SURVEY OF VESICULAR-ARBUSCULAR MYCORRHIZAE IN LETTUCE PRODUCTION IN RELATION TO MANAGEMENT AND SOIL FACTORS

The occurrence of vesicular-arbuscular mycorrhizae (VAM) root coloniza tion and spore number in soil was assessed for 18 fields under intensi ve lettuce (Lactuca sativa L.) production in California during July an d August of 1995. Data on management practices and soil characteristic s were compiled for each field, and included a wide range of condition s. The relationship between these factors and the occurrence of VAM in these fields was explored with multivariate statistical analysis. VAM colonization of lettuce tended to decrease with the use of chemical i nputs, such as pesticides and high amounts of P and N fertilizers. Add ition of soil organic matter amendments, the occurrence of other host crops in the rotation, and soil carbon:phosphorus and carbon:nitrogen ratios, were positively associated with VAM colonization of lettuce ro ots. The number of VAM spores in soil was strongly correlated with the number of other host crops in the rotation, the occurrence of weed ho sts and sampling date, but was more affected by general soil condition s than by management inputs. Higher total soil N, C and P, as well as CEC, were inversely related to soil spore number. A glasshouse study o f the two primary lettuce types sampled in the field showed no signifi cant differences in the extent of root colonization under similar grow ing conditions. The results of this study are compared with other stud ies on the effects of management and soil conditions on mycorrhizal oc currence in agriculture.