Effects of density of Helicotylenchus dihystera and Pratylenchus vulnus on American boxwood growing in microplots.

American boxwood, Buxus sempervirens var. globosum, was tolerant of Helicotylenchus dihystera [in field microplots] as measures of plant growth were similar to the control and nematode densities were maintained at high levels (1,705-1,810/500 cm(3) soil after 29 months). Boxwood was intolerant of Pratylenchus vulnus at initial densities of 163, 281, or 475 nematodes per 500 cm(3) soil. In comparisons with those of controls, vigor ratings of boxwood after 14 months were much lower at all densities of this nematode. Nematode density was not directly related to vigor rating. However, initial nematode density was directly proportional to growth suppression of boxwood as measured by the difference of the product of final plant height x width or by the difference of the plant surface area determined from a standardized photograph as compared to those of controls. A nematode density of 160/500 cm(3) of soil was found to suppress growth by 50%. Populations of P. vulnus declined, according to a linear function, with time after reaching over 7,100 nematodes/500 cm(3) of soil taken from the root zones of boxwoods. Ninety-five percent of the P. vulnus population died between 15 and 20 months after soil infestation.