Pratylenchus neglectus Reduces Yield of Winter Wheat in Dryland Cropping Systems

Rainfed wheat (Triticum aestivum L.) is planted each year on 1.5 million ha in the low-precipitation region (150 to 300 mm) of north-central Oregon and south-central Washington. Precipitation occurs mostly (75%) from late autumn (October) to early spring (April) and the amount is highly variable from year to year. Winters are cold and intervals of frozen soil are common. Warm to hot days and cool nights prevail during the dry summer period. Maturation of cereal crops is dependent upon healthy root systems capable of extracting water stored deeply in the soil profile. Ninety percent of hectares planted in this low-precipitation area are managed as a 24-month culture of winter wheat (10month growing season) alternated with a 14-month fallow (crop-free) period. This production system is known in dryland regions throughout the world as a winter wheat–summer fallow rotation (14) and is referred to as biennial winter wheat in this article. Most fallow in Oregon and Washington (19) is managed as a cultivated “dust mulch” that provides comparatively high yields because it retains as much as 70% of the precipitation occurring during the winter of the fallow period. Each winter wheat crop, therefore, has access to water equivalent to as much as 75% of the precipitation occurring during the rotational sequence. Direct seeding into nontilled (herbicide-maintained) fallow, called chemical fallow, is becoming increasingly popular but still lags behind cultivated fallow because direct-drill (no-till) systems continue to be less profitable than cultivated fallow systems (2,11,18,19). Threeyear rotations of winter wheat, a spring crop, and chemical fallow are of interest but have not been widely adopted because they are often less profitable and have greater year-to-year economic risk compared with biennial winter wheat. Directseeded fields planted annually to spring crops are also becoming increasingly practiced in the low-precipitation region. However, fields planted annually to no-till spring crops are planted mostly to wheat or barley (Hordeum vulgare L.) because these small-grain crops are more profitable than rotations that include broadleaf crops such yellow mustard (Sinapsis alba L.) or canola (Brassica napus L.). Although of increasing importance, annual no-till spring cropping is not yet widespread because it, too, is less profitable than biennial winter wheat (10,18). In continuous annual cropping systems in an intermediateprecipitation region, spring barley produces 34% greater yield than spring wheat in cultivated systems, 45% greater yield than spring wheat in direct-drill systems, 25% greater yield than biennial winter wheat planted into cultivated fallow, and 5% greater yield than biennial winter wheat planted into chemical fallow (11). Root-lesion nematodes (Pratylenchus spp.) are present in 90% of the fields in low-precipitation regions of the Pacific Northwest (PNW; southern Idaho, eastern Oregon, and eastern Washington) and effects of crop management practices on nematode populations have not been clearly defined (21). Much higher populations of Pratylenchus neglectus (Rensch 1924) Filipjev Schuurmanns & Stekhoven 1941 and P. thornei Sher & Allen 1953 were detected in annually cropped fields than in biennial winter wheat. Lower populations following fallow (12) and direct associations between Pratylenchus spp. population density and frequency of cereal cropping have been reported in other countries (7,17). Barley is generally considered a poorer host than wheat (21,29,32,33). P. neglectus and P. thornei cause substantial constraints to grain yield in rainfed cereals (4,22). P. neglectus and P. thornei have each reduced yields of annually cropped spring wheat by as much as 70% in the PNW (25,26) but these studies did not compare representative crop management systems for effects on or by species of Pratylenchus. Presumptive evidence was presented for reduction of winter wheat yield by P. neglectus but the effects of the rotational sequences on potential variability in stored soil water were not defined in that study (21). Thompson et al. (31) reported that the greater tolerance of barley to P. thornei compared with wheat enabled barley, in P. thornei-infested soil, to have greater efficiency in extracting and be more responsive than wheat to stored soil moisture in a year with limited in-crop rainfall. A replicated multiyear experiment was established during 2003 to examine the multidisciplinary aspects of eight representative cropping systems at a lowprecipitation site known to be infested with P. neglectus. This article reports associations between cropping systems, nematode populations, grain yields, and water extraction over the first 5 years of the experiment.