to cause yield loss, but requires very specific conditions for the pathogen to cause infection and yield loss.

Management of blackleg and the rotation of resistance sources

The blackleg fungus, Leptosphaeria maculans, has a high propensity to overcome resistance in Brassica napus (canola) cultivars. Major resistance breakdowns have been observed in regions including the Eyre Peninsula and Bordertown in South Australia. Experimental field trials in Victoria and South Australia have shown that canola cultivars with different resistance sources (e.g. different resistance genes or gene combinations) sown sequentially for three years into stubble from the previous year’s canola crop at two field sites resulted in decreased disease severity. In contrast, those canola cultivars with the same resistance source as the previous year’s canola stubble had high disease severity.
In order to be able to rotate canola cultivars the resistance of individual canola cultivars must be known. Two types of blackleg resistance are deployed in canola. These are major gene resistance (or SEEDLING RESISTANCE), which is usually effective at the point of entry of the pathogen into the plant (leaf or cotyledon) and polygenic (or quantitative) resistance (sometimes referred to as ADULT RESISTANCE), which allows initial infection, but slows growth of the pathogen within the plant.
Each Australian canola cultivar has been characterised for its major gene resistance using a set of blackleg isolates that are able to attack individual resistance genes at the seedling stage and thus, allow identification of the resistance genes present. Based on the type of seedling reaction (either susceptible or resistant) each cultivar has been categorised into a resistance group.
In order to determine the adult resistance of each cultivar, infected canola stubble has been collected from cultivars that represent each seedling resistance group and used to infect all Australian canola cultivars. The plants were grown to maturity and then assessed for stem canker. The results from this work have allowed each commercial canola cultivar to be placed into a rotation group that incorporates both the seedling and adult resistance tests.
The recommended canola rotation groups will be released to industry in spring 2012 after extensive feedback from industry to ensure the proposed new blackleg management system is both effective and easily adopted by growers.


Foliar fungicides – a new option for blackleg management

Canola plants are most susceptible to blackleg at the seedling stage. Fungicidal seed dressings provide approximately six weeks protection from blackleg, after which no protection from infection is available. Previous research conducted at Horsham has found that canola plants infected after the third to fifth leaf stage did not result in the formation of a stem canker.

In certain situations it may be economical to apply a foliar fungicide to extend the length of protection from blackleg, such as if disease severity is very high, if genetic resistance is inadequate or has been overcome by the fungus. Currently there are no foliar fungicides registered for use in canola, however this is likely to change in the near future.

Preliminary field trials were conducted in 2010 and 2011 investigating the use of foliar fungicides on canola to manage blackleg. The aim of this work was to investigate the use of foliar fungicides on canola seedlings as an alternative or in addition to the application of a seed dressing for blackleg control in areas of high disease pressure.
Results from these field experiments suggest that the addition of a seed dressing and the application of a foliar fungicide gave the best protection. Application timing was crucial with the 4-6 leaf growth stage greatly decreasing blackleg infection. However, the benefits were only found in those canola cultivars with a low level of resistance to blackleg and in situations of high disease pressure.
The use of foliar fungicides for blackleg management should not become a routine practice for most canola producers, but viewed as a post emergent tool to be used under high disease pressure conditions where preventative measures to decrease blackleg cannot be undertaken.

Blackleg Risk Assessor (BRA) – the support you need for making decisions about growing canola

 
The purpose of the BRA is to enable growers and advisors to determine the overall risk of blackleg development in an individual paddock by assessing all known factors that can influence blackleg. Growers can then alter individual factors to reduce risk; for example, sow cultivars with a different source of resistance, use a seed dressing or change paddock selection. The BRA does not use scores to determine the risk of disease severity, but serves to illustrate how crop and paddock management can be used to influence the potential for blackleg to develop.

Recommendations for managing blackleg in 2012

• In 2012 growers and advisors should consult a number of sources of information including the regional performance of commercial canola cultivars (NVT website), management practices to decrease blackleg severity (BRA – available from the GRDC website) and the National Blackleg Ratings which are published every year.
• Separating this year’s canola crop from last year’s stubble, rotating to a different canola cultivar every 2 years, and using either a fungicide seed dressing or fungicide amended fertiliser are all effective strategies for managing blackleg.
• In the future information on the rotation of canola cultivars and resistance grouping of cultivars will also be available.

Sclerotinia

The incidence of sclerotinia stem rot in canola crops in southern NSW was low in 2011. The relatively dry August and September (late winter/early spring) conditions provided few opportunities for the pathogen to complete the infection cycle for stem rot to occur. Hence, the occurrence of sclerotinia stem rot in canola crops tends to be sporadic, between years and between regions.

Of the few reported cases of sclerotinia stem rot in canola last year, many were found to be basal infection, which is infection of canola plants at the stem base where it meets the soil. This type of symptom is due to direct infection of plants by germinating sclerotia in the soil. In these instances sclerotia of the sclerotinia pathogen have not produced airborne spores, but rather softened, germinated and produced hyphae that have grown through the upper layers of the soil, directly infecting canola plants. The use of foliar fungicides will have no affect on this type of infection as this occurs below the soil surface.

Recommendations for managing sclerotinia stem rot in 2012

• Canola crops grown in high rainfall areas should be monitored closely for sclerotinia stem rot. Good growing conditions for canola usually mean good conditions for disease. If sclerotinia apothecia are detected within crops in late winter, this is a warning that the sclerotinia pathogen is present.
• Long periods of continual wet weather during flowering are ideal for sclerotinia infection to occur.
• If foliar fungicides are to be applied for managing sclerotinia, consider 2 applications of fungicide (first at 30% flowering, second 7 – 10 days later). Remember crop infection levels of at least 25% are necessary to recover the cost of applying foliar fungicides.
• Dense crops, grown in high rainfall districts with a history of sclerotinia stem rot are at most risk.
• If your canola crop developed sclerotinia in 2011, the 2012 canola crop should be regarded as at a risk of developing disease.

Contact details

Kurt Lindbeck,
Wagga Wagga Agricultural Institute, Pine Gully Road, Wagga Wagga, NSW 2650.
kurt.lindbeck@industry.nsw.gov.au