Mycorrhiza development in onion in response to inoculation with chitindecomposing actinomycetes

SUMMARY Twelve actinomycetes, originally isolated from spores of Glomus macrocarpum Tul. & Tul., were inoculated into nonsterilized soil containing G. macrocarpum and G. mosseae (Nicol. & Gerd.) Gerd. & Trappe. The chitin‐decomposing actinomycetes were strongly antagonistic toward fungi, bacteria or actinomycetes, or combinations of these based on prior studies. Eight Streptomyces , two Nocardia , and two unidentified species were tested for their effects on mycorrhiza development in onion ( Allium cepa L.), plant growth and nutrient content, soil hyphal density, and soil populations of bacteria and actinomycetes. None of the actinomycetes adversely affected plant growth or nutrient content. The lower foliar P content of two of the treatments could be explained as a dilution effect brought about by the significantly larger shoot growth. With few exceptions, the soil microbial populations and saprophytic fungal hyphal densities were not significantly different from the controls. Seven of the twelve actinomycetes significantly increased the percent mycorrhizal root colonization and four significantly increased the density of hyphae > 5μm in diameter, predominantly those of vesicular‐arbuscular mycorrhizal (VAM) fungi. This is the first study to demonstrate that actinomycetes isolated from spores of a VAM fungus can be beneficial to mycorrhiza development and growth of onion in a nonsterile soil system.