Resistance mechanisms against the root-rot nematode (Meloidogyne chitwoodi) in fodder radish (Raphanus sativus)

The root-knot nematode Meloidogyne chitwoodi, a quarantine organism in many European countries, can cause severe yield reductions in various important crops such as maize, potato, tomato, sugar beet, carrot and salsify. Controlling this nematode is difficult because of the lack of usable pesticides. By introducing a nematode-resistant green manure crop such as fodder radish (Raphanus sativus) in the crop rotation, the nematode population in the soil can be lowered to diminish the disease pressure in the following crop. Although various fodder radish cultivars with elevated resistance against M. chitwoodi have been released, breeding for resistance remains hampered by the lack of knowledge on the resistance mechanism. Therefore, we investigated the resistance mechanism of R. sativus to M. chitwoodi in this paper. Twelve populations were studied: three resistant R. sativus cultivars, five R. sativus populations with average or unknown resistance, two susceptible R. sativus cultivars, one Eruca sativa cultivar and one susceptible yellow mustard (Sinapis alba) cultivar. Seedlings all populations were inoculated with J₂ juveniles at age one week. After 5 days, we studied whether the J₂ juveniles entered the roots. After 12 days, the formation of a feeding site was studied while after 5 weeks the development of adult females was evaluated. Finally, we the number of produced egg masses was counted after 8, 10 and 13 weeks, as resistant plants may slow egg production down. Observing the disease development from infection to the production of egg masses in susceptible and resistant populations indicated what the mechanism of resistance may be. Our results may lead to a more rapid development of fodder radish cultivars with an elevated or possibly more durable resistance against the root-knot nematode.