Location of Helicoverpa zea (Lepidoptera: Noctuidae) larvae on different plant parts of determinate and indeterminate soybean

Helicoverpa zea (Boddie) is a damaging pest of many crops including soybean, Glycine max (L.), especially in the southern United States. Previous studies have concluded that oviposition and development of H. zea larvae mirror the phenology of soybean, with oviposition occurring during full bloom, younger larvae developing on blooms and leaves, intermediate aged larvae developing on varying tissue types, and older larvae developing on flowers and pods. In a field trial, we investigated the presence of natural infestations of H. zea larvae by instar in determinate and indeterminate soybean varieties. In complementary experiments, we artificially infested H. zea and allowed them to oviposit on plants within replicated cages (one with a determinate variety and two with an indeterminate variety). Plants were sampled weekly during the time larvae were present. In the natural infestation experiment, most larvae were found on blooms during R3 and were early to middle instars; by R4, most larvae were found on leaves and were middle to late instars. In contrast, in the cage study, most larvae were found on leaves regardless of soybean growth stage or larval stage. Determinate and indeterminate growth habit did not impact larval preference for different soybean tissue types. Our studies suggest H. zea larvae prefer specific tissue types, but also provide evidence that experimental design can influence the results. Finally, our finding of larval preference for leaves contrasts with findings from previous studies.

Propagule Densities of Macrophomina phaseolina in Soybean Tissue and Soil as Affected by Tillage, Cover Crop, and Herbicide

All current commercial soybean cultivars are susceptible to charcoal rot, a disease caused by Macrophomina phaseolina. Efforts to manage this disease through non-genetic means have not been effective. However, the combined effects of tillage, cover crop and herbicide use, and their roles in the population dynamics of this fungus have not been fully investigated. A field experiment was conducted in 2002 through 2004 at Stoneville, MS, to determine the population dynamics of Macrophomina phaseolina (Tassi) in soybean stem and root tissues at harvest and in soil at planting and harvest as affected by tillage, cover crop, and herbicide. The tillage treatments were conventional tillage (CT) and no-tillage (NT); the cover crops were hairy vetch, rye, and no cover crop; and the herbicide treatments were glyphosate- and non-glyphosate-based applications. Analysis of variance indicated that there was an effect due to tillage, cover crop, and tillage by year interaction on colony forming units (CFU) of M. phaseolina recovered from soybean tissue. Colony forming units in soybean tissue were greater under the CT than NT and were greater for hairy vetch and no cover crop than rye. Regardless of the cover crop system used, CFU in tissue was greater for CT in 2002 than in 2003 and 2004. Application of glyphosate did not affect the CFU in stem and root tissues or in the soil. The CFU from soil at harvest was significantly higher than at planting. The CFU in soil at planting and harvest was only affected by tillage and not by cover crop system. The CFU from stem and root tissues was greater than in soil suggesting that quantification of CFU in tissue may provide a better estimate of treatment effects at harvest. These results also suggest that charcoal rot may be better managed in the NT rather than in the CT system. Accepted for publication 11 December 2008. Published 30 January 2009.

Rapid and Sensitive Detection of Phytophthora sojae in Soil and Infected Soybeans by Species-Specific Polymerase Chain Reaction Assays

Root and stem rot caused by Phytophthora sojae is one of the most destructive diseases of soybean (Glycine max) worldwide. P. sojae can survive as oospores in soil for many years. In order to develop a rapid and accurate method for the specific detection of P. sojae in soil, the internal transcribed spacer (ITS) regions of eight P. sojae isolates were amplified using polymerase chain reaction (PCR) with the universal primers DC6 and ITS4. The sequences of PCR products were aligned with published sequences of 50 other Phytophthora species, and a region specific to P. sojae was used to design the specific PCR primers, PS1 and PS2. More than 245 isolates representing 25 species of Phytophthora and at least 35 other species of pathogens were used to test the specificity of the primers. PCR amplification with PS primers resulted in the amplification of a product of approximately 330 bp, exclusively from isolates of P. sojae. Tests with P. sojae genomic DNA determined that the sensitivity of the PS primer set is approximately 1 fg. This PCR assay, combined with a simple soil screening method developed in this work, allowed the detection of P. sojae from soil within 6 h, with a detection sensitivity of two oospores in 20 g of soil. PCR with the PS primers could also be used to detect P. sojae from diseased soybean tissue and residues. Real-time fluorescent quantitative PCR assays were also developed to detect the pathogen directly in soil samples. The PS primer-based PCR assay provides a rapid and sensitive tool for the detection of P. sojae in soil and infected soybean tissue.