| Summary: | This research has focused on the determination of natural populations in the fields, the effect of different inoculum densities on lettuce growth and a study of the association of this fungus with two nematodes (Pratylenchus penetrans Cobb and Meliodogyne hapla Chitwood). Under conditions of artificial infestation of soil the results were satisfactory, but in trials with naturally infested soil the fungus could not be detected. The effect of different inoculum densities was measured at different stages of growth, and only in those plants inoculated 2 weeks after seeding were differences significant and consistent. Some evidence of the detrimental effect of wounding the root system prior to attack by the fungus led to studies of the relationship between this fungus with either P. penetrans or M. hapla. In the first case a negative interaction seemed to exist; no significant increase of the damage caused to the lettuce was observed. In contrast, when the root-knot nematodes and P. tracheiphilum were combined there was a marked reduction of lettuce growth. The interaction was found to be additive.
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