Effects of Crop Rotations and Tillage on Pratylenchus spp. in the Semiarid Pacific Northwest United States

Abstract

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...

Selected Pacific Northwest Rangeland and Weed Plants as Hosts of Pratylenchus neglectus and P. thornei

“…in rangeland. Where nematode densities have become elevated in expiring conservation programs, it is suggested that poor or nonhost crop species such as barley, safflower, flax, brown mustard, or selected cultivars of pea or chickpea (30,37) be planted as a transition crop before planting the first crop of wheat. It is also likely that occurrences of specific weeds at any time in the cropping system can elevate densities of Pratylenchus spp.…”

Selected Pacific Northwest Crops as Hosts of Pratylenchus neglectus and P. thornei

“…Barley (Hordeum vulgare L.) cultivars are generally more resistant than commercial wheat cultivars (34)(35)(36)(37). While wheat is by far the dominant economically viable rainfed crop in the lowprecipitation zone of the PNW (32), field studies with potential rotation crops have provided insights of root-lesion nematode dynamics in dryland cropping systems (41). For instance, Pratylenchus spp.…”

“…For instance, Pratylenchus spp. densities were typically suppressed when barley was produced, as compared to canola (Brassica campestris L. and B. napus L.) or wheat (40,41,46). In low-precipitation regions, growers often report higher wheat yields when wheat follows barley compared to wheat planted after wheat, canola, or chickpea (Cicer arietinum L.).…”

“…If those same crops are grown to maturity for extraction of oil from seed, as almost always occurs in rainfed low-precipitation areas of the PNW, the Pratylenchus spp. density may remain elevated (40,41). Likewise, sorghum was reported as a host of P. thornei in South Australia (23) but not of P. neglectus in Kansas (7).…”

Influence of Semiarid Cropping Systems on Root Diseases and Inoculum Density of Soilborne Pathogens

“…Pythium diseases caused by multiple species are particularly prevalent in direct-seed systems when seed is planted into cool, wet soil (10,15,22,36,57). Root-lesion nematode {Pratylenchus neglectus and P. thornei) densities are strongly influenced by the frequency at which hosts are produced in the cropping system and are more prevalent in cultivated than in direct-seed systems and following wheat compared with barley (53,54). tritici) occurs regularly and is most prevalent in annual crops and in direct-seed systems (12,14,29,36,38,50).…”