Effects of benomyl and captan on growth and mycorrhizal colonization of Sitka-spruce (Picea sitchensis) and ash (Fraxinus excelsior) in Irish nurserysoil

The effects of two fungicides (benomyl and captan - at recommended doses an d up to three soil drenches) on root development and mycorrhizal colonizati on of Sitka-spruce (Picea sitchensis) and ash (Fraxinus excelsior) were exa mined after 20 weeks grow th, under environmentally controlled conditions, in soil from an Irish tree nursery. Although four mycorrhizal types have be en found on Sitka-spruce at the nursery, only one ectomycorrhizal type (Pic eirhiza horti-inflata) was identified on the short roots in this study. An inoculant (Vaminoc: MicroBio Ltd, Hemel Hempstead, UK) was used to inoculat e ash and 20-week-old seedlings had a higher arbuscular mycorrhizal (AM) co lonization in comparison with uninoculated controls. Multiple applications (2-3) of benomyl reduced the length of root and shoot and shout dry mass of Sitka-spruce, whereas in ash, it only depressed root length. Benomyl decre ased the numbers of ectomycorrhizas of Sitka-spruce and arbuscular mycorrhi zal colonization of Vaminoc-inoculated ash. A single application of captan stimulated root length and ectomycorrhizal colonization of Sitka-spruce and root dry mass in ash compared a ich uninoculated controls. Applications of captan reduced arbuscular mycorrhizal colonization of Vaminoc-inoculated a sh to levels near to those of uninoculated controls. Of the two fungicides used, benomyl had the most deleterious effect on root length and mycorrhiza l colonization of Sitka-spruce and ash.