The superior growth of wheat following Brassica crops compared to that following non-Brassica crops may be due to the suppression of soilborne fungal pathogens by volatile isothiocyanates (ITCs) released in the soil during hydrolysis of glucosinolates contained in Brassica tissues. We investigated the effects of volatile compounds released from the root, shoot and seed meal tissues of canola (Brassica napus) and Indian mustard (Brassica juncea) on the mycelial growth of five soilborne pathogens of cereals-Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Pythium irregulare and Bipolaris sorokiniana. Three isolates of each species, originally collected from the roots of wheat (Triticum aestivum) and barley grass (Hordeum leporinum ) in southern Australia, were exposed to volatiles released in vitro when sterile water was added to freeze-dried Brassica tissues. The root and shoot tissues of both Brassica species were more suppressive at flowering than maturity and mustard tissues were generally more suppressive than canola. The degree of fungal suppression by the various Brassica tissues was related to the concentration and type of isothiocyanates released, which varied with Brassica species, tissue age and tissue type. There were significant differences in the sensitivity of the fungal species and among isolates of each species. Gaeumannomyces and Rhizoctonia were generally the most sensitive to the volatiles released, Pythium and Bipolaris the least. The results indicate that the effectiveness of fungal suppression by Brassica crops will depend upon the species, age and type of Brassica tissue, which influence the type and concentration of isothiocyanates evolved, and the sensitivity of the pathogen.

Biofumigation refers to the suppression of soil-borne pathogens and pests by biocidal compounds released by Brassica crops when glucosinolates (GSL) in their residues decay in soil. We conducted field studies at 2 sites to investigate the hypothesis that biofumigation by Brassica break crops would reduce inoculum of the take-all fungus Gaeumannomyces graminis var. tritici (Ggt) to lower levels than non-Brassica break crops, and thereby reduce Ggt infection and associated yield loss in subsequent wheat crops. High and uniform levels of Ggt were established at the sites in the first year of the experiments by sowing wheat with sterilised ryegrass seed infested with Ggt. Ggt inoculum declined more rapidly under Brassica crops than under linola and this reduction coincided with the period of root decay and reduced root glucosinolate concentrations around crop maturity. There was no consistent difference in inoculum reduction between canola (Brassica napus) and Indian mustard (Brassica juncea), nor between cultivars with high and low root GSL within each species. Despite significant inoculum reduction attributable to biofumigation, there were no differences in the expression of disease and associated impacts on the yield of subsequent wheat crops across the sites. Seasonal conditions, in particular the distribution of rainfall in both the summer–autumn fallow following the break crops and during the subsequent wheat crop, influenced inoculum survival and subsequent disease development. In wet summers, inoculum declined to low levels following all break crops and no extra benefit from biofumigation was evident. In dry summers the lower inoculum levels following brassicas persisted until the following wheat crops were sown but subsequent development of the disease was influenced more by seasonal conditions than by initial inoculum levels. Significant extra benefits of biofumigation were observed in one experiment where wheat was sown within the break crops to simulate grass weed hosts of Ggt. Under these circumstances there was greater reduction in Ggt inoculum under canola than linseed and an associated decrease in disease development. For host-dependent pathogens such as Ggt, we hypothesise that the benefits of biofumigation to subsequent wheat crops will therefore be restricted to specific circumstances in which inoculum is preserved during and after the break crops (i.e. dry conditions, grass hosts present) and where conditions in the following wheat crop lead to significant disease development (early sowing, wet autumn and spring, dry periods during grain filling).

No abstract

The early growth of wheat was compared under direct drilled (DD) and cultivated treatments at 21 farm sites throughout southern New South Wales in 1997, 7 sites in 1998, and 11 sites in 1999. The experiments investigated the involvement of (1) paddock history/management, (2) common soil-borne fungal pathogens, and (3) populations of various microbial groups in early growth reductions associated with DD. Cultivation increased early vegetative growth at 62% of sites with an average increase in dry matter production of 33% compared with DD plots. Reduced shoot growth under DD was closely associated (r = 0.899) with decreased development of the root system. Early growth reductions under DD did not appear related to management practices such as different sowing points, herbicide or fertiliser application, or soil pH and fertility. Soil type and previous crop species, however, influenced the incidence but not severity of early growth reductions. Reduced growth under DD was not associated with the presence of any of the common soil-borne fungal pathogens of wheat, or with rhizosphere populations of total aerobic bacteria, total fungi, aerobic spore-forming bacteria, Gram-negative bacteria, and actinomycetes. On average, Pseudomonas populations in the rhizosphere of seedlings grown in cultivated soil were reduced by 61% compared with the DD treatment at sites where early growth reductions under DD were evident. An assessment of the inhibitory activity of pseudomonads towards wheat seedlings in a test-tube bioassay indicated that reduced growth in DD plots was more closely related with the inhibitory activity of Pseudomonas spp. to root growth than their population in the rhizosphere. A close relationship existed between the inhibitory activity of Pseudomonas spp. isolated from each site and the response of wheat seedlings to cultivation (r = 0.865). These results suggest that Pseudomonas spp. with inhibitory activity to root growth are involved in the reduced early growth of DD wheat in southern New South Wales.

S U M M A R YGlasshouse studies showed that the incidence of black point caused by Alternaria alternata in durum wheat was positively correlated with both the density of the inoculum and the growth stage of the wheat plants at the time of inoculation. A curvilinear relationship of the form Y = a + log X was found between inoculum 0 1980 Association of Applied Biologists