Damage Threshold of Meloidogyne hapla to Lettuce in Organic Soil

Lettuce was seeded in pots in the greenhouse and in field microplots in 1991 and 1992. Pots and microplots were filled with untreated or fumigated organic soil infested with Meloidogyne hapla at seven initial population densities (Pi) (0 to 32 eggs/cm(3) soil). Lettuce weight, severity of root galling, and number of eggs per root system (Pf) were determined after 8 weeks. At the highest Pi, M. hapla caused yield losses up to 64% in the microplots and plant death in the greenhouse tests. The Seinhorst equation was used to describe the relation between lettuce weight and Pi (r(2) = 0.73 - 0.98) and to calculate the damage threshold density (T). Values of T were 7 and 8 eggs/cm(3) soil in the greenhouse tests of 1991 and 1992, respectively. In the microplot tests, T was 1 egg/cm(3) soil in 1991 and 2 eggs/cm(3) soil in 1992. The damage threshold was the same in untreated and fumigated soils. At low Pi, root galling was more severe in the pots than in the microplots. Pf increased with increasing Pi of M. hapla in both tests, but declined at Pi > T in the greenhouse tests. The reproduction rate (Pf/Pi) of M. hapla was highest at the lowest Pi.