Assessment of the Detrimental Impact of Polyvalent Streptophages Intended to be Used as Biological Control Agents on Beneficial Soil Streptoflora

Streptophages are currently being investigated to control potato common scab, however, since a majority of streptophages are reported to be polyvalent, their potential to infect beneficial soil streptomycetes during the application process may have unintended consequences. To test this hypothesis, two phytopathogenic fungi, namely Fusarium solani and Rhizoctonia solani, were tested for their detrimental effect on the test crop wheat (Triticum aestivum cv. Gutha). F. solani caused a significant root weight reduction (34%) in the wheat plant and therefore was tested further in the pot trials with actinomycetes present. Sixty-seven streptomycete isolates from a Tasmanian potato farm were screened for their antifungal abilities against the two phytopathogenic fungi. Four actinomycetes found to be strongly antifungal were then tested for their disease-protective abilities against F. solani in pot trials again using wheat. Addition of the streptomycetes into the container media protected the plants against F. solani, indicating that streptomycetes have a disease-suppressive effect. A further pot trial was conducted to evaluate whether these beneficial streptomycete species would be affected by streptophage treatment and subsequently result in an increased risk of fungal infections. When streptophages were added to the pots, the shoot and root growth of wheat declined by 23.6% and 8.0%, respectively, in the pots with the pathogenic fungus compared to the control pots. These differences might suggest that removal of antifungal streptomycetes by polyvalent phages from plant rhizosphere when biocontrol of plant pathogenic streptomycetes (e.g. Streptomyces scabiei) is targeted might encourage secondary fungal infections in the farm environment. The presented data provide preliminary evidence that streptophage treatment of pathogenic streptomycetes may lead to an aggravated disease risk by soil-borne fungal pathogens when naturally present antagonists are removed. As a result, extensive farm site trials are required to determine the long-term detrimental impact of polyvalent streptophage treatments on beneficial soil streptoflora.