Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by Subject "Apple replant disease"

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    Effect of biological amendments on soil microbial properties and performance of pome fruit trees
    (Stellenbosch : Stellenbosch University, 2009-12) Van Schoor, Louise; Stassen, P. J. C.; Botha, Alfred; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The global movement in agriculture is towards more environmentally friendly, sustainable production practices, since the role of soil microbial functions in ensuring crop production and soil fertility has become more evident in agricultural systems. Furthermore, with the impeding phase-out of methyl bromide, apple replant disease (ARD) is becoming an increasingly important problem and biological management practises are needed. Since microbial activity is generally carbon-limited in agricultural soil, it is widely accepted that management practices providing a range of organic compounds on a regular basis will tend to maintain an active and diverse microbial population. It was hypothesised that the application of various biological amendments can affect soil microbial numbers and function, thereby having a positive effect on fruit tree growth and yield. The effect of continued applications of organic material, various microbial inoculants and biostimulants on tree performance were evaluated in conventional management systems. Field trials were established in a conventional pear orchard, potential apple replant disease sites, as well as an optimally managed, high density apple orchard under controlled fertigation. The use of compost, compost extracts, a Bacillus inoculant and humates were investigated intensively. Furthermore, to improve our understanding of soil biological systems a combination of simple, practical methods were used to evaluate the effect of biological amendments on soil microbial properties and effects were related to tree performance. Regular application of compost extract in combination with compost showed the most significant effect in improving tree performance in commercial pome fruit orchards under various conditions. In the pear orchard, cumulative yield over the first two seasons was improved by more than 50% compared to controls, while in the fertigated orchard yield was improved by 22%. Biological amendments also showed improved growth in orchards suffering from stunted growth symptoms typical of ARD. However, in severe ARD cases methyl bromide fumigation showed the most consistent effects. Other biological amendments which showed positive effects on yield were application of Bacillus inoculants (Biostart®) in combination with a labile C source and a low dosage humate product, as well as a combination of compost and humates. It was clear that a combination of labile organic matter and a diverse group of microorganisms showed most promise. Although for some specific treatments increased microbial numbers and activity may have resulted in improved tree performance, in general, changes in culture-based plate counts, soil enzyme activity and carbon utilisation profiles could not be used as an indicator of yield. It was suggested that improved synchronisation of nutrient release and plant uptake, as well as microbial phytohormone production, may play an important role in improving tree performance with application of biological amendments. More research is needed on the exact mechanisms through which compost extracts improve yield and studies on root growth proliferation, as well as effects in the rhizosphere are recommended.