Nematode-degree days, a density-time model for relating epidemiology and crop losses in perennials

The impact of Meloidogyne hapla on alfalfa (Medicago sativa) yield was described by a multiple point damage model as a function of current plant status, cumulative pest stress, and crop history. Nematode-degree days (NDD(female symbol)), calculated on a physiologic time scale as total area under the adult female population density curve, were used to express M. hapla parasitism as cumulative nematode dosage. NDD(female symbol) increased exponentially over physiological time at rates relative to M. hapla initial population density of eggs and second-stage juveniles (Pi). Dosage accumulation rates, varying from 213 to 272 NDD(female symbol) per degree day, did not differ (P = 0.05) among six Pi levels. Cumulative yield reductions, increasing linearly with NDD(female symbol) over time to 50% of the M. hapla-free controls, were well correlated for the six initial Pi levels (r(2) = 0.93). Progressive reductions in alfalfa yields to 65% of the nematode-free controls, reflective of the prolonged exposure of the crop to M. hapla, were adequately described by NDD model estimates of either combined population densities of eggs and juveniles or adult females. Cumulative area under the combined eggs (e) and juvenile (J) population curve (NDD(e+J)) and NDD(female symbol) were linearly related (r(2) = 0.97).