VIABILITY OF VA-MYCORRHIZAL FUNGI FOLLOWING SOIL SOLARIZATION AND FUMIGATION

Two field experiments were conducted to examine the effect of soil sol arization on the survival of arbuscular mycorrhizal (AM) fungi and roo t colonization of three crops. The experiments were carried out ina lo amy sand soil (Rehovot) and a silty soil (Bet She'an Valley). For both experiments, assessment of indigenous AM fungal populations by the mo st probable number (MPN) method indicated that populations were reduce d to zero after 2 or 4 weeks of solarization treatment. However, Glomu s intraradices inoculum applied to the soil prior to solarization rema ined viable even after 8 weeks of solarization. After soil fumigation with methyl bromide both indigenous and applied AM fungi were nondetec table. Percentage root colonization by the indigenous AM fungal popula tions, together with plant-growth parameters, were assessed for three crops: onion and wheat (Rehovot), and carrot (Bet She'an). When sown o n solarized field plots, onion and carrot seedlings showed a plant gro wth retardation, whereas wheat showed an increased growth response. Ro ot colonization by indigenous AM fungi was not evident until 6 weeks a fter seedling emergence. Fumigation with methyl bromide reduced root c olonization by indigenous AM populations, and reduced onion and wheat plant development at early growth stages. In a laboratory experiment, a temperature of 45 degrees C for up to 24 h did not affect AM spore v iability, 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 colo nization by indigenous AM fungi until 6-8 weeks after plant emergence.