Title : Influence of crop rotation and soil temperature on the trophic levels of nematodes and population densities of Heterodera carotae in a highveld carrot producing region of South Africa
Abstract:
The carrot cyst nematode, Heterodera carotae Jones, 1950 has been recorded across the European carrot growing regions, along with the Americas and Africa. Estimated carrot crop losses related to H. carotae damage range from 20 to 90% across the globe. Since the host range of this nematode is limited to cultivated- and wild carrots, crop rotations are economically and ecologically preferred management strategies, but with limited empirical support data. Following much deliberation on crop rotation cost-benefits, carrot monoculture, brassica, sorghum, wheat and fallow were chosen to monitor the changes in the populations of H. carotae under field conditions in the Highveld region of South Africa during the 2024/2025 seasons. the aim of the study was to assess whether the use of carrot-monoculture, Brassica sp., sorghum, wheat and fallow in context of crop rotation will reduce nematode population densities of H. carotae and other nematodes under field conditions and further accessing whether seasonal nematode biodiversity would enhance the possibility of predicting yield in carrot production. The trials were conducted on a 2-ha carrot field in the Tarlton area, with a Hutton Soil Form and a Loamy Sand Soil. Initial nematode population density was established during the first year of planting to carrots. randomised complete block design with four replications. Four crop rotation cycles, namely carrot, Brassica species, wheat and fallowing with six unknown IP treatments. Soil samples were collected weekly from emergence to harvest or incorporation for 2024/2025 seasons. Ten subsamples were taken per block and combined to make up a composite sample which was eventually processed to extract nematodes. Soil temperatures with two probes were measured at 20 cm depth, using an Aquacheck probe under fallow and crop blocks for the duration of the experiment. During three seasons (winter, spring and summer) of the first year all plots were planted with carrots. During the second year, plots were planted to either brassica, wheat or left fallow, depending on the rotation cycle applied. During the third year, carrots will be cultivated as sole crop during summer, winter and spring. The cultivar Allyance will be used during summer whilst the cultivar Soprano will be used in winter and spring. Generalized Linear Mixed Models with Template Model Builder (glmm TMB) showed that crop rotation had a significant influence on nematode populations across the 2024/2025 seasons. Heterodera carotae was the most dominant plant parasitic species across all crop rotations. Average monthly temperatures had significant influence on the populations of H. carotae, where for every unit increase of temperature, there was a corresponding decrease in the population. Furthermore, the biodiversity of nematode trophic levels was also established where PPN were the most prominent. This were observed across four rotation cycles, with bacterial and fungal feeders contributing respectively. In Conclusion brassica species could be used in crop rotation with carrot to manage population densities of H. carotae and average monthly temperature has shown to have a significant influence on the population of H. carotae, where for every unit increase of temperature, there was a corresponding decrease in population under different treatments.
