Developing a partial resistance test for fodder radish (Raphanus Sativus var. olieformis) against meloidgyne chitwoodi

Since its detection in The Netherlands in 1990, M. chitwoodi became a major threat to seed and ware potato production. An integrated approach to control this nematode is needed and first steps have been taken. One of these is the availability of resistant green manure crops. The EU project QLRT-­-1999-­-1462 (DREAM), proved the feasibility to produce resistant green manure crops against M. chitwoodi and M. fallax. As a result, breeders in The Netherlands and Germany introduced partially resistant fodder radish varieties. Partial resistance in fodder radish is the result of a mixture of susceptible and resistant seeds. Current methods do not reflect the real host-­-status of these varieties, they provide resistance based on Pf/Pi ratio, which is known to be density dependent. Moreover, they are tested in an artificial system that ignores the relevant conditions set by the population dynamics of this nematode. Therefore, the development of a reliable method to estimate the partial resistance of these fodder radish varieties is of major importance. Teklu et al., (2014) indicated that studying the population dynamics using a range of Pi values and a specific experimental setup made it possible to estimate partial resistance of fodder radish. A second experiment with two new varieties (Melotop and POR1101), two previously tested ones (Defender and Contra) and two standard controls (Radical and Siletina) was carried out to ascertain the reproducibility of the first results, check the Pi independence of the partial resistance estimator and assess the proportion of resistant seeds in the seed mixture. Ultimately, this research has to lead to a simplified screening method at one/two Pi densities while maintaining the validity and precision of the estimator. The biomass of all plant parts were assessed and correlated with the Pi to investigate the magnitude of any negative effect of plant growth on the sanitary effect of this green manure. Not all tested varieties showed tolerance to M. chitwoodi. Minimum yield (m) varied between 0 and 0.8. The number of roots with galls increased with Pi and was highest in both controls. The results confirm the use of population dynamic models to define partial resistance. Maximum multiplication rates (a) and maximum population densities (M) were very low. The two standard varieties, Radical and Siletina, (a = 0.116 and 0.136 and M = 1.85 and 1.12 J2 (g dry soil)-­-1, respectively), proved to be bad hosts. Partial resistance based on the more reliable parameter M was high (RS m <1%) for tested varieties. RSa and RSM 85 were not always equal, as estimation of a was difficult, due to the low nematode counts and higher standard errors at low Pi values of the resistant varieties. Possibilities to use fewer nematode densities will be discussed.