Methodology development for partial resistance testing of potato cultivars resistant to M. chitwoodi

M. chitwoodi was first described in 1995 in The Netherlands (Karssen, 1995). It is now listed as a quarantine organism in the EPPO region with 4 EU member states officially infested. Since 1996, research has been initiated to identify resistant genes against M. chitwoodi from wild species of tuber bearing potatoes and integrate these genes into cultivated potatoes. Currently, several breeding companies successfully produced resistant genotypes against M. chitwoodi. Parallel to this, research was started to develop a standard methodology to screen the partial resistance of these genotypes. Population dynamical models were used to estimate their level of resistance, expressed as percentage of relative susceptibility (rs). This methodology provides farmers with quantitative information on the effect of growing resistant potatoes at any initial population density in their field. The models were first tested in a pilot project in 2010 with 3 resistant potato genotypes in (5 kg) pots at a range of 13 nematode densities. In 2011, another 8 genotypes were tested in (10 kg) pots at a range of 12 densities. The results showed that Seinhorst’s population dynamical models for nematodes with multiple generations fitted well, except in one genotype tested in 2011which lacked resistance to M. chitwoodi, and a reduction of the number of densities used seems possible. In 2012, research was initiated to investigate whether the pot size can be downscaled from 10 to 5 or even to 2 kg pots – also at 12 densities -, without loss of quality of the estimated relative susceptibility. Also, growth, yield loss and quality damage as root knot index (RKI) were assessed and compared. Genotypes 2011M1, MDG2 and cv. Désirée (control) were the tested potatoes. The population dynamical model fitted well for the genotypes tested. The maximum multiplications rate “a” and the maximum population density “M” at 2, 5 and 10 kg pots were estimated and used to calculate the rsa and rsM values . Despite a decrease in “a” and “M” values with increasing pot size the rsa values were relatively stable. The rsM values were a bit higher in 5 kg pots. Seinhorst yield models used to describe the fresh tuber weight also fitted well. The RKI values obtained from the three pots sizes were also stable as a quality measure for industrial processing. Overall results indicate the possibility of downscaling the resistance test for M. chitwoodi in potato in terms of pot size and number of densities. Implications of the current research in the development of a cheap and reliable resistance test will be discussed.