Modelling population densities of root-lesion nematode (Pratylenchus thornei) from soil profile temperatures to choose an optimum sowing date for wheat in a subtropical region

The root-lesion nematode Pratylenchus thornei is widely distributed in many wheat growing countries and is particularly damaging to wheat in subtropical environments. This study aimed to investigate by simulation the effects of soil profile temperatures after different sowing dates on reproduction of P. thornei in susceptible and moderately resistant wheat cultivars in the subtropical grain region of eastern Australia. A quadratic regression model relating P. thornei population densities to thermal time was produced from experimental data for susceptible, intolerant wheat cv. Gatcher and moderately resistant, tolerant wheat cv. GS50a, and applied to soil profile temperatures after four sowing dates from 25 April to 24 July. Simulated final population densities of P. thornei throughout the soil profile to 60 cm depth were least at 18 weeks after 25 May sowing. For Gatcher and GS50a, respectively, there were 5.7 and 3.5 times as many nematodes after 24 July sowing as after 25 May sowing, 2.0 and 1.6 after 24 June, and 1.4 and 1.3 after 25 April sowings. GS50a had 78% (May) to 86% (July) fewer nematodes in the soil profile than Gatcher. These simulations indicated that an optimum sowing date can be chosen to limit P. thornei reproduction in both susceptible and moderately resistant wheat cultivars. Gatcher had a 61% increase in measured grain yield and GS50a had an 8% increase from sowing in late May rather than in the third week of June. Sowing wheat at an optimum time in subtropical grain regions to ensure roots grow in cool soil can be a useful component of integrated management by reducing the rate of P. thornei reproduction and increasing grain yield.