Predicting the slow decline of root lesion nematodes (Pratylenchus thornei) during host-free fallow to improve farm management

Pratylenchus thornei is a major pathogen of cereal and legume crops around the world, especially in the northern grains region of eastern Australia. The dominance of host species within the rotation has seen soil pathogen population densities increase. Long weed-free fallows combined with sorghum production (non host crop) to reduce population densities has been successful. However, little is known about the rate of population decline during the fallow or how long this non–host period should continue in order to reduce the population below an accepted damage threshold. The rate of decline from a range of initial starting populations (high, medium, low and very low) were monitored over a 30 month weed free fallow. Fallows were initiated in November (late Spring) for three consecutive years. Nematode population densities and soil moisture were measured at eight depths down the soil profile to 1.5 m and used to describe the rate of population decline over time in each soil layer. Dynamic populations of P. thornei existed within the upper layers (<0.6 m)of the soil and these declined at a rate that could be described by the negative exponential model Y = ae−bt. The time taken for a population to decline was dependent on the initial population density at harvest of the previous host crop. Generally, between 300–600 days of host-free fallow was required to reduce a moderately high initial population of 80P. thornei/cm3 soil to the damage threshold of 2 P. thornei/cm3. The rate of decline varied between soil layers, particularly in the surface layer (0–0.15m), but remained constant from year to year for each layer. There was no interaction between year and soil layer. Knowing the expected rate of decline of a P. thornei population at the start of a fallow allows better management of the crop rotation to ensure populations do not continue to rise and thus reduce the yield potential of future crops.