Occurrence of root-lesion nematodes (Pratylenchus thornei and P. neglectus) and stunt nematode (Merlinius brevidens) in the northern grain region of Australia

Pratylenchus thornei, P. neglectus and Merlinius brevidens were respectively present in soil from 67, 32 and 73% of 795 wheat fields sampled in the northern grain region of Australia. Population densities of P. thornei (31% of samples with >2000 nematodes/kg soil) were higher than those of P. neglectus (5%) or M. brevidens (11%). The mean proportion of plant-parasitic to total nematodes was 0.40. The incidence of P. thornei and P. neglectus increased significantly with the number of years that fields had been cropped since clearing natural vegetation. All three species occurred in soil samples across a wide range of texture and pH. Maximal incidence of P. thornei was in heavier textured soils (48% clay, 37% sand) than P. neglectus (39% clay, 45% sand). The incidence of M. brevidens increased linearly with increasing clay and decreasing sand contents. M. brevidens occurred in the pH range 6.0-9.5 and the Pratylenchus spp. in the range 6.5-9.5, with maximal incidence of P. neglectus at pH 8.2, P. thornei 8.4 and M. brevidens 8.8. Population densities of P. neglectus and especially non-parasitic nematodes were higher in the 0-15 cm soil layer than 15-30 cm. Overall the soil conditions of the northern grain region appear more favourable for P. thornei and M. brevidens than for P. neglectus.