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Soil water pressure and Verticillium dahliae interactions on potato.

Foliar and root growth of potato (cv. Russet Burbank) and colonization of roots and stem vascular sap by Verticillium dahliae were monitored weekly for 4 weeks beginning at emergence in soils maintained at different water pressures. A significant soil water pressure x V. dahliae interaction resulted in reduced aerial biomass and increased root-to-shoot ratio under excessive soil moisture conditions (-0.01 MPa). At -0.01 MPa, the percent increase in root-to-shoot ratio of plants grown in infested compared with noninfested soil ranged from 40 to 82% over 2 years. In contrast, both root length and colonization of roots by V. dahliae were significantly suppressed at -0.01 MPa compared with the lowest soil water pressure (-0.15 MPa). Roots of plants grown at -0.01 compared with -0.15 MPa were 19.6 to 44% shorter at emergence, and suppression of root growth at this high soil water pressure persisted through the observation period. Root colonization by V. dahliae was suppressed consistently at -0.01 compared with -0.15 MPa. Reduction in CFU per 100 cm of root length for these respective soil water pressures ranged from 60 to 100%. Soil water pressure had no effect on the population size of V. dahliae in stem sap. High soil water pressures may enhance potato early dying caused by V. dahliae by slowing root growth and/or by indirectly increasing the rate of microsclerotial germination. Both would facilitate entry of the pathogen into the stele through the vulnerable tissue of the root apex.