Richard W. Smiley

Insights into the prevalence and management of soilborne cereal pathogens under direct seeding in the Pacific Northwest, U.S.A.

Direct seeding or no-till leaves the soil undisturbed, except where the seed is planted and the soil fertilized. It offers several advantages in small-grain cereal production, including reduction in labor and other operating costs, reduction of soil erosion, and improvement of soil quality. However, only about 10% of small grains in the U.S.A., and 6% of the small grains in the Pacific Northwest region of the U.S.A. are currently direct seeded. Root diseases are major constraints to adoption of direct seeding; they increase because of lack of tillage, increased crop residue left on the surface, and typically cooler and wetter soil conditions in the spring. This review covers some recent research on the four most important root diseases of cereals in the Pacific Northwest and their causal agents. These diseases are rhizoctonia root rot and bare patch [Rhizoctonia solani AG-8, Rhizoctonia oryzae], pythium damping-off and root rot [Pythium spp.], take-all [Gaeumannomyces tritici var. tritici], and fusarium foot rot [Fusarium pseudograminearum and Fusarium culmorum] We discuss how these diseases are affected by direct seeding and the impact of management strategies, including crop rotation, residue management, control of inoculum from volunteers and weeds, fertilizer placement, genetic tolerance, biological control, development of natural suppressiveness, and prediction of risk through DNA-based detection methods.Key words: soilborne pathogens, cereal, direct seeding, management strategies, no-till.

Soil C and N changes under tillage and cropping systems in semi-arid Pacific Northwest agriculture

Soils in semi-arid regions are highly susceptible to soil organic matter (SOM) loss when cultivated because of erratic yield, removal of crop residue for feed or fuel, uncontrolled soil erosion, and frequent fallowing to increase water storage. It is important to quantify the effect of each factor to be able to identify agoecosystems that are sustainable and recognize the management practices that best sequester C in soil. We identified changes in SOM in long-term experiments, some dating from the early 1900s, by evaluating tillage and crop rotation effects at several locations in semi-arid regions of the US Pacific Northwest. The major factors influencing changes in organic C and N were the frequency of summer-fallow and the amount of C input by crop residue. Soil erosion was low in long-term studies, but even limited soil loss can have a substantial impact on C and N levels if allowed over many years. Yearly crop production is recommended because any cropping system that included summer-fallow lost SOM over time without large applications of manure. We conclude that most of the SOM loss was due to high biological oxidation and absence of C input during the fallow year rather than resulting from erosion. Decreasing tillage intensity reduced SOM loss, but the effect was not as dramatic as eliminating summer-fallow. Crop management practices such as N fertilization increased residue production and improved C and N levels in soil. SOM can be maintained or increased in most semi-arid soils if they are cropped every year, crop residues are returned to soil, and erosion is kept to a minimum. SOM loss may be more intense in the Pacific Northwest because fallowing keeps the soil moist during the summer months when it would normally be dry. Our experiments identify two primary deficiencies of long-term studies to measure C sequestering capability: (1) soil C loss can be partitioned between erosion and biological oxidation only by estimation, and (2) C changes occurring below 30 cm in grassland soils cannot be quantified in many instances because samples were not collected.

Crop Damage Estimates for Crown Rot of Wheat and Barley in the Pacific Northwest

Crown rot of wheat and barley in the Pacific Northwest is caused by a complex of Fusarium pseudograminearum, F. culmorum, F. avenaceum, Bipolaris sorokiniana, and Microdochium nivale. Yield-loss estimates were made by evaluating yield components on tillers collected from commercial fields and sorted by disease severity classes, and by comparing yields for field plots inoculated with F. pseudograminearum with yields in naturally infested soil. Increasing crown rot severity caused an increase in grain protein content and reduction in grain yield, kernels per head, kernel weight, test weight, tiller height, and straw weight. Crown rot reduced winter wheat yield as much as 1,550 kg/ha (35%,

Influence of glyphosate on Rhizoctonia root rot, growth, and yield of barley

219/ha) in commercial fields, with a 13-field mean of 9.5% (

Diseases of wheat in long-term agronomic experiments at Pendleton, Oregon

51/ha). Inoculation reduced yields as much as 2,630 kg/ha (61%,

Pathogenicity of Fungi Associated with the Wheat Crown Rot Complex in Oregon and Washington

372/ha) over that caused by the native pathogen flora. Rain-induced crusting of the soil surface greatly amplified preemergence damping-off caused by F. pseudograminearum. Crown rot caused the greatest losses during seasons of lowest precipitation but also damaged crops under wet conditions. Aboveground symptoms were not always apparent under conditions of moderate infection and yield constraint. Damage from crown rot in the Pacific Northwest is more widespread and damaging than previously recognized.

Suppression of Wheat Growth and Yield by Pratylenchus neglectus in the Pacific Northwest

Time intervals between applying glyphosate to kill volunteer cereals and weeds and planting spring barley by direct drilling (no-till) into Rhizoctonia-infested soil were evaluated in field plots at Pendleton, Oregon, and Lacrosse and Lind, Washington. As the interval was shortened from autumn to spring application or from 3 wk to 3 days before planting in the spring, severity of Rhizoctonia root rot increased and grain yield decreased (.)

Crop sequences for managing cereal cyst nematode and fungal pathogens of winter wheat

Diseases of winter wheat were evaluated over 3 years in four long-term (27- to 60-year) cropping system experiments. Disease incidence and severity were evaluated with respect to seasonal precipitation and soil chemical and microbiological parameters. Take-all and eyespot were associated with increasing precipitation, and Rhizoctonia root rot and Fusarium crown rot were favored by drought. Eyespot and crown rot increased with rate of applied nitrogen and were inversely proportional to soil pH. Surface residue from previous crops had variable effects on diseases. Crown rot increased with amount of surface residue and was directly correlated with soil organic nitrogen and carbon. Surface residue also had a variable effect on Rhizoctonia root rot, depending on the magnitude of soil microbial respiration; root rot increased directly with amount of residue in a wheat-summer fallow rotation and was unaffected by residue or tillage in a wheat-pea rotation. Repeated burning of wheat stubble caused variable disease response, depending on precipitation and nitrogen rate. At high fertility, burning suppressed Pythium root rot and Rhizoctonia root rot, and enhanced eyespot and take-all. Effects of crop rotations on diseases appeared related to soil microflora effects on pathogen survival or virulence. Rhizoctonia root rot was most damaging in wheat-fallow rotation, Pythium root rot in wheat-fallow and annual wheat, and eyespot and crown rot in annual wheat. Diseases were collectively least prevalent where nitrogen in a wheat-fallow rotation was applied as pea vines or manure, rather than as inorganic fertilizer. Diseases also were generally less damaging in a wheat-pea rotation than in an annual wheat or wheat-fallow rotation. Soilborne plant pathogenic fungi appeared to suppress wheat yield by 3 to 12%. Long-term experiments provided insights to crop management and seasonal effects that are unlikely to be identified in short-term experiments.

Detection and discrimination of Pratylenchus neglectus and P. thornei in DNA extracts from soil

Crown rot of wheat in the Pacific Northwest is caused by a complex including Bipolaris sorokiniana, Fusarium avenaceum, F. culmorum, F. pseudograminearum, and Microdochium nivale. Relative pathogenicity was examined under greenhouse conditions for 178 isolates of the five species, and under field conditions for 24 isolates of B. sorokiniana, F. culmorum, and F. pseudograminearum. In the greenhouse, all five species reduced (P < 0.05) plant height relative to noninoculated controls. Disease severity was inversely correlated with plant height for the three Fusarium spp. In one or more of four experiments with spring wheat in the field, all three species reduced stand establishment and density of mature heads and increased the incidence and severity of crown rot. F. culmorum and F. pseudograminearum caused the greatest disease severity and plant damage and were the only pathogens that reduced grain yield. Virulence ratings were variable among isolates for each species in all greenhouse and field experiments. Isolate variability was especially high for the location and year variables in field experiments. Mixtures of multiple isolates are required for future research.

Detection and Quantification of Pratylenchus thornei in DNA Extracted from Soil Using Real-Time PCR

Many wheat (Triticum aestivum) fields planted annually in the Pacific Northwest are infested by high populations of the lesion nematode, Pratylenchus neglectus. Spring wheat cultivars varying in tolerance and resistance to P. neglectus were treated or not treated with aldicarb to examine relationships between the nematode and growth and yield of annual direct-seeded (no-till) wheat. Increasing initial density of P. neglectus in soil was more strongly associated with declining growth and yield of intolerant (Machete and Spear) than moderately tolerant (Frame and Krichauff) cultivars. Yield suppression by P. neglectus was generally 8 to 36% for intolerant cultivars, but reached 71% in soil also harboring Heterodera avenae, Rhizoctonia solani AG-8, and Gaeumannomyces graminis var. tritici. Intolerant cultivars had lower yields than Krichauff in rainfed but not in irrigated experiments. Density of P. neglectus in mature roots was generally lower for moderately resistant Krichauff than for susceptible Machete and Spear. Aldicarb improved yields in irrigated but not in rainfed experiments, and increased plant height and reduced variability in tiller height, canopy temperature, and density of P. neglectus in roots. This is the first report of damage to wheat by P. neglectus in the Pacific Northwest. Breeding wheat for tolerance and resistance to P. neglectus is suggested.