Seasonal fluctuations of soil and tissue populations of Ditylenchus dipsaci and Aphelenchoides ritzemabosi in alfalfa

Population dynamics of A. ritzemabosi and D. dipsaci were studied in two al falfa fields in Wyoming. Symptomatic stem-bud tissue and root-zone soil fro m alfalfa plants exhibiting symptoms of D. dipsaci infection were collected at intervals of 3 to 4 weeks. Both nematodes were extracted from stem tiss ue with the Baermann funnel method and from soil with the sieving and Baerm ann funnel method. Soil moisture and soil temperature at 5 cm accounted for 64.8% and 61.0%, respectively, of the variability in numbers of both nemat odes in soil at the Big Horn field. Also at the Big Horn field, A. ritzemab osi was found in soil on only three of the 14 collection dates, whereas D. dipsaci was found in soil on 12 dates. Aphelenchoides ritzemabosi was found in stem tissue samples on 9 of the 14 sampling dates whereas D. dipsaci wa s found on all dates. Populations of both nematodes in stem tissue peaked i n October, and soil populations of both peaked in January, when soil moistu re was greatest. Numbers of D. dipsaci in stem tissue were related to mean air temperature 3 weeks prior to tissue collection, while none of the clima tic factors measured were associated with numbers of A. ritzemabosi. At the Dayton held, soil moisture plus soil temperature at 5 cm accounted for 98. 2% and 91.4% of the variability in the soil populations of A. ritzemabosi a nd D. dipsaci, respectively. Aphelenchoides ritzemabosi was extracted from soil at two of the five collection dates, compared to extraction of D. dips aci at three dates. Aphelenchoides ritzemabosi was collected from stem tiss ue at six of the seven sampling dates while D. dipsaci was found at all sam pling dates. The only environmental factor that was associated with an incr ease in the numbers of both nematodes in alfalfa stem tissue was total prec ipitation 1 week prior to sampling, and this occurred only at the Dayton fi eld. Numbers of A. ritzemabosi in stem tissue appeared to be not affected b y any of the environmental factors studied, while numbers of D. dipsaci in stem tissue were associated with cumulative monthly precipitation, snow cov er at time of sampling, and the mean weekly temperature 3 weeks prior to sa mpling. Harvesting alfalfa reduced the numbers of A. ritzemabosi at the Big Horn field and both nematodes at the Dayton field.