New root diseases of canola in Australia

In Australia, canola is grown as a winter oilseed crop and is adapted to most arable areas with fertile soil and good drainage (Potter et al. 1999). Canola is grown in rotation with winter cereals and legumes to break the disease cycle in these crops. Improved yield and quality of wheat is often observed when grown after canola, which has been partially attributed to the fact that canola is not a host for take-all (Norton et al. 1999). Canola enjoyed a resurgence in popularity in Australia as a break crop in the 1980s, after its early failure in the 1960s and 1970s (Murray and Stovold 1970; Bokor et al. 1975; Salisbury et al. 1995).

It has been recognised that the two major diseases of canola in Australia are blackleg (caused by Leptosphaeria maculans) and Sclerotinia stem rot (caused by Sclerotinia sclerotiorum) (West et al. 2001; Hind et al. 2003). In the 1990s there was an apparent yield decline phenomenon, which has been attributed to an increase in both of these diseases (Kirkegaard et al. 2006). Although these are the major diseases of canola in Australia, our hypothesis is that there are several other soilborne diseases that could be contributing to this yield decline.

Canola plants were collected at flowering from commercial crops in the Wagga Wagga district. Plants were returned to the laboratory and isolations were made onto quarter-strength potato dextrose agar from lesions on roots. From these initial isolations, pure cultures and single spore isolates were established. Initial identifications were made from these cultures, which were confirmed by Dr Brett Summerell (Botanic Gardens Trust, Sydney) and Dr Michael Priest (NSWDPI, Orange). Pathogenicity of all isolates was determined on canola cv. Skipton using a Petri dish assay. Twenty seeds were placed on the surface of a 4-day-old subculture colony of the isolated fungi growing on water agar and incubated at 25°C. The proportion of seedlings that were killed by the fungus was determined after 7 days. All tests were replicated four times. The fungi were reisolated from the infected seedlings to satisfy Koch’s postulates.

The fungi isolated from the roots of the canola from Wagga Wagga included Macrophomina phaseolina (31.4%), Fusarium spp. (30.0%) (including F. oxysporum, F. acuminatum, F. semitectum, F. solani, F. nivalis and F. equiseti), Phoma lingam (12.3%), Sclerotium spp. (2.7%) and other fungi (23.6%). Pathogenicity of all of these fungi was confirmed.

The majority of diseases recorded on canola in Australia are of the leaves, stem or crown. These include blackleg, Sclerotinia stem rot and various diseases caused by species of Alternaria. Of these, blackleg is by far the most important disease and is capable of devastating the entire crop (West et al. 2001). There are fewer diseases of the roots of canola documented in Australia; however, blackleg has also been found to be able to attack the roots of canola (Sosnowski et al. 2001).

In this work we have demonstrated that there are a large number of soilborne fungi capable of causing disease in canola in the high rainfall areas of southern New South Wales. Surprisingly, there were a large number of isolates of M. phaseolina, a common pathogen that infects a large number of plant species worldwide (Dhingra and Sinclair 1978). Although this pathogen has been recorded affecting canola in Argentina and the USA (Baird et al. 1994; Gaetán et al. 2006) this is the first record of it attacking canola in Australia. There are no documented yield losses attributed to this pathogen in canola. An isolate of this fungus has been lodged with the NSW DPI herbarium at Orange (DAR 78096). Furthermore, there were six species of Fusarium found attacking canola, two of which, F. acuminatum and F. semitectum, have not been recorded from canola previously. Isolates of these fungi have been lodged with the NSW DPI herbarium at Orange (DAR 78097 and DAR 78098 respectively). Previously, F. acuminatum has been associated with crown rot of wheat (Williams et al. 2002) while F. semitectum has been shown to cause disease in cotton, kangaroo paw and asparagus in Australia (Elmer et al. 1997; Satou et al. 2001; Wang et al. 2004). Our results indicate that in addition to the major diseases such as Sclerotinia stem rot and blackleg, there are several pathogens in the soil which could be contributing to yield decline in canola in south-eastern Australia.