Influences of soil amendments on rhizosphere microbial diversity and potential of fluorescent Gram negative bacteria in grapevine disease suppression

Soil amendments in viticulture enhance both grapevine growth and soil chemistry. However, their role in soil microbial diversity and disease suppression in the vineyard has not yet been thoroughly explored. The purpose of this thesis was to explore the influences of mulches and composts on vineyard rhizosphere microbial populations and diversity, specifically the effects on the microbial populations living in the grapevine rhizosphere. One NSW site in Wagga Wagga and two South Australian vineyard trial sites, one in the Clare Valley and one at Langhorne Creek, were assessed. The application of commercially prepared Grape Marc (GM) compost at Langhorne Creek significantly increased bacterial and fungal rhizosphere populations. This result prompted the initiation of a vineyard trial at Wagga Wagga, in which two types of GM based composts (pine bark based and pine sawdust based) were designed, produced and applied for the investigation of shorter term effects on grapevine rhizosphere microbial population and diversity.

The application of GM compost in viticulture has great potential to improve vineyard soil health and contribute to grapevine disease suppression but its use must be economically viable. Grape marc (GM) is a waste by-product produced in the wine making process, so it is a relatively inexpensive potential component of composts for vineyard soil. As pinebark, a widely used compost component, is expensive, whereas sawdust is a relatively inexpensive industry waste product, there was a need to investigate differences in the effects on the rhizosphere microbial community of GM compost produced with these two different types of carbon source.

To gain a clear insight into the effects of soil amendments on the vineyard rhizosphere microbial communities, three different methods were used to analyse microbial populations and diversity. Plate culturing (Chapter 3), using seven different agar types, successfully evaluated population changes of microbial groups. To investigate differences in functional diversity, the Community Level Physiological Profile (CLLP) method using Biolog Ecoplates™ (Chapter 4) was utilised. Possible changes in genetic diversity were then explored using the Terminal Restriction Fragment Length Polymorphism (TRFLP) technique (Chapter 5). Finally, to investigate the potential for disease control, 28 isolates of Gram negative fluorescent bacteria were isolated from the GM composts and screened for antagonistic and defence enzyme activity in Chardonnay cuttings co-inoculated with the fungal pathogen, Rhizoctonia solani (Chapter 6).

In general, the mulch and compost soil amendments influenced bacterial and fungal grapevine rhizosphere populations differently. Plate culturing showed that organic amendments such as mulches and composts increased the rhizosphere bacterial and fungal populations at all three sites investigated. The application of glyphosate decreased bacterial and fungal populations. Some differences in population and diversity responses were noted for the pinebark and sawdust based GM composts. The application of pinebark GM compost increased the bacterial populations whereas the sawdust based GM compost produced higher populations of fungi. Furthermore, both the TRFLP and the Biolog Ecoplates™ methods showed that the pine bark GM compost increased rhizosphere functional and genetic diversity than the sawdust GM compost.

Understanding the effects of GM composts on grapevine disease suppression is of great relevance for successful vineyard management. After the addition of the GM composts at the Wagga Wagga site, the largest bacterial population increases were colonies growing on Pseudomonads agar. Results from the final experimental chapter in this thesis showed that two of these Gram negative bacterial had antagonistic traits towards the fungal pathogen Rhizoctonia solani, thus indicating the potential of GM compost in vineyard floor management to increase suppressive bacterial populations and potentially contribute to grapevine disease management.