Substrate Media, Fertilizer Rate, and Mycorrhizal Inoculum Source Affect Azalea Plant Root Mycorrhizal Colonization and Severity of Phytophthora Cinnamomi Infection

Liners of two azalea cultivars Delaware Valley White ‘DVW’ and Silver Sword ‘SS’ were container grown in bark- or peat-based substrate media fertilized with Nutricote at full (FR, Control) and half rate (HR) using commercial and natural mycorrhizal sources for each treatment. They were compared to FR and HR treated plants. Two sets of treated plants were arranged in a RBCD with four replications at two commercial nurseries in southern New Jersey. One set was infected with the pathogen Phytophthora cinnamomi after plant establishment, and the other set was left without infection for comparison. Both sets were irrigated using overhead irrigation during the growing season. Plant top and root biomass of both ‘DVW’ and ‘SS’ cultivars was reduced in plants infected with P. cinnamomi and was more pronounced in ‘DVW’ plants that were grown in bark-based substrate media with FR fertilizer irrespective of the mycorrhizal inoculum source. The incorporation of natural mycorrhizae not only improved ‘DVW’ plant biomass at HR fertilizer in comparison to other treatments but also reduced the severity of pathogen root infection. Overall mycorrhizal colonization of DVW roots was greater than for SS. There was little difference between mycorrhizal colonization between fertilizer treatments as all plants became mycorrhizal. However, it appears that natural inoculum (containing a complete mix of mycorrhizae and bacteria) had a more stimulatory effect on plant growth than commercial inoculum.