Charcoal rot of soybean is caused by the fungal pathogen Macrophomina phaseolina. Effective and reliable techniques to evaluate soybean for resistance to this fungus are needed to work toward a management scheme that would utilize host resistance. Three experiments were conducted to investigate the use of a cut-stem inoculation technique to evaluate soybean genotypes for resistance to M. phaseolina. The first experiment compared aggressiveness of M. phaseolina isolates collected from soybean on different soybean genotypes. Significant (P < 0.05) differences among the isolates and genotypes for relative area under disease progress curve (RAUDPC) were found without a significant isolate–genotype interaction. The second experiment compared 14 soybean genotypes inoculated with M. phaseolina in multiple trials conducted in two environments, the greenhouse and growth chamber. Significant (P < 0.05) differences among environments and highly significant (P < 0.001) differences among soybean genotypes for RAUDPC were found. The environment–genotype interaction was nonsignificant (P > 0.05). Soybean genotypes DT97-4290, DT98-7553, DT98-17554, and DT99-16864 had significantly (P < 0.05) lower RAUDPC than 7 of the 14 genotypes. The third experiment evaluated resistance in selected Phaseolus spp. and soybean genotypes. The range of RAUDPC for Phaseolus spp. was similar to that of soybean. The Phaseolus lunatus ‘Bush Baby Lima’ had significantly (P < 0.05) lower RAUDPC than P. vulgaris genotypes evaluated. The cut-stem inoculation technique, which has several advantages over field tests, successfully distinguished differences in aggressiveness among M. phaseolina isolates and relative differences among soybean genotypes for resistance to M. phaseolina comparable with results of field tests.

Charcoal rot, caused by the fungal pathogen Macrophomina phaseolina, is an economically important disease on soybean and other crops including maize (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and sunflower (Helianthus annuus L.). Without effective cultural or chemical options to control charcoal rot in soybean [Glycine max (L.) Merr.], finding sources of genetic resistance is of high interest. In this study, 70 ancestral soybean genotypes were screened for resistance to M. phaseolina using a cut‐stem inoculation technique under semi‐controlled greenhouse conditions. Lesion progression on the stems in the first experiment was measured 7 to 15 d after inoculation. Three follow‐up experiments were conducted to select and confirm the genotypes with the strongest partial resistance. Two experiments evaluated lesion lengths and the third experiment evaluated seedling survival. In the two experiments measuring lesion lengths, PI 548302 (42 and 38 mm) and PI 548414 (36 and 52 mm) had significantly shorter lesion lengths than the moderately resistant genotype, DT97‐4290 (58 and 87 mm). In the fourth experiment, percentage survival of PI 548414 (88%), PI 548302 (81%), and PI 548178 (66%) were significantly higher than survival of DT97‐4290 (32%). These three genotypes may be useful as parents for developing soybean cultivars with charcoal rot resistance.

Soybean is the most produced and consumed oil seed crop worldwide. In 2013, 226 million metric tons were produced in over 70 countries. Organically produced soybean represents less than 0.1% of total world production. In the USA, the certified organic soybean crop was grown on 53 thousand ha or 0.17% of the total soybean acreage in the USA (32 million ha) in 2011. A gradual increase in production of organically grown soybean has occurred since the inception of organic labeling due to increased human consumption of soy products and increased demand for organic soybean meal to produce organic animal products. Production constraints caused by pathogens and insect pests are often similar in organic and non-organic soybean production, but management between the two systems often differs. In general, the non-organic, grain-type soybean crop are genetically modified higher-yielding cultivars, often with disease and pest resistance, and are grown with the use of synthetic pesticides. The higher value of organically produced soybean makes production of the crop an attractive option to some farmers. This article reviews production and uses of organically grown soybean in the USA, potential constraints to production caused by pathogens and insect pests, and management practices used to reduce the impact of these constraints.

Inadequate plant nutrition and biotic stress are key threats to current and future crop yields. Zinc (Zn) deficiency and toxicity in major crop plants have been documented, but there is limited information on how pathogen and pest damage may be affected by differing plant Zn levels. In our study, we used soybean plants as a host, a soybean pest, and three soybean pathogens to determine whether plant Zn levels change pest and disease assessments. Two soybean cultivars were grown in sand culture with a soluble nutrient solution that ranged from Zn-deficient to toxic. Detached leaves from these plants were either inoculated with Aphis glycines, the soybean aphid, Xanthomonas axonopodis pv. glycines, a bacterium that causes bacterial pustule, Sclerotinia sclerotiorum, the necrotrophic fungus responsible for stem rot, or Phakopsora pachyrhizi, a biotrophic obligate pathogen that causes soybean rust. There were significant (P < 5%) effects on aphid colonization, positive counts for bacterial pustule, S. sclerotiorum leaf area affected, and numbers of rust lesions associated with the Zn treatments. Plants grown with the physiologically optimal levels of Zn (2 μM) had less (P < 5%) soybean aphids cm -2 leaflet than plants grown without Zn, at 0.1 Zn (0.2 μM), or at 100 Zn fertilization (200 μM). Plants grown with the normal fertilization of Zn or 100 Zn had fewer (P < 5%) positive counts for bacterial pustule and less lesion area affected by S. sclerotiorum than plants grown without Zn or fertilized with 0.1 Zn. For soybean rust, plants grown with the physiologically optimal fertilization of Zn or 100 Zn had higher (P < 5%) lesions cm -2 on leaflets from plants grown without Zn or fertilized with 0.1 Zn. These results indicate different Zn nutrition levels in soybean significantly affected aphid and disease development.

Soybean hairy roots transformed with the resveratrol synthase and resveratrol oxymethyl transferase genes driven by constitutive Arabidopsis actin and CsVMV promoters were characterized. Transformed hairy roots accumulated glycoside conjugates of the stilbenic compound resveratrol and the related compound pterostilbene, which are normally not synthesized by soybean plants. Expression of the non-native stilbenic phytoalexin synthesis in soybean hairy roots increased their resistance to the soybean pathogen Rhizoctonia solani. The expression of the AhRS3 gene resulted in 20% to 50% decreased root necrosis compared to that of untransformed hairy roots. The expression of two genes, the AhRS3 and ROMT, required for pterostilbene synthesis in soybean, resulted in significantly lower root necrosis (ranging from 0% to 7%) in transgenic roots than in untransformed hairy roots that had about 84% necrosis. Overexpression of the soybean prenyltransferase (dimethylallyltransferase) G4DT gene in soybean hairy roots increased accumulation of the native phytoalexin glyceollin resulting in decreased root necrosis.