minor and are similar to those caused by M. chitwoodi
and M. fallax on potato; this was also reported by de
Weerdt et al. (2011). However, the damage threshold
that we determined for M. minor on potato is markedly
higher compared with M. chitwoodi and M. fallax. Based
on the Seinhorst model we found a threshold of Pi =
41 J2 (100 cm3 soil)−1. Santo et al. (1981) reported a
damage threshold of less than 1 J2 (250 cm3 soil)−1 for
M. chitwoodi on potato cv. Russet Burbank and in The
Netherlands a threshold of 10 J2 (100 cm3 soil)−1 is
applied for this species (Norshie et al., 2011). For M.
chitwoodi it was shown in a field experiment that degreeday
accumulation during the growing season was more
important than initial population densities for damage
development on potato tubers cv. Russet Burbank (Griffin,
1985). The damage is caused by the second- and thirdgenerations,
which are formed when tubers are present.
Possibly, the same reasons apply for damage of M. minor
on tubers. The damage was determined after 3 months in a
pot test giving ample time for at least a second generation
to develop