Organic amendments enhance biocontrol of soil-borne diseases and soil dwelling pests
Fungal biocontrol agents (BCAs) such as Trichoderma species have been used to suppress diseases caused by soil-borne pathogens such as Sclerotium cepivorum causing white rot in onions. Similarly, BCAs termed entomopathogenic fungi (EPF) have been used against a range of soil dwelling pests such as larvae of vine weevil (Otiorhyncus sulcatus). The main problem in obtaining effective disease or pest control has been the development of a cheap delivery method that can achieve and maintain sufficiently high inoculum levels of BCAs in root zone soil or growing media. BCAs are conventionally produced in sterile media containing various carbon and nitrogen sources so there is no competition with other fungi that may be present on unsterilised substrates. The volume cost of BCA inocula produced on sterile substrates is typically x104 greater than the volume cost of growing media, which restricts the economic application rate of BCAs in growing media or soil. Several fungal BCAs have the capacity not only to parasitise their pathogen or pest hosts, but also to colonise suitable organic matter saprophytically. Applying BCAs in a low cost organic carrier material could potentially lead to high and sustained soil populations and effective disease or pest control. Green waste compost (GWC) is an abundant source of carbon-rich organic matter, a potential nutrient source for Trichoderma species and similar fungal BCAs. Spent mushroom compost (SMC) and brewery yeast waste are widely available sources of fungal chitin, the same structural material of insect cuticle, and therefore a potential nutrient source for EPF. The aim of research at EMR has been to investigate the effect of co-application of BCAs and organic amendments on the subsequent population of BCAs in soil or growing media, and their potential for sustained levels of soil-borne disease and soil dwelling pest control.
Field application of compost and Trichoderma for control of white rot in onions
Rye grain inoculum of Trichoderma viride produced by Sylvan Bio was mixed into screened GWC at 0.25% w/w. An onion set planting machine modified by Moulton Bulb Company to apply the Trichoderma inoculated compost at along planting rows was used at a field site in Bedfordshire (F. Parrish Ltd). In nitrate vulnerable zones, the EU Nitrates Directive limits the amount of compost which can be applied by uniformly broadcasting in fields to around 25 tonnes/ha. The machine enabled a compost application of 50 tonnes/ha to be achieved along the planting rows, although the amount applied over the entire area was only 12 tonnes/ha. The soil population of Trichoderma propagules in treated rows remained above 3 x 106 g-1 throughout the entire onion growing season. This level of Trichoderma propagules in soil has been shown to give good control of white rot in previous onion pot and field experiments. However, the level of white rot disease in onions in the experimental field was less than 3% so that further trials are needed to test the control efficacy of the technology using a range of BCAs that are now registered for use in field crops.
Amending growing media with spent mushroom compost and EPF for vine weevil control
Several organic materials were compared for their ability to support the growth and persistence of two commercially available EPF: Met52 (Metarhizium anisopliae) and Naturalis-L (Beauveria bassiana). These included GWC, crushed crab shells, brewery yeast waste, coffee grindings, mushroom stipes and SMC. Of these, the latter material produced the best results and minimised competition from contaminants such as Aspergillus species.
An experiment was conducted in which a peat-based growing medium was amended at 10 and 20% v/v with SMC containing Naturalis-L. Growing media containing only Naturalis-L added at 0.1 ml/litre, or without additives were used as controls. Ten late instar vine weevil larvae were added to pots containing each of the above treatments, together with a food source.
After 19 days, all the larvae in the unamended growing medium were still alive as larvae or pupae, whereas in growing medium containing both SMC and Naturalis-L, all larvae were dead. In the Naturalis-L only treatment, 50% of the larvae were dead. In growing medium containing both SMC and Naturalis-L, the B. bassiana population declined slowly, but was still higher after 49 days than in growing medium where only Naturalis-L was added (Figure). At the end of this period, there was no difference between the 10 and 20% rates. No B. bassiana propagules were detected in growing media without Naturalis-L.
Amending soils or growing media with a combination of BCAs and organic carrier materials has the potential to increase the initial population of BCAs and their subsequent survival, resulting in improved disease or pest control over BCAs used in isolation. Disease and pest control in the field needs to be tested on a range of pathogen infected or pest infested soil types. Potted plant experiments are needed to validate the results of the laboratory EPF bioassays on pest populations.
Ralph Noble, Jean Fitzgerald and Andreja Dobrovin-Pennington