Standardised production of aerobic compost extract for disease management in sustainable viticulture

Disease management in conventional viticulture involves regular applications of synthetic fungicides. There is, however, significant pressure from grape and wine markets to reduce inputs of synthetic fungicides because of concern about their safety to humans and the environment, and due to increasing evidence of pathogen populations developing fungicide resistance. Aerobic compost extract (ACE) is an oxygenated watery extract of compost that favours the persistence of aerobic microorganisms extracted from compost. Some horticultural practitioners claim that ACE is a sustainable alternative to synthetic fungicides when applied to the soil or to the fruit and foliage for crop protection. Scientific evidence supporting the effectiveness and safety of ACE is limited. Moreover, wide variation in production systems for ACE has made comparison of the few refereed reports available difficult. The primary aim of this research was to standardise production of ACE for safety to humans, and for consistent and high levels of suppression of two grapevine diseases caused by fungal pathogens of different biology; namely, powdery mildew, caused by Erysiphe necator, and botrytis bunch rot, caused by Botrytis cinerea. Production variables for ACE from three composts with variable raw ingredients were evaluated systematically by quantifying the growth and reproduction of B. cinerea on detached bean leaflets treated with different ACEs. Bacterial-dominant ACEs produced with a compost weight to water volume ratio of 1 :3 to 1: 10, and from compost sampled in the very early secondary mesophilic stage of composting inhibited B. cinerea colonisation of bean leaflets to a greater extent than ACEs produced from compost sampled in the later mesophilic stages. There was evidence to suggest that the magnitude of pathogen suppression was associated positively to the number of bacterial and fungal Terminal Restriction Fragments (T-RFs) or microbial taxon diversity in ACE. This association will need to be tested further by measuring T-RFs in ACEs prepared from a variety of compost windrows. ACE directly inhibited the germination of B. cinerea conidia in vitro and removal of microorganisms from ACE by filtration reduced but did not eliminate its capacity to inhibit conidial germination. Water-soluble antibiotics were not detected in filtered ACE following an in vitro assay for the inhibition of B. cinerea colony growth. Under glasshouse conditions, the mean powdery mildew severity on Cabernet Sauvignon leaves was less than 0.1 % when ACE was applied up to 4 days before or up to 7 days after inoculation with E. necator conidia; mean severity on non-treated, inoculated leaves was 22%. This result suggested that ACE had curative as well as protective properties. ACE or ACE amended with fish hydrolysate and/or_liquid kelp was prepared using standardised methods and applied nine or 12 times at 10-14 day intervals to Chardonnay or Riesling vines grown commercially in different growing seasons in southern Tasmania. Powdery mildew was controlled by ACE or amended ACE to a commercially acceptable level on Chardonnay leaves and bunches under conditions of high disease severity. The incidence of latent B. cinerea in Chardonnay bunches at harvest, after moist incubation, was nearly half that observed in nontreated bunches. The incidence and severity of sporulation of B. cinerea on Riesling grape bunches was reduced significantly by ACE or amended ACE relative to a dechlorinated water control treatment; these control bunches (not leaves) escaped visible infection by E. necator but powdery mildew was controlled on leaves treated with ACE or amended ACE. Treatment of Riesling leaves with ACE increased the number of culturable microorganisms on leaves 100-fold, 1 h after application. By 13 days post-application the number of culturable microorganisms remained higher than pre-application counts. The human pathogenic bacteria Escherichia coli, Listeria monocytogenes and Bacillus cereus were not detected in compost used to prepare standardised ACE. Production conditions for ACE did not favour re-growth of a non-pathogenic streptomycin resistant strain of E. coli. However, there was an increase in E. coli numbers when fish hydrolysate or molasses were introduced to ACE. Further experimental work is necessary to ensure negligible growth and persistence of human pathogens in standardised ACE amended with nutrients. In the interim, standardised ACE should be prepared without the addition of nutrients to prevent danger to human health. The effectiveness of standardised ACE can now be evaluated across _a range of viticultural conditions and for its impact on grape and wine quality. It is envisaged that ACE will be integrated with other measures to reduce the severity of diseases of grapevines and other horticultural crops.