Gregory L. Tylka

Regional Assessment of Soybean Brown Stem Rot, Phytophthora sojae, and Heterodera glycines Using Area-Frame Sampling: Prevalence and Effects of Tillage.

ABSTRACT The prevalence of brown stem rot (caused by Phialophora gregata), Heterodera glycines, and Phytophthora sojae in the north central United States was investigated during the fall of 1995 and 1996. Soybean fields were randomly selected using an area-frame sampling design in collaboration with the National Agricultural Statistics Service. Soil and soybean stem samples, along with tillage information, were collected from 1,462 fields in Illinois, Iowa, Minnesota, Missouri, and Ohio. An additional 275 soil samples collected from Indiana were assessed for H. glycines. For each field, the incidence and prevalence of brown stem rot was assessed in 20 soybean stem pieces. The prevalence and recovery (expressed as the percentage of leaf disks colonized) of P. sojae and the prevalence and population densities of H. glycines were determined from the soil samples. The prevalence of brown stem rot ranged from 28% in Missouri to 73% in Illinois; 68 and 72% of the fields in Minnesota and Iowa, respectively, showed symptomatic samples. The incidence of brown stem rot was greater in conservation-till than in conventional-till fields in all states except Minnesota, which had few no-till fields. P. sojae was detected in two-thirds of the soybean fields in Ohio and Minnesota, whereas 63, 55, and 41% of the fields in Iowa, Missouri, and Illinois, respectively, were infested with the pathogen. The recovery rates of P. sojae were significantly greater (P </= 0.05) in conservation-till than in conventional-till fields in all states except Iowa. H. glycines was detected in 83% of the soybean fields in Illinois, 74% in Iowa, 71% in Missouri, 60% in Ohio, 54% in Minnesota, and 47% in Indiana. Both the prevalence and population densities of H. glycines were consistently greater in tilled than in no-till fields in all states for which tillage information was available.

Effects of Weed Management Systems on Canopy Insects in Herbicide-Resistant Soybeans

Abstract The effects of transgenic herbicide-resistant soybean varieties and their corresponding weed management strategies on canopy insects were examined in studies at two locations in Iowa in 1997 and 1998. Weed management systems that allowed more weed escapes typically had higher insect population densities. However, systems with fewer weeds seemingly were preferred by potato leafhoppers. Bean leaf beetles and potato leafhoppers showed preferences for certain soybean varieties, but these effects were attributed to soybean plant height. These findings indicate that although the transgenic soybean varieties did not strongly affect insect populations, weed management systems can affect insect populations in soybean. However, this impact is likely related more to weed suppression effectiveness than to a direct effect of the herbicides on the insects.

Soybean brown stem rot, Phytophthora sojae, and Heterodera glycines affected by soil texture and tillage relations

ABSTRACT Investigations were conducted to determine whether the effects of tillage practices on the prevalence of brown stem rot of soybean (caused by Phialophora gregata), Heterodera glycines, and Phytophthora sojae were confounded by soil texture in samples collected in the fall of 1995 and 1996. Soil and soybean stem samples, along with tillage information, were collected from 1,462 randomly selected fields in Illinois, Iowa, Minnesota, Missouri, and Ohio in collaboration with the National Agricultural Statistics Service. The incidence of brown stem rot was determined from 20 soybean stem pieces collected from each field in a zigzag pattern. The detection frequency of P. sojae (expressed as percent leaf disks colonized) and population densities of H. glycines were determined from soil cores also collected in a zigzag pattern. The soil samples were grouped into various textural classes, and the effect of soil texture and tillage relations on the activities of each pathogen were determined. Both tillage and soil texture affected the incidence of brown stem rot; however, there was no interaction between tillage and soil texture. Conservation tillage had a greater (P < 0.05) incidence of brown stem rot in clay loam and silty clay loam than did conventional tillage. The detection frequency of P. sojae was not affected by tillage, but a tillage x texture interaction (P = 0.013) indicated that the effect of tillage depended on soil texture. There was a greater (P < 0.05) detection frequency of P. sojae in conservation tillage than in conventional tillage in silt loam and loam soils. However, in sandy loam, the detection frequency of P. sojae was greater (P = 0.0099) in conventional tillage than in conservation tillage. Population densities of H. glycines were significantly affected by both tillage and soil texture, but overall, there was no tillage x texture interaction. There was an inverse relationship between population densities of H. glycines and percent clay (r = -0.81, P = 0.01) in no-till fields, but little or no change in nematode densities was observed with increasing clay content in tilled fields. Population densities of H. glycines were less (P < 0.05) in no-till fields than in tilled fields in silty clay loam and clay soils. There was no difference in H. glycines densities between the tillage categories in soils sandier than silty clay loam or clay. The findings emphasize the need for cautious interpretation of the effects of tillage practices on diseases and pathogens in the absence of information on soil texture.

Soybean Cyst Nematode Reduces Soybean Yield Without Causing Obvious Aboveground Symptoms

Field experiments were conducted at locations in northern and southern Illinois, central Iowa, and central Missouri from 1997 to 1999 to investigate the effects of Heterodera glycines on soybean growth, development, and yield. A wide range of infestation levels was present at all locations. Two locally adapted cultivars, one resistant to H. glycines, were grown at each location. Cultivars were planted in alternating four-row strips with 76 cm between rows. For each cultivar, 20 1-m-long single-row plots were sampled every 2 weeks starting 4 weeks after planting. Infection by H. glycines reduced plant height and leaf and stem weight on the resistant cultivars in the first 12 weeks after planting, and delayed pod and seed development 12 to 14 weeks after planting. Biomass accumulation was not reduced on the susceptible cultivars until 10 weeks after planting; reduction in pod and seed development occurred throughout the reproductive stages. Susceptible cultivars produced significantly lower yields than resistant cultivars, but the yield reductions were not accompanied by visually detectable symptoms.

Relationships between tillage and spatial patterns of Heterodera glycines

ABSTRACT The dynamics of Heterodera glycines spatial patterns were studied under different tillage systems in two naturally infested soybean fields in Iowa from 1994 to 1997. At each location, there were four different tillage treatments (conventional tillage, reduced tillage, ridge tillage, and no tillage). Soil samples were taken from 98 contiguous quadrats (5.2 m(2)) per plot in the fall of 1994, before any tillage was performed, and in the spring of the following 3 years shortly after planting. Cysts were extracted from soil samples by elutriation and counted, and eggs were extracted from cysts and enumerated. Spatial patterns of H. glycines populations were characterized by geostatistical analysis and variance-to-mean (VM) ratios. Semivariance values were calculated for cyst and egg densities and semivariograms were constructed. In general, there was greater spatial dependence among cyst populations than egg populations. In one field with a strongly aggregated initial H. glycines population, tillage practices resulted in changes in spatial patterns of H. glycines populations, characterized by spherical-model semivariogram parameters (sill, nugget effect, and range of spatial dependence). These parameters indicated increasing aggregation over time in no tillage and ridge tillage treatments, but decreasing aggregation in reduced and conventional tillage treatments. There was an increase of 350% in sill values (maximum semivariance) for cyst populations after 3 years of no tillage, but in the conventional tillage treatment, sill values remained unchanged or decreased over time as tillage was implemented. Semivariograms for cyst and egg population densities revealed strong anisotropy (directional spatial dependence) along soybean rows, coincident with the direction of tillage practices. VM ratios for cyst counts increased each year in the no tillage and ridge tillage treatments, but decreased for 2 years in reduced tillage and conventional tillage treatments. Final VM ratios for cyst and egg counts were highest in the no tillage treatment. In a second field, with low initial aggregation of H. glycines populations, there was little measurable change in semivariogram parameters after 3 years of no tillage, but in the conventional tillage treatment, populations became less aggregated, as the range, sill, and the proportion of the sill explained by spatial dependence decreased for cyst population densities. Our results indicated that in soybean fields with initially aggregated populations of H. glycines, no tillage and ridge tillage systems promoted aggregation of the nematode population, whereas conventional and reduced tillage systems resulted in a less aggregated spatial pattern.

Effect of tillage practices on vertical distribution of Phytophthora sojae

The vertical distribution of Phytophthora sojae was investigated in soil samples collected in the spring of 1994 from soybean fields at 62 locations in Illinois, Indiana, Iowa, and Minnesota. In the fall of 1995, soil samples were collected from 18 additional locations in Illinois and Iowa. Each location consisted of a pair of no-till and conventional-till fields, and soil samples were collected from arbitrarily selected locations in each field at 0- to 7.5-cm and 7.5- to 15-cm depths. Separate intensive sampling was made in the spring of 1995 from two pairs of adjacent no-till and conventional-till fields at the Iowa State University Northeast Research Farm, in which samples were collected from 0- to 30-cm depth in increments of 5 cm. Samples were assayed for P. sojae with the use of a leaf-disk bioassay. In the 1994 regional samples, there was greater recovery of P. sojae (P ≤ 0.05) at 0- to 7.5-cm depth in the no-till samples than in the conventional-till samples for all states except Minnesota. The fall 1995 samples from Illinois followed a similar trend (P = 0.05); whereas samples from Iowa showed no significant difference between tillage systems. At depths greater than 7.5 cm, there was generally no difference in detection frequency of P. sojae between tillage systems. Samples from the Northeast Research Farm followed patterns of vertical distribution similar to those of the regional samples. In no-till fields, the detection frequency of P. sojae was greatest near the soil surface; two to three times greater than that of the conventional-till fields at this depth. In the conventional-till fields, however, the frequency of recovery peaked at 20 cm and was comparable at these depths to those of no-till fields. There was a positive correlation between the percentage of leaf disks colonized and residue dry weights in the no-till fields (r = 0.84, P = 0.04; and r = 0.86, P = 0.03) but not in the conventional-till fields (r = -0.06, P = 0.90; and r = -0.60, P = 0.17). The recovery of P. sojae in greater frequency near the soil surface in no-till fields than in conventional-till fields suggests that the potential for damping-off may be greater in no-till fields than in conventional-till fields.

Heterodera glycines Infection Increases Incidence and Severity of Brown Stem Rot in Both Resistant and Susceptible Soybean

Growth chamber experiments were conducted to investigate whether parasitism by Heterodera glycines, the soybean cyst nematode, increases incidence and severity of brown stem rot (BSR) of soybean, caused by Phialophora gregata, in both resistant and susceptible soybean cultivars. Soybean genotypes with various combinations of resistance and susceptibility to both pathogens were inoculated with P. gregata alone or P. gregata plus H. glycines. In most tests of H. glycines-susceptible genotypes, incidence and severity of internal stem discoloration, characteristic of BSR, was greater in the presence than in the absence of H. glycines, regardless of susceptibility or resistance to BSR. There was less of an increasing effect of H. glycines on stem symptoms in genotypes resistant to both BSR and H. glycines; however, P. gregata colonization of these genotypes was increased. Stems of both a BSR-resistant and a BSR-susceptible genotype were colonized earlier by P. gregata in the presence than in the absence of H. glycines. Our findings indicate that H. glycines can increase the incidence and severity of BSR in soybean regardless of resistance or susceptibility to either pathogen.

Soybean Cyst Nematode Reproduction in the North Central United States

An experiment was conducted in Heterodera glycines-infested fields in 40 north central U.S. environments (21 sites in 1994 and 19 sites in 1995) to assess reproduction of this nematode. Two resistant and two susceptible soybean cultivars from each of the maturity groups (MG) I through IV were grown at each site in 6.1 m by 4 row plots. Soil samples were collected from each plot at planting and harvest and processed at Iowa State University to determine H. glycines initial (Pi) and final (Pf) population densities as eggs per 100 cm3 of soil. Overall, reproduction (Pf/Pi) of H. glycines on susceptible cultivars in all MG was similar. Reproduction was higher on MG III and IV susceptible cultivars than on those in MG I and II. Resistant MG I and II cultivars reduced nematode population densities more consistently than those in MG III and IV. Reproduction of the nematode was similar among sites within the same maturity zone (MZ), defined as the areas of best adaptation of the corresponding MG. Nonetheless, careful monitoring of nematode population densities is necessary to assess changes that occur over time in individual fields.

Effects of fertilizers on virulence of Steinernema carpocapsae

Abstract The effects of three fertilizers (fresh cow manure, composted manure, and urea) were determined on the virulence of Steinernema carpocapsae (Weiser) against the greater wax moth, Galleria mellonella (L.). Nitrogen levels were standardized among fertilizer treatments at two levels (280 and 560 kg ha−1). Urea and fresh manure decreased nematode virulence in laboratory experiments. In field experiments, however, only the fresh manure treatment reduced nematode virulence. In both laboratory and field experiments, composted manure did not affect nematode virulence. Fertilizer effects on virulence of S. carpocapsae were more rapid in a soil with reduced organic matter than in a soil with higher levels of organic matter. Factors causing reduced nematode virulence are discussed.

A nematode, fungus, and aphid interact via a shared host plant: implications for soybean management

Soybean, Glycine max (L.) Merrill (Fabaceae), is an introduced crop to America and initially benefited from a small number of pests threatening its production. Since its rapid expansion in production beginning in the 1930s, several pests have been introduced from the native range of soybean. Our knowledge of how these pests interact and the implications for management is limited. We examined how three common economic soybean pests, the nematode Heterodera glycines Ichinohe (Nematoda: Heteroderidae), the fungus Cadophora gregata Harrington & McNew (Incertae sedis), and the aphid Aphis glycines Matsumura (Hemiptera: Aphididae), interact on soybean cyst nematode‐susceptible (SCN‐S) and soybean cyst nematode‐resistant cultivars carrying the PI 88788 resistance source (SCN‐R). From 2008 to 2010, six soybean cultivars were infested with either a single pest or all three pests in combination in a micro‐plot field experiment. Pest performance was measured in a ‘single pest’ treatment and compared with pest performance in the ‘multiple pest’ treatment, allowing us to measure the impact of SCN resistance and the presence of other soybean pests on each pest’s performance. Performance of H. glycines (80% reduction in reproduction) and A. glycines (19.8% reduction in plant exposure) was reduced on SCN‐R cultivars. Regardless of cultivar, the presence of multiple pests significantly decreased the performance of A. glycines, but significantly increased H. glycines performance. The presence of multiple pests decreased the performance of C. gregata on SCN‐S soybean cultivars (20.6% reduction in disease rating).