Relationships of initial population densities of Meloidogyne incognita and M. hapla to yield of tomato.

Microplots 80 x 100 cm, infested with varying initial population densities (P(i)) of Meloidogyne incognita or M. hapla, were planted to tomato at two locations. Experiments were conducted in a sandy loam soil at Fletcher, N. C. (mountains) where the mean temperature for May to September is ca 20.7 C, and in a loamy saml at Clayton, N. C. (coastal plain) where the mean temperature for May to Septemher is ca 24.8 C. In these experimentally infested plots, M. incognita and M. hapla caused maximum yield losses of 20-30%, at lhe mountain site with Pi of 0-12,500 eggs and larvae/500 cm(3) of soil. In the coastal plain, M. incognita suppressed yields up to 85%, and M. hapla suppressed yields up to 50% in comparison with the non infested control. A part of the high losses at this site apparently was due to M. incognita predisposing tomato to the early blight fungus. In a second experiment, in which a nematicide was used to obtain a range of P(i)s (with P(i) as high as 25,000/50 cm(3) of soil) at Fletcher, losses due to M. incognita were as great as 50%, but similar densities of M. hapla suppressed yields by only 10-25%. Approximate threshold densities for both species ranged from 500 to 1,000 larvae and eggs (higher for surviving larvae) for the mountain site, whereas numbers as low as 20 larvae/500 cm(3) of soil of either species caused significant damage in the coastal plain. Chemical soil treatments proved useful in obtaining various initial population densities; however, problems were encountered in measuring effective inoculum after such treatments, especially in the heavier soil.