Target spot, caused by Corynespora cassiicola, has recently emerged as a problematic foliar disease of cotton. This pathogen causes premature defoliation during boll set and maturation that can subsequently impact yield, and on certain cotton cultivars loss can be substantial. This study sought to better understand target spot epidemics and disease-incited yield losses on cotton. In order to establish a range of disease, varying numbers of fungicide applications were made to each of two cotton cultivars in each of four site-years. Target spot intensity was rated over several dates beginning in late July or early August and continuing into September. Yield of seed plus lint (seed cotton) was recorded at harvest. When analyzed across cultivars, a second or third fungicide application increased yield compared with no treatment. Lack of significant yield response with a single fungicide application may have been due to timing of that application which preceded disease onset. The cultivar PhytoGen 499 WRF had consistently greater defoliation than any of the three Deltapine cultivars grown in each site-year. However, yields of both cultivars responded similarly to the fungicide regimes. Yield loss models based on late August defoliation were only predictive at site-years where conditions favored target spot development, i.e., abundant rain and moderate temperatures. Epidemic development fit the Gompertz growth model better than it did a logistic model. Knowledge of the underlying mathematical character of the epidemiology of target spot will prove useful for development of a predictive model for the disease.
Rust, putatively caused by Puccinia emaculata, is a widespread and potentially damaging disease of switchgrass, a crop produced as feedstock for livestock and bioenergy. Azoxystrobin, chlorothalonil, and myclobutanil were applied at 1-, 2-, 3-, or 4-wk intervals for 12 to 14 wks to the vegetatively propagated switchgrass cultivar ‘Cloud Nine’ to assess fungicide selection and application interval for the control of rust as well as the impact of this disease on switchgrass biomass yield. While rust severity significantly differed among study years, azoxystrobin and myclobutanil were often equally and more effective than chlorothalonil at controlling rust, with superior disease control coming at the shorter compared to extended application intervals. Year, product, application interval, and product × interval significantly impacted dry biomass yield, which was greatest in 2016 and lowest in 2014. Dry biomass yield protection was significantly better with azoxystrobin and myclobutanil applications than with chlorothalonil or no fungicide. Linear regression models with the final disease rating, as well as with AUDPC in each year, were significant but coefficients of determination were low to moderate (0.21 < R2 < 0.60), indicating that rust response and subsequent disease impact on dry biomass yield were impacted by other factors. From our models, an estimated 3 to 5% biomass decline was calculated for each 10% increment in rust-related leaf necrosis observed at the final September rating date. With rust-related leaf necrosis > 80% by 1 Sept in each of four study years, biomass yield may be reduced by 24 to 40% if rust problems are not managed in switchgrass crops.
In Coastal Plain soils of the southeastern United States, the root-knot nematode (RKN) Meloidogyne incognita race 3 causes significant yield loss in corn. Impact of abamectin + thiamethoxam and clothianidin + Bacillus firmus I-1582 nematicide seed treatments along with terbufos granular nematicide on RKN reproduction, plant populations, plant growth, and yield was assessed at two Alabama sites. Thiamethoxam and clothianidin insecticide seed treatments were included as controls. A factorial arranged as split-split plot with year as the main plot, seed treatment as the split plot, and granular nematicide as the split-split-plot treatment was used. Lower plant populations were noted for terbufos-treated corn at one study site. Reduced RKN reproduction was observed with terbufos granular nematicide at both study sites. Fresh seedling weights and yields were usually higher for the terbufos- than non-terbufos-treated corn. At both sites, both nematicide seed treatments and their insecticide seed treatment counterparts had similar plant populations, RKN reproduction rate, and fresh seedling weights. Yield differences were noted at both locations between abamectin + thiamethoxam and thiamethoxam but not clothianidin + Bacillus firmus I-1582 and clothianidin. With mean yield gains up to 13.5%, terbufos was superior to both of the nematicide seed treatments for managing RKN and protecting corn yields. Accepted for publication 19 October 2015. Published 1 November 2015.
Native American tribes with bountiful harvests. Today it is widely recognized that this associated intercropping system derives much of its success from symbiotic bacteria (e.g. Rhizobium). These bacteria colonize the roots of leguminous plants, allowing them to fix atmospheric nitrogen into ammonia. However, the effect of this intercropping practice on the microbial community, independent of the effect of the symbiotic nitrogen-fixing bacteria, is not well understood. Therefore, a study was designed to model the effects of simultaneously intercropping bean and corn on the abundance of aerobic heterotrophic, free-living nitrogen-fixing, and symbiotic nitrogen-fixing bacteria, as well as plant growth and fecundity markers. In parallel, the benefits mediated by rhizobia were evaluated by inoculating a duplicate set of treatments with N-Dure, a rhizobia-containing inoculum. Native American varieties of pole-bean (Phaseolus vulgaris L.) and corn (Zea mays mays L.) were planted in monoculture and biculture treatments. All cultivations were maintained under greenhouse conditions for 52 days with daily watering and no additional fertilizer or microbial amendments. Although a significant increase in weight per plant was noted for the inoculated biculture when compared to either the inoculated bean or corn monocultures (p ≤ 0.05), the abundance of heterotrophic and free-living nitrogen-fixing bacteria did not show a significant change from the related controls, with or without inoculation. However, symbiotic nitrogen-fixing bacteria, as measured by root nodulation, increased significantly (p ≤ 0.05) for the inoculated biculture and single planting. Thus, these data confirm that corn benefited from this associated intercropping system as shown by an increase in plant biomass that can be attributed to Rhizobium. However, neither the legume-bacteria symbiotic relationship nor the increase in plant biodiversity resulting from this intercropping practice appears to have had significant effects on the abundance of the two common soil-associated bacterial groups evaluated, though further research would be necessary to fully assess the changes to heterotrophic bacterial diversity at the species level. KEYWORDS: Three Sisters; Nitrogen-fixing Bacteria; Inoculation with Rhizobia; Plant Growth Promoting Bacteria; Soil Microbial Biota; Corn and Bean Simultaneous Planting.
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