Use of beneficial microorganisms and elicitors for Psa-V control in kiwifruit

Abstract

The kiwifruit disease Pseudomonas syringae pv. actinidiae (Psa-V) was first identified in New Zealand in 2010 and currently affects over 70% of the countrys kiwifruit orchard area. Because no single agent is likely to provide complete control of Psa-V, this project aimed to identify a combined treatment of beneficial microorganisms, commercially available elicitors and natural products with the potential to control Psa-V. A key focus of this project was the use of novel microorganisms, specifically root endophytic Trichoderma, which were assessed for control of Psa-V on young seedlings under environmentally controlled conditions and for persistence in situ by re-isolation from potted kiwifruit plants in an orchard trial. These fungi have the potential to colonise root surfaces and penetrate roots, release a range of metabolic compounds (including antibiotics), induce plant systemic resistance, and improve general plant health to limit damage from pathogens. Root colonisation with Trichoderma can also enhance plant growth, crop productivity, nutrient uptake and resistance to abiotic stresses. To identify promising Trichoderma isolates for the control of Psa-V and plant growth promotion, we isolated root endophytic Trichoderma from healthy kiwifruit vines and a wide variety of other healthy plant species in areas with high Psa-V incidence. In laboratory screening assays, roots of young kiwifruit seedlings of different cultivars were inoculated with different Trichoderma isolates as mixtures or single isolates. After 8 to 10 weeks, seedlings were challenged with Psa-V by stab inoculation. Several promising mixtures and single isolates were identified and our results indicate that a single Trichoderma inoculation early during plant development has a significant effect on kiwifruit plant health and survival. Field trials in Psa-V infected kiwifruit orchards using container-grown kiwifruit have also confirmed the efficacy of the best treatments alone and in combination with a plant elicitor and other microorganisms to reduce Psa-V symptoms on two kiwifruit cultivars. When re-isolating Trichoderma at the end of the orchard trial from the roots of plants that had or had not received a combination of yeast, Trichoderma and Actigard treatments before being exposed to Psa-V in the field, we found significantly higher numbers of Trichoderma isolates in the roots of Trichoderma-treated plants compared to the untreated controls. The results suggest that inoculation promotes the establishment of a strong root endophytic Trichoderma community that contributes to the control of Psa-V on kiwifruit, even when older plants are treated.