Smiley, R. W., Machado, S., Gourlie, J. A., Pritchett, L. C, Yan, G. R, and Jacobsen, E. E. 2013. Effects of crop rotations and tillage on Pratylenchus spp. in the semiarid Pacific Northwest United States. Plant Dis. 97:537-546.There is interest in converting rainfed cropping systems in the Pacific Northwest from a 2-year rotation of winter wheat and cultivated fallow to direct-seed (no-till) systems that include chemical fallow, spring cereals, and food legume and brassica crops. Little information is available regarding effects of these changes on plant-parasitic nematodes. Eight cropping systems in a low-precipitation region (<330 mm) were compared over 9 years. Each phase of each rotation occurred each year. The density of Pratylenchus spp. was greater in cultivated than chemical fallow, became greater with increasing frequency of host crops, and was inversely associated with precipitation (R^ = 0.92, a < 0.01). Densities after harvesting mustard, spring wheat, winter wheat. and winter pea were greater (a < 0.01) than after harvesting spring barley or spring pea. Camelina also produced low densities. Winter wheat led to a greater density of Pratylenchus neglectus and spring wheat led to a greater density of P. thornei. Density of Pratyienchus spp. was correlated (R^ = 0.88, a < 0.01) but generally higher when detected by real-time polymerase chain reaction on DNA extracts from soil than when detected by a traditional method. Selection of different Pratylenchus spp. by different wheat cultivars or growth habit must be addressed to minitnize the level of nematode risk to future plantings of intolerant crops.The root-lesion nematodes Pratylenchus neglectus (Rensch) Filipjev Schuurmanns & Stekhoven and P. thornei Sher & Allen suppress yields of rainfed cereals in temperate regions worldwide (5,43). These species are present in 90% of fields in low-precipitation regions ( 150 to 300 mm) of north-central Oregon and southcentral Washington, and the nematode density in soil is above a potentially damaging level in perhaps 60% of the fields (42). Rainfed wheat {Triticum aestivum L.) in the region is planted on 1.5 million ha annually. In all, 90% of the hectares in the <330-mm annual precipitation area are managed as a monoculture "rotation" of winter wheat (10 months) and summer fallow (14 months) because precipitation occurs mostly from late autumn (October) to early spring (April) and the amount is highly variable from year to year (31,32). Winters are cold and intervals of frozen soil are common. Warm to hot days and cool nights prevail during the dry summer period when crops are maturing. Optimal grain yield for cereal crops is dependent upon healthy root systems capable of extracting water stored deeply in the soil profile. Most fallow is managed as a mechanically prepared "dust mulch".Direct seeding into non-tilled chemical summer fallow is of strong interest but still lags behind mechanical cultivation in the low-precipitation region because direct-seed systems have historically been less profitable than mecha...