The need for a strategic perspective in crop rotation

Free-living plant-parasitic nematodes (PPN), including migratory endoparasites such as Pratylenchus spp., cause yield reduction in agriculture and horticulture world-wide. In Norway, nematicides are banned due to their adverse effect on human health and the environment. Thus, management of plant-parasitic nematodes rely on cultural practices, such as crop rotation. Free-living PPN tend to have broad host-ranges, which complicates the design of effective crop rotations. Information on the reproductive rate and damage potential of nematode species on different crops is of crucial importance when designing a successful crop rotation. Results from several experiments indicate that in order to reduce the numbers of free-living PPN, the sequence of crops is more important than the length of the rotation. The crop rotation should aim at protecting the most economically valuable crop. An oat (Avena sativa) field in Norway was heavily infested with Tylenchorhynchus dubius (1200 ind/250 ml soil) and a low population of Pratylenchus crenatus (10 ind/250 ml soil). The primary goal was to reduce T. dubius by growing turnip rape (Brassica rapa ssp. oleifera), with carrot as the following crop. T. dubius was reduced with 77-85% after turnip rape. In contrast, the population of P. crenatus increased by more than tenfold. The increased numbers of P. crenatus could be damaging to the carrot crop. This illustrates that crop rotation should be a long-term strategy, with carefully designed rotations to protect the most economically valuable crop (e.g. carrot). This also illustrates the challenges of designing a crop rotation that effectively reduces multiple nematode populations. In a started project, we will use photography with Unmanned Aerial Vehicles and transects to monitor nematode populations and damage in several fields throughout the growing season, and over several seasons. These fields will serve as naturally occurring experiments. We want to develop decision-making tools for nematode management in Norway.