DISTRIBUTION IN THE WESTERN UNITED-STATES ON ALFALFA AND CULTIVAR REACTION TO MIXED POPULATIONS OF DITYLENCHUS-DIPSACI AND APHELENCHOIDES RITZEMABOSI

Ditylenchus dipsaci and Aphelenchoides ritzemabosi were extracted from 29 of 40 plant samples (72.5%) collected from Arizona, California, Co lorado, Idaho, Montana, Oregon, South Dakota, Utah, Washington, and Wy oming, Percentages of A, ritzemabosi in tissue of the 29 samples range d from 1.77 to 67.82%. Only Ditylenchus dipsaci was recovered from the remaining 11 samples. All of the 16 fields sampled in Wyoming contain ed both nematodes. Percentages of A. ritzemabosi in the Wyoming sample s ranged from 0.7-30.0%, with an overall mean of 10.3%. Individual pla nts collected from a field in Big Horn, Wyoming, all contained both ne matodes. Percentages of A. ritzemabosi in tissue ranged from 5-70%. Al falfa stem nematode symptomatic plants in 17 of 18 alfalfa cultivars c ollected from a screening nursery in California contained both nematod es, of which 10-94% were A. ritzemabosi. Only one cultivar had D. dips aci only, and no entries had A. ritzemabosi only. Under environmentall y controlled conditions, A. ritzemabosi reproduced in all nine alfalfa cultivars tested at 6 weeks of age with a mean reproductive factor (f inal population/initial population) of 4.1. There were more (P less th an or equal to 0.05) A. ritzemabosi in stem and bud tissue of the susc eptible cultivars at harvest than in the resistant cultivars with comb ined cultivar means of 238, 42,78, and 4 A. ritzemabosi/g tissue for t he susceptible, moderately resistant, resistant, and highly resistant cultivars, respectively. Percentage A, ritzemabosi in tissues decrease d over time in seedlings but increased in older plants.