An agar--gel immunodiffusion test for detection of Brucella antibodies in human serum.

A comparison was made of results obtained with a Brucella agar--gel immunodiffusion (AGID) test and the standard tube-agglutination test on 612 human sera. Agreement between the tests was 97% when the titer was 1:160 or higher. Of 448 sera that showed no agglutination titer, 447 were negative with the AGID test. Results of the AGID test were also compared to those obtained with the 2-mercaptoethanol (2-ME) agglutination test on 148 sera that demonstrated a standard tube-agglutination titer of 1:20 or higher. All sera with a 2-ME-agglutination titer of 1:40 or higher were positive with the AGID test. Of 123 sera that showed no 2-ME-agglutination titer, 21 were positive with the AGID test. Two of these 21 sera were obtained from patients with bacteriologically proven brucellosis, and eight were from abattoir employees with suspected but not bacteriologically proven brucellosis.     

Oat residue and soil compaction influences on common root rot (Aphanomyes euteiches) of peas in a fine-textured soil

Management of common root rot (Aphanomyces euteiches Drechs.) in peas (Pisum sativum L.) is sought primarily by host crop avoidance for several years. Soil compaction is known to aggravate A. euteiches disease in peas but effects on infection and subsequent symptom development are not sufficiently known to assist in cultural control. Several isolated observations have noted that oat crop residues may suppress A. euteiches infection and disease in pea roots. The individual and combined influence (a factorial combination of two factors each at two levels) of a prior oat crop and soil compaction were studied for their effects on common root rot severity in processing peas grown in an A. euteiches disease nursery on a fine-textured soil in the northern Corn Belt of the USA. A previous crop of summer oats relative to prior-year peas significantly suppressed common root rot and increased pea fresh vine weight 210% at peak bloom stage. Both fresh vine weight and green pea yield were reduced as much as 63% by soil compaction and increased as much as 48% by a prior oat crop. Greater soil bulk density at the 10 to 25-cm depth identified wheel traffic compaction patterns in each year. A 10-fold reduction of saturated hydraulic conductivity in the 10 to 25-cm compacted zone and high soil-water potentials within the upper 60 cm both confirmed an impaired water drainage, especially during infiltration events. These observations support the use of a previous full season or summer oat crop jointly with chisel plowing, plus the prevention of excessive traffic during secondary tillage and planting, to reduce common root rot in a field infested with A. euteiches. Shallow incorporation of oat shoot and root residue by chiseling could be a crucial component of the cultural control of the disease. R Rodriguez Kabana Section editor     

Soil strength and water content influences on corn root distribution in a sandy soil

Initial field observations revealed a shallow corn (Zea mays L.) root system on a Zimmerman fine sand in a corn/soybean (Glycine max L.) rotation. Since root distribution influences crop water and nutrient absorption, it is essential to identify factors limiting root growth. The objective of this study was to determine the factor(s) limiting corn rooting depth on an irrigated fine sand soil. Bulk density, saturated hydraulic conductivity, and soil water retention were measured on undisturbed soil cores. Corn root distribution assessed at tasseling over a 3-yr period showed an average of 94% of total root length within the upper 0.60 m of soil with 85% in the upper 0.30 m of soil. Mechanical impedance was estimated with a cone penetrometer on two dates with differing water contents. Cone penetrometer measurements greater than 3 MPa indicated mechanical impedance in soil layers extending from 0.15 to 0.35 m deep. Penetration resistance decreased as soil water content increased. However, soil water contents greater than field capacity were required to decrease penetration resistance below the 3 MPa threshold. Such water saturated conditions only occurred for short periods immediately after precipitation or irrigation events, thus roots usually encountered restrictive soil strengths. The soil layer from 0.15 to 0.60 m had high bulk density, 1.57 Mg m^-3. This compacted soil layer, with slower saturated hydraulic conductivities (121 to 138 mm hr-1), held more water than the soil above or below it and reduced water movement through the soil profile. Crop water use occurred to a depth of approximately 0.75 m. In conclusion, a compacted soil layer confined roots almost entirely to the top 0.60 m of soil because it had high soil strength and bulk density. The compacted layer, in turn, retained more water for crop use.     

Green manures of oat,rape and sweet corn for reducing common root rot in pea (Pisum sativum) caused by Aphanomyces euteiches

Green manure crops of sweet corn, soybean, alfalfa, snap bean, rape, pea and of the two oat cultivars Dane and Troy were incorporated into the same soil and containers in which the crop had grown for five weeks. The soil was then evaluated for suppression of common root rot (Aphanomyces euteiches) of pea grown in infested pasteurized and non-pasteurized soils in the greenhouse. Pea biomass reduction and a plant bioassay for A. euteiches were used to measure the green manure suppression of disease. Green manures of sweet corn cv.Jubilee, oat cv.Troy, and rape cv.Humus significantly reduced pea biomass losses over the non-amended control soil treatments. Oat cv.Troyand sweet corn cv.Jubilee green manures significantly reduced inoculum density of A. euteiches over the corresponding fallow controls in inoculated pasteurized soil by 87% and 76%, respectively, and in inoculated non-pasteurized soil by 67% and 66%, respectively. Only the green manure of oat cv. Troy reduced inoculum density significantly below fallow.     

Influence of cultural practices on edaphic factors related to root disease in Pinus nursery seedlings

Conifer seedlings grown in bare-root nurseries are frequently damaged and destroyed by soil-borne pathogenic fungi that cause root rot. Relationships between nursery cultural practices, soil characteristics, and populations of potential pathogens in the soil were examined in three bare-root tree nurseries in the Midwestern USA. Soil-borne populations of Fusarium spp. and Pythium spp. were enumerated as a function of soil depth in the upper 42 cm; red and white pine seedling root systems were assessed visually for signs of root rot. Soil organic carbon and resistance to cone penetration (as a function of depth) were augmented by saturated hydraulic conductivity (K_sat), water retention characteristic, texture and pH at selected depths. Cone index (CI) provided accurate ‘fingerprints’ of cultural practices in each nursery. A tillage pan due to rotary tillage was detected by CI in the Minnesota and Wisconsin nurseries, but no such tillage pan was indicated in the Michigan nursery, which did not use rotary tillage. Curves of CI also indicated differing maximum depth of tillage disturbance between nurseries; maximum rooting depth based on 3 MPa CI were different among nurseries. Vertical distribution of soil-borne Fusarium spp. reflected the vertical incorporation pattern associated with the type of tillage implement used to incorporate cover crop residue prior to Pinus seedling establishment. Peak numbers of Fusarium spp., from 250 to 950 colony-forming units (cfu g-1 dry soil) were recorded between 12 – 24 cm depth in two nurseries using a moldboard plow for incorporation while steadily decreasing populations, from 1800 to 250 cfu g-1 dry soil, were found from 0 to 15 cm in the third nursery using a disc. Vertical distribution of the Fusarium spp. also correlated with organic carbon levels, which suggested that cover-crop incorporation and conifer rooting had determined the location of soil-borne Fusarium spp. propagules. K_sat suggest that tillage pans caused by rotary tillage may impede drainage during nearly daily irrigation enough to cause physiological stress to the seedlings and predispose them to disease. Low levels of mortality (from < 1% to 5%) were observed in two-year-old Pinus seedlings while disease severity varied by nursery and seedling species. Tillage should be used to control depth placement of biomass residue and pathogenic fungal propagules, and adjusted to prevent tillage pans within the seedling root zone. More studies are needed to determine the impact of these cultural controls on the need and application depth of fumigation for pathogen control. This revised version was published online in August 2006 with corrections to the Cover Date.