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THE DISTRIBUTION OF PLANT PATHOGENS IN THE SYDNEY BASIN, A REPORT TO THE DEPARTMENT OF PRIMARY INDUSTRIES.

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Introduction

Vegetable growing is disappearing in the Sydney Basin due to the expansion of housing...

Vegetable growing is disappearing in the Sydney Basin due to the expansion of housing. However, the close proximity to the main market makes vegetable growing in the Sydney Basin very attractive. Financial constraints especially the cost of land place enormous pressure on vegetable growers. Thus growers attempt to use land as efficiently as possible. One method now widely used is repeat cropping; new seedlings are planted while the remnants of the previous crop remain in the soil (DPI 2008). One possible consequence of this practice is that plant pathogens may proliferate.

A suite of plant pathogens has been described from vegetable production (Risk and Bellamy, 2001). Three generalist fungi, Thielaviopsis spp, Rhizoctonia spp and Fusarium spp, probably have greatest potential effect because they may colonise the seedlings of many hosts (Angus, 1992). Because all the fungi are pathogens of seedlings, they enable comparative examination of their presence in soil. This investigation examines the abundance and distribution of seedling pathogens of vegetable crops of the Sydney Basin, with view to predicting their potential influence on vegetable production.

Materials and Methods:

Sites across the Sydney Basin were randomly selected...

Sites across the Sydney Basin were randomly selected, except that in each of the three regions, at least 15 sites was selected that currently grows vegetables and five sites at each of three other different lands uses (total 15). Replicated soil samples were collected at each site. On return to the laboratory, fungi were isolated from each sample and plated on to PDA and identified using morphological methods.

Results of the Survey:

A total of 90 sites were examined, including 45 vegetable farms. Across the Sydney Basin, 10 different fungi were identified...

A total of 90 sites were examined, including 45 vegetable farms. Across the Sydney Basin, 10 different fungi were identified. Of the fungi identified, Thielaviopsis spp was found at 44 farms, Rhizoctonia spp at 28 farms and Fusarium spp at 15 farms. With one exception, these pathogenic fungi were not detected at any other site (Table 1). The three pathogens were evenly distributed across all four regions of the basin, except for Fusarium spp which was only found in the southern region (Table 2).

Discussion and Recommendations:

The ten fungi isolated from soils used for vegetable growing across the Sydney Basin have been recorded...

The ten fungi isolated from soils used for vegetable growing across the Sydney Basin have been recorded in soil before (Domsch et al., 1980). Of the 10 fungi, three are important pathogens of seedlings (Domsch et al., 1980), and these were commonly recovered from soils used to grow vegetables and extremely rare in soil used for other purposes (Table 1). The regional distribution of each of the pathogens differed. Thielaviopsis spp and Rhizoctonia spp appear to be evenly spread across the Sydney Basin. Fusarium spp differed in that it was only recovered from the southern region. Thielaviopsis spp and Fusarium spp were common when they were present, close to 100% of all sites samples in the region. Rhizoctonia spp was present in approximately half of the vegetable growing sites of all regions.

The reason why Fusarium spp was absent from the central and northern region requires further investigation. Presumably the rainfall of the three regions is similar (BOM, 2008), the same soil types are found in all three regions (Clegg, 1999) and the vegetable growing practices are similar (DPI, 2008). It may be that Fusarium spp has only just arrived in the southern region and will spread to the other regions as inoculum is dispersed, much as has happened with this pathogen in the cotton growing regions of eastern Australia (Nehl et al., 2006). Spread of this pathogen may be a major detriment to vegetable production in the Sydney Basin, as we are seeing recent reports of alarming seedling deaths in lettuce and capsicum from the southern region (DPI, 2008).

These data indicate an alarmingly high prevalence of seedling pathogens in vegetable growing regions, especially the southern region where all pathogens are present. Not only are seedling pathogens common on farms, but no farm appears to be free of seedling pathogens. The precise interaction between the pathogens and their host plants require clarification. Reports of plant disease in vegetables rarely mention seedling pathogens (DPI, 2008) and it may be that the diseases have not been considered in reports or that seedling diseases are not recognised. Further, measures that aim to decrease seedling disease, such as seed dressing, do not appear to be having an effect on the prevalence of the pathogens. If seedling pathogens are reducing plant productivity, research on measures to reduce the impact of the pathogens, or increase plant tolerance to pathogens will be necessary.

It is also alarming that pathogens are being found only at vegetable growing soils. All other fungi appeared to be broadly and predictably distributed. If vegetable production is selecting for pathogens, then increasing pathogenicity may evolve, with loss of economic benefit the local production now allows.

This report presents data on the distribution of three seedling pathogens detected in soil used to grow vegetables in the Sydney Basin. Two pathogens appear to be widespread, and one is at present limited to the southern region. The prevalence of these fungi indicates a major potential problem that requires further investigation of their impact, dispersal and control.

Distribution of fungi in soil used to grow vegetables, fruit trees and gardens collected from across the Sydney basin.


Table 1.

Regional distribution of seedling pathogens in soils with different land use.


Table 2.

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References

  1. Angus, B.B. (1992) Seedling pathogens of commonly grown vegetables in NSW, Australia. Australasian Plant Pathogy 42: 164-198. BOM: website of the Bureau of Meteorology, accessed Jan 2008.
  2. Clegg, A.A. (1999) Soils of the eastern escarpment of Australia. Soil Science 159: 271 � 280.
  3. DPI: website of the Department of Primary Industry, accessed Jan 2008.
  4. Domsch, K.H., Gams, W, and Anserson T_H. (1980) Compendium of Soil Fungi. IHW Verlag.Berlin.
  5. Nehl, D.J., Anderson, C.J.M., Allen, S.J. and Methuselah, D.D. (2006) Spread and pathology of the seedling disease in cotton caused by Fusarium oxysporum. Plant Disease Reports 198: 56 � 59.
  6. Risk, D.A. & Bellamy J.M. (2001) Host Pathogen Index of Plant Diseases in Australia. CSIRO press, Canberra.
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