Use of Beneficial Microorganisms and Elicitors for Control of Pseudomonas syringae pv. actinidiae in Kiwifruit (Actinidia spp.)

The kiwifruit pathogen Pseudomonas syringae pv. actinidiae (Psa-V or biovar 3) was first identified in New Zealand in 2010 and currently affects over 75% of the country’s kiwifruit orchards. Our research focused on the use of root endophytic Trichoderma isolates for control of this disease. These fungi have the potential to limit damage from pathogens by colonising root surfaces, penetrating roots, releasing metabolic compounds and inducing plant systemic resistance, thereby improving general plant health and growth. To identify promising Trichoderma isolates for the control of Psa-V, we obtained root endophytic Trichoderma from healthy kiwifruit vines and a wide variety of other healthy plant species in areas with high Psa incidence. In laboratory screening assays, young kiwifruit seedlings of different cultivars were inoculated with selected isolates. Challenging seedlings with Psa-V by stab inoculation allowed the identification of several promising Trichoderma mixtures and single isolates. Field trials in Psa-V infected orchards using container-grown kiwifruit also confirmed the efficacy of the best treatments (both alone and in combination with a plant elicitor and other microorganisms) to reduce Psa-V symptoms. 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 Trichoderma and other 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. Our results indicate that a single Trichoderma inoculation early during plant development has a significant effect on kiwifruit plant health and survival and promotes the establishment of a strong root endophytic Trichoderma community, which contributes to the control of Psa-V on kiwifruit, even when older plants are treated.