Microbiological aspects of apple replant disease

The apple industry represents a major part of Tasmanian, Australian and world wide fruit production. Continuing demand for new varieties in the changing global marketplace presents growers with an ongoing requirement for removal of uneconomic varieties and their replacement with the more currently favoured varieties, this particularly being the case for Tasmanian and Australian growers supplying competitive overseas markets. Planting designs using closely spaced early fruiting trees on dwarfing rootstocks, leading to quicker economic returns, also encourages faster turn-around of orchards. These factors have placed increasing pressure on land available for apple orchards. Replanting of many plant species, including apple (Malus domestica), often presents growers with a well known management problem commonly referred to as \replant disease\". Trees planted in locations previously occupied by earlier plantings show reduced growth and an atypical growth pattern. Severely affected plants can show significant stunting often resulting in a characteristic \"wave\" pattern across affected fields with weakest growth corresponding with tree positions in the previous orchard. Reductions in growth can adversely impact on orchard coverage and hence yield per unit area with consequent economic impacts. Dwarfing rootstocks popular under current management strategies tend to be more susceptible to the effects of replant making effective management of apple replant disease of significant importance to growers. While the underlying cause of this condition has yet to be established it has been found to be associated with a range of biotic and abiotic factors. Fumigation with aggressive fumigants such as methyl bromide has been found to consistently prevent occurrence of the problem. Other lower impact or cheaper alternatives are available but they have not gained wide acceptance due in part to inconsistent results and the economic need for young trees to achieve uniform growth to the appropriate size for the orchard design. In 1990 parties to the Montreal Protocol 1987 agreed to a phase out of a range of ozone depleting substances including methyl bromide. Consequently the present thesis reports on components of research seeking an alternative control measure for the local (Tasmanian) industry. A range of possible measures were investigated on both potted and field conditions trees. Treatments included: sterilants methyl bromide Telone C35¬¨vÜ a mixture of Telone¬¨vÜ 65% and chloropicrin 35% Basamid¬¨vÜ (dazomet) metham and PerIke (calcium cyanamide) organic matter addition in the form of peat and biocontrol agents including two Trichoderma sp. Trichopel¬¨vÜ and a locally isolated strain Td22 together with a commercially available Bacillus subtilis agent known by the trade name Companion¬¨vÜ. In addition dominant microbiota were isolated from these treatments in order to find potential pathogens or biocontrol agents that may be involved in causing or countering apple replant disorder. It was found that Telone C35¬¨vÜ was the only alternate treatment that produced consistent growth results equivalent to methyl bromide. Given both fumigants utilize the same equipment and management strategies a change to Telone C35¬¨vÜ provides growers with a suitable replacement for methyl bromide. Basamid¬¨vÜ in these trials did not produce a consistent effect in countering apple replant. Other potential sterilants did not produce consistent effective results. Addition of organic matter produced some positive effects but results did not suggest it as a consistent and suitable countermeasure for apple replant. Treatment of soil with commercial and locally isolated biocontrol agents did not produce conclusive results. Application of mono-ammonium phosphate was effective under glasshouse conditions confirming extensive earlier work but was ineffective in field trials reported here possibly due to enhanced weed competition. Plant water stress is often said to be associated with replant disease but there is little published evidence supporting the assertion. A trial comparing root hydraulic conductivity over the initial 6 weeks from planting into replant and non-replant soil demonstrated a reduction in conductivity in replant soil. The result is discussed in terms of a link between the replant status of the soil and a measurable physiological response in plants."