SEEDLING MORTALITY AND DEVELOPMENT OF ROOT-ROT IN WHITE-PINE SEEDLINGS IN 2 BARE-ROOT NURSERIES

Seedling mortality and development of root rot in white pine (Pinus st robus) were followed across locations and over time within three opera tional nursery fields with loamy sand soils at a provincial nursery in southwestern Ontario, Canada, and a state nursery in southern Wiscons in, USA. One Ontario field was fumigated with dazomet; the other was n ot fumigated. The Wisconsin field was fumigated with methyl bromide-ch loropicrin. Mortality and disease severity data were collected; the la tter were based on visual assessment of seedling roots. White pine roo t rot (10 to > 30% incidence) was observed in the fields regardless of soil fumigation treatment. Systematically placed plots in the outer t wo seedling beds in the Wisconsin field had higher mortality levels th an those in the other beds during the second growing season, and they had higher disease severity in July and October of the same year. Plot locations in the Ontario fields were stratified according to topograp hic features. Cumulative seedling mortality level during the second gr owing season in the Ontario fumigated field was highest in the low are as (P < 0.001) and equal in the mid-slope and high areas; cumulative m ortality in the nonfumigated field did not differ among location types . Higher disease severity was observed in the low areas of the nonfumi gated field in July and October of the same growing season (P = 0.03) compared to pooled mid-slope and high areas. Seedling mortality was hi gher than expected in mid-spring and less than expected in October of the second growing season in the outermost bed of the Wisconsin field and for all topographic areas in the Ontario fields.