EXPLOITING THE BIOFUMIGATION POTENTIAL OF BRASSICAS IN FARMING SYSTEMS
1J.A. Kirkegaard, 2J.N. Matthiessen2, 3P.T.W. Wong, 4A Mead, 1M. Sarwar and 1B.J. Smith
1CSIRO Plant Industry, GPO Box 1600, Canberra ACT 2601.
2CSIRO Entomology, Private Bag PO Wembley WA 6014
3NSW Agriculture Agricultural Institute, PMB Wagga Wagga 2650.
4NSW Agriculture Cowra Agricultural Research Station, PO Box 242, Cowra 2794
Biofumigation refers to the suppression of soil-borne pests and pathogens by biocidal compounds, principally isothiocyanates (ITCs) released from Brassicaceous rotation and green manure crops when glucosinolates (GSLs) in their tissues are hydrolysed. Recent research in Australia has investigated the potential for biofumigation to contribute to the control of soil-borne pathogens in broad acre cropping as well as high value horticultural crops. In broad acre cropping, Brassica oilseeds such as canola are thought to be superior break-crops for subsequent cereal crops, partly due to their biofumigation effects on cereal pathogens. We have identified the compounds in Brassica roots suppressive to cereal pathogens and demonstrated their toxicity to fungal inoculum. However the impact of this suppression on disease development in subsequent cereals was highly dependent on seasonal conditions. In horticultural systems, a wide range of Brassica green manures can be used; they can be grown immediately prior to commercial crops, and both shoot and root material can contribute to biofumigation. Variation in GSL types and concentrations, and biomass production provides opportunities to select or develop varieties with increased biofumigation potential. Despite the considerable potential to enhance the production of ITCs by brassicas, the inefficiency in the conversion of tissue derived GSLs to ITCs and various loss processes would suggest that biofumigation is unlikely to match synthetic fumigants in efficacy and will form part of an integrated control package. Some examples from current Australian research are reviewed.
KEYWORDS: isothiocyanate, disease, wheat, green-manure, take-all
Field experiments during the early 1990’s demonstrated that wheat crops grew more vigorously following Brassica break crops such as canola and Indian mustard than other break crops such as linseed or oats (Angus et al. 1991, Kirkegaard et al. 1994). Improved vigour could not be explained by residual water or N and could not be matched by applied N. The beneficial effects of brassicas were not obvious in all situations (Heenan 1995) and the magnitude of the effect varied significantly with site and seasonal conditions (Kirkegaard et al. 1994).
One hypothesis regarding the beneficial effects of brassicas is that the Brassica crops release biocidal compounds, principally isothiocyanates (ITCs) during the breakdown of glucosinolates in their residues, which reduce disease infection in following crops. The term “biofumigation” was coined to describe this suppression of soil pathogens by compounds released from Brassica tissues, and implies a greater reduction in disease inoculum than that resulting from the simple absence of a host. In broad-acre cropping, the Brassica crops are grown to maturity and the dry shoot residues remaining after harvest are low in GSL and are not usually incorporated by cultivation. Thus the decaying root system was the likely source of the suppressive compounds. Field studies identified 2-phenylethyl GSL (2PE-GSL) as the major GSL present in the roots of canola, comprising around 80% of the total GSL profile (Kirkegaard and Sarwar 1999).
In vitro toxicity tests using fresh or re-hydrated Brassica root tissues demonstrated suppression of a range of soil-borne cereal pathogens (Angus et al. 1994, Kirkegaard et al. 1996). Further studies using different ITCs dissolved in agar showed that 2-phenylethyl isothiocyanate (2PE-ITC) was the most toxic of several ITCs (including the commercial soil fumigant methyl ITC). This was consistent with previous studies indicating the generally higher toxicity of the aromatic ITCs (Sarwar et al. 1998). Although these studies provided evidence for the toxicity of ITCs under laboratory conditions, their effectiveness in soil remained uncertain, as significant losses of ITCs due to sorption and other processes may occur (Brown and Morra 1997). The efficiency of conversion of GSLs in Brassica tissues into ITCs can be as low as 15% in soil (Borek et al. 1997).
In pot studies, Smith et al. (1999) showed that incorporation of ground, freeze-dried canola root residues at realistic field rates (0.1 – 0.5% w/w) caused a 50% reduction in the infection of wheat seedlings by several root fungal pathogens, a result consistent with that found for Pratylenchus nematodes (Potter et al. 1998). In another study, Kirkegaard et al. (1998) grew several different crops in pots of soil with added inoculum of the take-all fungus (Gaeumannomyces graminis var. tritici), and incorporated the root residues after harvesting the tops at flowering. Wheat seedlings were sown into the pots eight weeks later and the level of take-all infection was assessed. The results showed lower levels of infection on wheat after the brassicas compared with linseed and higher root GSL levels increased the level of suppression. These results demonstrate that significant suppression of fungal inoculum can be achieved in soil by incorporated canola roots or root residues, and that higher levels of GSL increase suppression.
In the field, canola roots are generally left undisturbed after harvest and there is a 4-5 month period from canola harvest (December) until the following wheat crops are sown (May). We investigated the impact of a range of crops including brassicas with high and low root GSLs on the levels of take-all inoculum in soil during the growth of the crops, at maturity, and during the summer fallow. The experiments were conducted at sites where high initial levels of take-all inoculum had been established in the previous year. The levels of inoculum were measured using a wheat seedling bioassay on soil sampled from plots. The results showed that at harvest, the Brassica crops were more suppressive to the take-all fungus than linseed and that the level of suppression was greater for the high root GSL variety (Table 1). These differences were not apparent during crop growth suggesting that the suppression occurred as a result of root decay around maturity rather than exudation from live roots. This is consistent with the fact that most 2PE-GSL is present in the large tap and lateral roots and is likely to be released during the decay of these larger roots as the crop matures (Kirkegaard and Sarwar 1999). The impact of this fungal suppression on the disease development in following wheat crops was dependent on the seasonal conditions in the subsequent summer fallow (Table 1). In the wet summer of 1996/97 the differences in take-all inoculum between different crops had disappeared by March due to the wet summer reducing inoculum to low levels after all break crops. In contrast, the dry summer in 1997/98 preserved inoculum and significant differences were apparent until mid April, when the following crops were sown. These results indicate that the benefits of biofumigation will be highly dependent on seasonal conditions.
Table 1. Take-all inoculum levels assessed using a wheat seedling bioassay (% infection) following different crops in two seasons as Ginninderra Experiment Station, Canberra ACT.
Wheat 97 56 92 76
Linola 43 8 46 25
Canola (low root GSL) 29 5 50 18
Canola (high root GSL) 18 6 34 6
Mustard 24 3 31 17
The potential of Brassicaceous green manures to suppress a range of soil-borne pests and diseases is supported by considerable empirical field evidence (Table 2).
Table 2. Suppression of a range of pests by Brassica green manures (from Matthiessen and Kirkegaard 1998)
Soldier Fly Kale, Radish 76-86 1982
Bacterial wilt Mustard 77 1999
Root knot nematode Rapeseed 53-86 1994-1996
Aphanomyces (fungus) White mustard 29-54 1990
However there is difficulty in interpreting the role of ITCs in the suppressive effects reported in many studies because no information is provided on the type or concentration of the pre-cursor GSLs in the incorporated tissues. The suppression of pests and diseases achieved to date with usually little or know knowledge of GSL types or concentrations suggests that improvements could be expected with purposeful selection for biofumigant types and more information on the fate and activity of the compounds in soil.
Having established that suppression by incorporated Brassica tissues is associated with GSL hydrolysis products, and indications of which hydrolysis products are most toxic, there are significant opportunities to enhance the biofumigation potential of Brassicaceous green manures. Firstly, by selecting brassicas which produce the greatest amount of the GSL-precursors most toxic to the target organisms, and secondly by managing the incorporation process to maximise the exposure of the organisms to the toxic compounds at the most vulnerable stage. Strategies to increase the production of GSL hydrolysis products by brassicas have been summarised elsewhere (Kirkegaard and Sarwar 1998), and rely upon; (1) significant variation in the type and concentration in individual GSLs among different species, cultivars and plant parts, (2) independent variation in biomass production and (3) differential toxicity of different hydrolysis products to particular organisms. Together these represent opportunities to select or develop biofumigant types which may provide up to 105-fold increase in biofumigation potential over varieties selected at random (Table 3).
Table 3. Opportunities to enhance biofumigation potential based on reported variation in relevant parameters.
GSL concentrations 50 fold Kirkegaard and Sarwar (1998)
Biomass production 20 fold Kirkegaard and Sarwar (1998)
Differential toxicity of ITCs 50 fold Brown and Morra (1997)
The potential efficacy of ITCs present in Brassica green manures can be considered by comparison with amounts of the synthetic soil fumigant methyl ITC (MITC) which are applied (517 – 1294 nmol/g). Assuming a maximum biomass of 15 t/ha, a tissue GSL concentrations of 100 mmole/g and an incorporation depth of 10 cm (soil bulk density 1.4), the potential ITC production (assuming 100% conversion) would be equivalent to 1070 nmol/g, which is in the range of commercial MITC application. Although the efficiency of conversion from incorporated tissues can be as low as 15% (Borek et al. 1997), several ITCs have been shown to be up to 10 times more toxic than MITC. It is difficult however, to predict the impact of the lower concentrations of ITC released over an extended time period relative to commercial fumigant applications. More information is required on the fate, persistence and efficacy of the biocidal compounds released from incorporated tissues.
The impact of the biocidal compounds released from the tissue can also be increased by matching the timing of incorporation and release of biocides to the most vulnerable stages of the pest organisms life-cycle. While sufficient time must be provided for the organic material to decompose to avoid both physical and potential allelopathic interferences in the following crop, delaying for too long may allow some pathogens to recover.
Further studies are in progress to improve our understanding of the accumulation of GSLs in Brassica plants, the fate and activity of the biocidal hydrolysis compounds released from incorporated tissue in the soil, and the most effective ways of incorporating biofumigant crops into integrated pest management strategies.
Funding for biofumigation research in Australia is provided by Grains Research and Development Corporation and the Horticultural Research and Development Corporation.
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