Australasian Association of Nematologists

Mark Potter, Department of Plant Science and Crop Protection, University of Adelaide, Waite Campus

Thesis Summary

The inclusion of canola (Brassica napus) in the southern Australian cereal cropping rotation has led to much discussion regarding the qualities of the crop as a 'disease break'. Canola has been reported to 'clean up' the soil, reducing populations of pests and pathogens and therefore providing a healthier soil environment for subsequent crops. However, while many growers and agronomists agree that the crop can be used as a disease control tool, the effect is not reliable, often exacerbating soil disease problems, due primarily to the apparent susceptibility of canola to a range of pests and pathogens.

Field studies reported within this thesis showed that canola (Brassica napus) led to at least comparable reductions in soil levels of the root lesion nematode (Pratylenchus spp.) as other rotation crops grown in parallel. Brassica crops were observed to be particularly effective when green manured (incorporated into the soil), suggesting the release of nematicidal agents as tissue broke down in the soil. However, the disease break effect of the crop was limited by its susceptibility to infestation by the nematode. Efforts to improve the disease break effectiveness of the crop against Pratylenchus spp. must therefore consider both the susceptibility of the plants and the nematicidal potency of the tissues as they degrade in the soil.

Degrading leaf tissues were observed to be more potent against P. neglectus than root tissues. However, HPLC studies revealed no relationship between the nematicidal potency of Brassica leaves and the glucosinolates within them, suggesting that an alternative allelopathic system was active within these tissues. As leaf tissues are not incorporated into the soil within the cropping system under study, further scrutiny of these tissues was considered beyond the scope of this thesis.

A close association was observed between the levels of glucosinolates within the roots and the nematicidal potency of these tissues. Further study revealed that a single glucosinolate, 2-phenylethyl (2-PE), was entirely responsible for the nematicidal qualities of the root tissue, despite the presence of comparable levels of other glucosinolates, such as 2-propenyl, within some tissues. In vitro studies of purified 2-PE isothiocyanate confirmed the nematicidal properties of the molecule. Studies also revealed that root levels of 2-PE glucosinolates were closely associated with the susceptibility of Brassica roots to P. neglectus. Plants containing above a critical level of 2-PE glucosinolate were both more resistant to P. neglectus.

The mean 2-PE glucosinolate levels in the most commonly grown canola variety of the southern Australian cereal region, Dunkeld, were below the observed critical level, perhaps explaining the unpredictable nature of the crop as a disease break. However, considerable variation was observed in the levels of 2-PE glucosinolate within its roots, suggesting the potential to increase the mean levels beyond the threshold. Self pollination studies of Dunkeld plants revealed that the variation in 2-PE glucosinolate levels was heritable, with S₁ and S₂ populations maintaining the parental phenotype. S₂ progeny of a segregating selection exhibited a 'high' : 'low' 2-PE glucosinolate phenotype in approximately a 3:1 ratio, suggesting single gene inheritance following Mendelian principles. 'High' 2-PE glucosinolate S₂ sub-populations were significantly less susceptible to P. neglectus than 'low' 2-PE glucosinolate counterparts.

These same self pollination studies revealed a varied distribution pattern for total glucosinolate levels, suggesting a second heritable character which may have been linked to total seed glucosinolate levels. However, these findings were not further pursued within this thesis, and served only to stress the need to select for % 2-PE glucosinolate levels to minimise the risk of impacting on seed glucosinolate levels and thus quality.

It is likely that 'high' 2-PE glucosinolate sub-populations could be developed from many of the current canola varieties. Such sub-populations should maintain the agronomic characters of the parent line, occupying the same rotational niche as the original varieties. However, being less susceptible and more nematicidal, these lines should perform reliably as disease breaks against P. neglectus within the cereal rotation.

More detailed information may be found in the following publications:

Potter, Davies & Rathjen 1998 - Suppressive impact of glucosinolate in Brassica vegetative tissues on root lesion nematode (Pratylenchus neglectus). Journal of Chemical Ecology 24:67 - 80.

Potter, Davies, Kirkegaard & Rathjen 1999 - Improved resistance to root lesion nematode (Pratylenchus neglectus) in canola (Brassica napus) containing elevated root levels of 2-phenylethyl glucosinolate. Journal of Nematology (in press).

Potter,Davies & Rathjen 1999 - Breeding to increase the disease break strength of canola (Brassica napus). The inheritance of 2-phenylethyl glucosinolate in the root. Plant Breeding (in press).

(Vivien Vanstone, University of Adelaide, vvanston@waite.adelaide.edu.au).