Multiple Diseases and Disease Reviews

Smiley, R. W., Machado, S., Rhinhart, K. E. L., Reardon, C. L., and Wuest, S. B. 2016. Rapid quantification of soilborne pathogen communities in wheat-based long-term field experiments. Plant Disease 100:(in press). (pre-published online as

Rainfed experiments operated continuously for up to 84 years in semiarid eastern Oregon are among the oldest agronomic trials in North America. Disease incidence and severity had been quantified visually but quantification of inoculum density had not been attempted. Natural inoculum of 17 fungal and nematode pathogens were quantified for each of two years on eight trials using DNA extracts from soil. Crop type, tillage, rotation, soil fertility and year, and their interactions, had large effects on the pathogens. Fusarium culmorum and Pratylenchus thornei were more dominant than F. pseudograminearum and P. neglectus where spring crops were grown, and the opposite species dominances occurred where winter wheat was the only crop. Bipolaris sorokiniana and Phoma pinodella were restricted to presence of spring cereals and pulse crops, respectively. Helgardia spp. occurred in winter wheat-fallow rotations but not in annual winter wheat. Gaeumannomyces graminis var. tritici was more prevalent in cultivated than in non-cultivated soils and the opposite generally occurred for Rhizoctonia solani AG-8. Densities of Pythium spp. Clade F were high but were also influenced by treatments. Significant treatment effects and interactions were more prevalent in two long-standing (> 50 yr) annually cropped experiments (29%) than two long-standing 2-yr wheat-fallow rotations (14%). Associations among pathogens occurred mostly in an 84-yr-old annual cereals experiment. This survey provided guidance for research on dynamics of root-infecting pathogens of rainfed field crops and identified two pathogens (D. tritici-repentis and Phoma pinodella) not previously identified at the location.

Paulitz, T.C., R.W. Smiley, and R.J. Cook. 2002. Insights into the prevalence and management of soilborne cereal pathogens under direct seeding in the Pacific Northwest, U.S.A. Canadian Journal of Plant Pathology 24:416-428.

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 (PNW) 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 PNW and their causal agents.  These diseases are rhizoctonia root rot and bare patch (Rhizoctonia solani AG-8, R. oryzae), pythium damping-off and root rot (Pythium spp.), take-all (Gaeumannomyces tritici var. tritici), and fusarium foot rot (Fusarium pseudograminearum and F. 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.

Smiley, R.W. 1996. Diseases of Wheat and Barley in Conservation Cropping Systems of the Semi-Arid Pacific Northwest. American Journal of Alternative Agriculture 11:95-103. [proceedings of a meeting of the U.N. Middle East Peace Initiative]

Diseases continue to be important constraints to production of wheat and barley in conservation cropping systems of the semi-arid Pacific Northwest. Several diseases are more damaging in high- than low-residue seedbeds, and in crops planted during early autumn to reduce soil erosion during winter. This is particularly true for nonirrigated winter wheat in rotation with summer fallow in low rainfall zones (250-400 mm). Changes in cropping systems in the region have made management of diseases and maintenance of yield goals and farm profitability more challenging because disease management is often more complex and expensive for conservation- than inversion-tillage systems. Management practices being developed to meet this challenge are reviewed for diseases that are particularly troublesome in conservation farming systems of the Pacific Northwest.

Smiley, R.W., H.P. Collins, and P.E. Rasmussen. 1996. Diseases of wheat in long-term agronomic experiments at Pendleton, Oregon. Plant Disease 80:813-820.

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 magnitude of soil microbial respiration; root rot increased directly with amount of residue in a wheat-summer fallow rotation and was unaffected by residue and/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 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.
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