Investigating the use of marigolds (Tagetes spp.) and brassicaceous cover crops as a tool for the management of lesion nematodes in apple orchards. Phase I - Pot trials.

Lesion nematodes (Pratylenchus spp.) are the most common plant-parasitic nematodes present in apple orchards in South Africa. Apple trees, especially younger trees, infected with lesion nematodes have poor growth and yield declines gradually. As yet, there are no effective alternative control measures available for nematode control in apple orchards. Marigold (Tagetes spp.) is one of the most widely studied plant genera due to its allelopathic potential against plant-parasitic nematodes (PPN), yet limited cover cropping with marigold is being practiced in commercial perennial cropping systems. The biofumigation technique which involves the incorporation of mechanically chopped brassicaceous plant material into the soil to control soilborne nematodes offers an attractive alternative for PPN management. In this study we will investigate the efficacy of marigolds and brassicaceous cover crops as pre-plant treatments for diminishing lesion nematode populations when replanting apple orchards, as well as their ability to reduce lesion nematode populations in infected apple plants.

Root-lesion nematodes were cultured using the in vitro carrot disc method. Plants were inoculated and maintained in a glasshouse, set at 25°C. Treatments consisted of co-planting and incorporation of a range of biofumigation and allelopathic cover crops. Five replicates of each treatment were covered with a transparent plastic bag to create a microbiome or solarization effect, while 5 were left uncovered. Each treatment was replicated 5x and arranged in a randomized block design. Nematode numbers were assessed before, during and after each experiment. Nematodes were extracted from roots and soil using an adapted sugar flotation method.

After growing apple rootstocks in treated soils for 12 weeks, significant differences occurred between treatments, especially between soil subjected to solarization and those without. Without solarization, the only treatment that had significantly less RLN per gram roots than the fallow control was the T. patula treatment. With solarization, numbers of RLN in all the treatments, except T. patula, were significantly lower than those in the fallow treatment. When infected apple rootstocks were treated, all the treatments, except T. patula significantly reduced nematodes over the course of the season. Tagetes erecta established well in a commercial apple orchard and was able to reduce the RLN populations in the soil and root micro-environment of apple trees.

Biofumigation with both brassicaceous cover crops and marigolds demonstrates effective RLN suppression in infested soil, resulting in reduced infection in apple roots, given soil temperatures are conducive to biofumigation (25°C – 35°C). RLN suppression through biodisinfestation is significantly more effective than biofumigation alone, under the same environmental conditions as biofumigation. Co-planting brassicaceous cover crops with an apple rootstock infected by RLN and macerating and incorporating aerial plant parts of these crops into the soil as a biofumigant results in reduced reproduction of RLN and under favourable conditions. Using marigolds as a cover crop in a commercial apple orchard, had a suppressive effect on RLN population densities in this apple orchard.

This study provides comprehensive evidence to support the use of marigolds and brassicaceous cover crops as a tool for managing RLN in apple orchards. Potential applications arising from this study includes the use of marigolds and brassicaceous plants as pre-plant treatments for diminishing lesion nematode populations when replanting apple orchards, as well as intercropping allelopathic plants to reduce lesion nematode populations in infested orchards.