Effect of initial nematode population density on the interaction of Pratylenchus penetrans and Verticillium dahliae on 'Russet burbank' potato

Four similar growth chamber experiments were conducted to test the hypothes is that the initial population density (Pi) of Pratylenchus penetrans influ ences the severity of interactive effects of P. penetrans and Verticillium dahliae on shoot growth, photosynthesis, and tuber yield of Russet Burbank potato. In each experiment, three population densities of P. penetrans with and without concomitant inoculation with V. dahliae were compared with nem atode-free controls. The three specific Pi of P. penetrans tested varied fr om experiment to experiment but fell in the ranges 0.8-2.5, 1.8-3.9, 2.1-8. 8, and 7.5-32.4 nematodes/cm(3) soil. Inoculum of V. dahliae was mixed into soil, and the assayed density was 5.4 propagules/gram dry soil. Plants wer e grown 60 to 80 days in a controlled environment. Plant growth parameters in two experiments indicated significant interactions between P. penetrans and V. dahliae. In the absence of V. dahliae, P. penetrans did not reduce p lant growth and tuber yield below that of the nematode-free control or did so only at the highest one or two population densities tested. In the prese nce of V. dahliae, the lowest population density significantly reduced shoo t weight and photosynthesis in three and four experiments, respectively.. H igher densities had no additional effect on shoot weight and caused additio nal reductions in photosynthesis in only one experiment. Population densiti es of 0.8 and 7.5 nematodes/cm(3) soil reduced tuber yield by 51% and 45%, whereas higher densities had no effect or a 15% additional effect, respecti vely. These data indicate that interactive effects between P. penetrans and V. dahliae on Russet Burbank potato are manifested at P. penetrans populat ion densities less than 1 nematode/cm(3) soil and that the nematode populat ion density must be substantially higher before additional effects are appa rent.