Viability of VA-mycorrhizal fungi following soil solarization and fumigation

Two field experiments were conducted to examine the effect of soil solarization on the survival of arbuscular mycorrhizal (AM) fungi and root colonization of three crops. The experiments were carried out in a loamy sand soil (Rehovot) and a silty soil (Bet She'an Valley). For both experiments, assessment of indigenous AM fungal populations by the most probable number (MPN) method indicated that populations were reduced to zero after 2 or 4 weeks of solarization treatment. However, Glomus intraradices inoculum applied to the soil prior to solarization remained viable even after 8 weeks of solarization. After soil fumigation with methyl bromide both indigenous and applied AM fungi were nondetectable. Percentage root colonization by the indigenous AM fungal populations, together with plant-growth parameters, were assessed for three crops: onion and wheat (Rehovot), and carrot (Bet She'an). When sown on solarized field plots, onion and carrot seedlings showed a plant growth retardation, whereas wheat showed an increased growth response. Root colonization by indigenous AM fungi was not evident until 6 weeks after seedling emergence. Fumigation with methyl bromide reduced root colonization by indigenous AM populations, and reduced onion and wheat plant development at early growth stages. In a laboratory experiment, a temperature of 45° C for up to 24 h did not affect AM spore viability, indicating that temperatures reached during the solarization treatment cannot solely account for the reduced AM fungi viability in the field. Apparently, soil solarization temporarily delays root colonization by indigenous AM fungi until 6-8 weeks after plant emergence.