Current management of sudden death syndrome (SDS) of soybean, caused by Fusarium virguliforme, focuses on planting resistant varieties and improving soil drainage; however, these measures are not completely effective. A 6-year study evaluated the effects of cropping system diversification on SDS and soybean yield. SDS, root health, yield, and F. virguliforme density in soil were assessed in a naturally infested field trial comparing a 2-year cropping system consisting of a corn-soybean rotation and synthetic fertilizer applications with 3- and 4-year cropping systems consisting of corn-soybean-oat + red clover and corn-soybean-oat +alfalfa-alfalfa rotations, respectively, with both manure and low synthetic fertilizer rates. In 5 of 6 years, SDS incidence and severity were lower and yield higher in the 3- and 4-year systems than in the 2-year system. SDS severity and incidence were up to 17-fold lower in the diversified systems than in the 2-year system. Incidence and severity of SDS explained 45 to 87% of the variation in yield. Plants in the 2-year system generally showed more severe root rot and lower plant weights than plants in the diversified systems. F. virguliforme density in soil was up to fivefold greater in the 2-year system compared with the 4-year system. The processes responsible for the suppression of SDS and yield protection in the diversified cropping systems still need to be determined.

No abstract

Goss’s bacterial wilt and leaf blight of maize is caused by Clavibacter michiganensis subsp. nebraskensis. Infested residue is the primary source of inoculum and infection occurs via wounds caused by sand blasting, hail, or wind damage. The pathogen survives as an epiphyte on maize leaves and, because the disease has been observed on plants in the field with no obvious wounding, we wondered whether infection by epiphytic C. michiganensis subsp. nebraskensis and disease development could occur in the absence of severe wounding. Consequently, greenhouse experiments were done to evaluate disease development in the absence of wounding in ambient and increased humidity conditions. Maize plants at the V4 to V5 crop development stage were spray inoculated with a suspension of C. michiganensis subsp. nebraskensis (108 cells ml−1). Leaf blight incidence was assessed on whole plants and individual leaves; epiphytic populations of C. michiganensis subsp. nebraskensis were monitored by dilution plating of leaf washes; and epiphytic C. michiganensis subsp. nebraskensis colonization was visualized using scanning electron microscopy (SEM). Goss’s leaf blight symptoms were observed on nonwounded plants in ambient (37.0% plant incidence) and increased humidity conditions (60.0% plant incidence). Populations of epiphytic C. michiganensis subsp. nebraskensis survived and increased on maize leaves, particularly at increased humidity. We observed C. michiganensis subsp. nebraskensis colonizing maize leaves in localized sites that included epidermal junctions, cuticle depressions, in and around stomata, and at the base of trichomes. Single cells and aggregates of C. michiganensis subsp. nebraskensis were observed within substomatal chambers using SEM. These data indicate that severe wounding is not necessary for C. michiganensis subsp. nebraskensis infection of maize, and stomata or trichomes may serve as entry points for the bacterium.

A statewide survey was carried out from 2005 through 2007 to quantify, map, and analyze the spatial dynamics and seasonal patterns of Bean pod mottle virus (BPMV) prevalence and incidence within Iowa. In all, 8 to 16 soybean fields were arbitrarily sampled from 96 counties in 2005 and all 99 counties in 2006 and 2007. Field- and county-scale BPMV prevalence and incidence data were mapped using geographic information systems software. BPMV prevalence was highest in the 2006 soybean growing season, when BPMV was detected in 38.7% of all soybean fields, 91.9% of all counties, and 100% of the agricultural climate districts. BPMV incidence at the field scale was highest in 2006, when mean statewide end-of-season incidence was 24.4%. Spatial analyses indicated that BPMV incidence was spatially clustered at the county scale in all three growing seasons. Prevalence at the county scale was clustered in 2005 and 2007 but not in 2006. Semivariogram analyses at the field scale indicated the presence of significant (P ≤ 0.05) spatial dependence (clustering) at distances ≤23.4 km in 2005, 297.7 km in 2006, and 45.2 km in 2007. Data for county-scale incidence displayed a north (low incidence) to south (high incidence) BPMV gradient in each year of the survey. High county-scale BPMV prevalence and incidence levels in 2006 were significantly associated with BPMV prevalence and incidence in 2007 (P ≤ 0.05). Soybean fields with narrow row spacings (≤38 cm) were associated with higher levels of BPMV incidence. Soybean fields infected with BPMV had a higher probability of infection by Phomopsis pod and stem blight than did non-BPMV-infected fields. This study provides new quantitative tools and information to better understand the seasonal, temporal, and geographical distribution of BPMV disease risk at several spatial scales.

Goss’s wilt of corn, caused by Clavibacter michiganensis subsp. nebraskensis, has reemerged since 2006 as an economically important disease of corn in in the Midwestern United States. In 2012 and 2013, field plot studies were conducted with a pathogenic, rifampicin-resistant C. michiganensis subsp. nebraskensis isolate and a Goss’s wilt-susceptible corn hybrid to monitor epiphytic C. michiganensis subsp. nebraskensis population densities and the temporal and spatial spread of Goss’s wilt incidence originating from inoculum point sources. The randomized complete block trial included three treatments: noninoculated control, inoculum point sources established by wound inoculation, and inoculum point sources consisting of C. michiganensis subsp. nebraskensis-infested corn residue. Epiphytic C. michiganensis subsp. nebraskensis was detected on asymptomatic corn leaves collected up to 2.5 m away from inoculum sources at 15 days after inoculation in both years. The percentage of asymptomatic leaf samples on which epiphytic C. michiganensis subsp. nebraskensis was detected increased until mid-August in both years, and reached 90, 55, and 35% in wound-, residue-, and noninoculated plots, respectively, in 2012; and 50, 11, and 2%, respectively, in 2013. Although both growing seasons were drier than normal, Goss’s wilt incidence increased over time and space from all C. michiganensis subsp. nebraskensis point sources. Plots infested with C. michiganensis subsp. nebraskensis residue had final Goss’s wilt incidence of 7.5 and 1.8% in 2012 and 2013, respectively; plots with a wound-inoculated source had final Goss’s wilt incidence of 16.6 and 14.0% in 2012 and 2013, respectively. Our findings suggest that relatively recent outbreaks of Goss’s wilt in new regions of the United States may be the result of a gradual, nondetected buildup of C. michiganensis subsp. nebraskensis inoculum in fields.