Infection and Colonization of Potato Roots by Verticillium dahliae as Affected by Pratylenchus penetrans and P. crenatus

Factorial greenhouse experiments were conducted to test hypotheses concerning the nature of the interaction between Verticillium dahliae and Pratylenchus penetrans or Pratylenchus crenatus in potato early dying. Soil was infested with known densities of V. dahliae and P. penetrans or V. dahliae and P. crenatus, and plants were destructively harvested 3, 5, 7, and 9 weeks after planting. Root samples were excised using a grid method and stained using an immunoenzymatic procedure. Root infection by V. dahliae was through the root tip and the zone of elongation, regardless of the presence of nematodes. Discrete colonies in the cortex were not observed, but rather the hyphae progressed into the cortex and grew towards the developing vascular tissues. Hyphae penetrated and grew in the young xylem elements and then colonized the vascular cylinder, or they were prevented from further development by a visible host response. Five weeks after planting, roots grown in soil infested with V. dahliae alone had a very low percentage of infected root tips (1.2%), which was significantly less (P < 0.02) than infected roots growing in soil infested with either V. dahliae and P. penetrans or V. dahliae and P. crenatus (2.3 and 2.5%, respectively). Roots were colonized by V dahliae to a significantly greater extent (P < 0.01) when grown in soil infested with V. dahliae and P. penetrans (0.13 cm of colonized root/m of root) than in soil infested with V. dahliae alone or with V. dahliae and P. crenatus (0.05 and 0.02 cm of colonized root/m of root, respectively). This trend continued with differences among treatments more pronounced after 7 weeks. Low levels of initial root infection and colonization resulted in high incidences of stem colonization by V. dahliae in treatments with V. dahliae and P. penetrans (58% after 5 weeks and 100% after 7 weeks). Infection by V. dahliae was not observed to be associated with the site of nematode feeding, and the effect of nematodes on initial infection may not be species-specific. The interaction between V. dahliae and P. penetrans in potato early dying may occur within the root early in the infection process, resulting in an altered or delayed host response to colonization by V. dahliae.