Spatial and Temporal Variability of Sorghum Grain Yield: Influence of Soil,Water, Pests,and Diseases Relationships

This study was conducted to determine relationships between biotic and abiotic factors and to generate information needed to improve the management of site-specific farming (SSF). The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil texture, soil NO₃--N, soil pH, and greenbugs (Schizaphis graminum) (Gb) on sorghum grain yield were investigated at Halfway, TX on geo-referenced locations on a 30-m grid in 1997, 1998, and 1999. Grain yields were influenced by interrelationships among many factors. Grain yields were consistently high under 80% ET treatment and in the upper slopes where the clay and silt fractions of the soil were high. Soil NO₃--N, rainfall, hybrid, and Gb effects on grain yields were seasonally unstable. Soil NO₃--N increased grain yield when water was abundant and depressed grain yields when water was limiting. Plant density effects on grain yield were confounded with hybrid responses to drought and Gb infestation. Managing seasonally unstable factors is a major challenge for farmers and better ways to monitor crop growth and diagnose causes of poor plant growth are needed. To improve the management of SSF, effects of the relationships between biotic and abiotic factors on crop yield must be integrated and evaluated as a system. Based on our study, information on seasonally stable factors like elevation and soil texture is useful in identifying management zones for water and fertilizer application. Water and fertilizers management should be complemented by in-season management of seasonally unstable factors like soil NO₃--N, rainfall, hybrid, and Gb effects on grain yield.     

Uptake and distribution of [14C]metalaxyl by detached tobacco leaves

When the petioles of detached tobacco leaves (10–17 cm²) were incubated in aqueous solutions containing [¹⁴C]metalaxyl, uptake of the fungicide was dependent on the temperature and photoperiod. Detached leaves took up 78% more [¹⁴C]metalaxyl at 26°C than at 16°C. The rate of uptake in the light at 21°C was linear, but after an additional 20h in the dark, there was only twice as much fungicide in the leaves. Different sized leaves contained the same amount of fungicide per cm² area. Uptake by detached leaves of the ¹⁴C-labelled anilide lactones ofurace and RE-26940 [2-methoxy-N-(tetrahydro-2-oxo-3-thienyl)acet-2′,6′-xylidide] was similar to that of metalaxyl. At the concentration of metalaxyl (66 ng ml^?1) that controlled blue mould (Peronospora tabacina) on detached tobacco leaves, the amount of fungicide in the leaves was found to be 7.25 ng. Autoradiography showed that the distribution of [14C]metalaxyl in detached leaves after incubation for 23h was uniform, although higher concentrations of the label were present in the smaller veins of the leaves.     

Distribution of Pseudosclerotia of Monilinia vaccinii-corymbosi and Risk of Apothecial Emergence Following Mechanical Cultivation

ABSTRACT Pseudosclerotia (infected, mummified fruit) of Monilinia vacciniicorymbosi overwinter on the orchard floor and germinate to produce apothecia in early spring, providing the only source of primary inoculum for mummy berry disease of blueberry. Three experiments were carried out to develop a model for the relative efficacy of mechanical cultivation in reducing the risk associated with primary inoculum. In the first experiment, apothecial emergence from pseudosclerotia buried 0, 1.5, 3, 6, and 10 cm below the soil surface was monitored to determine the critical depth necessary to inhibit emergence. No apothecia emerged from pseudosclerotia buried at depths of >/=3 cm, and the critical depth of burial was determined at 2.6 cm by regression analysis. In the second experiment, pseudosclerotia or plastic beads (used as surrogates for pseudosclerotia) were placed on the soil surface of experimental plots before cultivation with an in-row rotary cultivator, a disc harrow, or a rotary cultivator with each implement operated in a single pass. Vertical distribution profiles of pseudosclerotia or beads in the topsoil were characterized after excavation with a custom-built sampling device. The proportion of pseudosclerotia placed below the critical depth of 2.6 cm was 20.9, 52.6, and 78.6% for the in-row rotary cultivator, the disc harrow, and the rotary cultivator, respectively. For all three implements, vertical distribution profiles of pseudosclerotia and plastic beads were very similar, allowing the latter to be used in subsequent experiments in commercial fields. In the third experiment, two blueberry plantings were surveyed to determine the horizontal distribution of pseudosclerotia on the orchard floor with distance from the crowns of the plants. The greatest frequency of pseudosclerotia occurred between 30 and 40 cm from the plants. Based on measurements of the distance from plants within which different implements can operate, the proportion of pseudosclerotia accessible by cultivation ranged from 58.7% for the disc harrow to 87.2% for the in-row rotary cultivator. Taken together, results from the three experiments indicated that cultivation with a single implement can reduce risk of apothecial emergence by about 50%. More effective risk reductions may be obtained by combining implements that result in deep burial of pseudosclerotia with those that have access to pseudosclerotia near the plants. This was demonstrated by a commercial cultivation method that utilized three passes of different implements and resulted in extensive reshaping of plant beds, placing 88.2% of beads below the critical depth of 2.6 cm.     

Cocultures of Peronospora tabacina and Nicotiana Species to Study Host-Pathogen Interactions

ABSTRACT Long-term cocultures of the tobacco blue mold pathogen, Peronospora tabacina, with Nicotiana tabacum and N. repanda callus were derived from infected host plant tissue. In this apparently contaminant-free system, sporulation occurred under similar conditions as in intact plants. Sporangia were collected from cocultures and used to complete Koch's postulates. The cocultures were grown under two light regimes. One consisted of 23 h of light followed by 1 h of darkness and the second comprised total darkness. Sporulation occurred frequently in the 23 h light-grown cocultures but resulted in production of abnormal sporangiophores and sporangia. Production of normal sporangiophores and sporangia was achieved by transferring light-grown cocultures to overnight darkness and resulted in necrosis of the callus. Cocultures of Peronospora tabacina with either host species, grown in total darkness, frequently sporulated with minimal necrosis over the course of 1 year. Thus, cocultures should prove useful as a source of Peronospora tabacina over extended periods of time at low risk of pathogen release, for studying the physiology of Peronospora tabacina- Nicotiana interactions, maintaining Peronospora tabacina lines for genetic studies, and providing a reliable source of axenic inoculum for research.     

Interactions of Peronospora tabacina with Roots of Nicotiana spp. in Gnotobiotic Associations

ABSTRACT Peronospora tabacina is an obligate plant pathogen that causes downy mildew disease on several species of Nicotiana, including N. tabacum (tobacco). The primary objective of this study was to use gnotobiotic associations to describe interactions between the pathogen and roots of either N. tabacum (cv. KY14) or N. repanda. We found that the pathogen was capable of moving systemically from shoots to roots of both host species and emerged from the root tissues as hyphae. We also demonstrated that root-associated hyphae were infectious on roots of nearby plants and resulted in new systemic infections. Following overnight darkness, sporulation of the pathogen was observed on infected roots exposed to air on both host species. We also found that within 2 months in culture, structures resembling resting stages of Peronospora tabacina were produced on hyphae emerging from roots of N. repanda but not N. tabacum. These findings appear relevant to both the epidemiology of the disease and to future studies of this and other downy mildew pathosystems.     

Bioassays using detached tobacco leaves to determine the sensitivity of Peronospora tabacina to fungicides

Two bioassay methods are described which use detached tobacco leaves to measure the sensitivity of Peronospora tabacina to systemic fungicides. Tobacco leaves (13–15 cm²), treated with fungicides before or after detachment from the plant, were inoculated with sporangia in water drops and, after incubation in beakers and Petri plates, the disease severity and/or production of sporangia was determined 4–7 days after treatment with the fungicides. Of 15 systemic fungicides applied to detached leaves, eight N-phenylamides at 0.066?1.0 μg ml^?1 controlled blue mould; metalaxyl was the most effective fungicide. Isolates of P. tabacina, collected in the field from tobacco plants grown in soil treated with metalaxyl, were not resistant to the fungicide applied to detached leaves prior to inoculation. The fungicide, applied to leaves before detachment, was used to measure the efficacy of five systemic N-phenylamide fungicides sprayed on the basal and unsprayed distal portions of the leaves. Blue mould was controlled on the basal portion of the leaf by all the fungicides at 0.66?1.0 μg ml^?1, but it required the application of 3–30 times more chemical on the basal portion to achieve comparable blue mould control on the distal part of the leaf.     

Removal of duvatrienediols from the surface of tobacco leaves increases their susceptibility to blue mold

Dipping leaf strips of greenhouse or field-grown tobacco (Nicotiana tabacum L., cv. Ky-14) plants into acetone for 1 s, prior to inoculation with sporangia of Peronospora tabacina Adam, increased their susceptibility to blue mold. Disease severity and sporangial production on leaf discs from acetone-treated leaves were markedly increased compared to those on discs from untreated leaves. Treatment with acetone also decreased variation in susceptibility of leaves from plants of various ages. Disease severity on discs obtained from attached leaves which were dipped in acetone for 1 s was three times greater up to 15 days after dipping than on discs from leaves that were not dipped. TLC and GLC analyses of the acetone extracts indicated that 95% or more of the major cuticular diterpenoids, α- and β-4,8,13-duvatriene-1,3-diols (DVT), were removed from the surface by dipping for 1 s. These compounds had not reappeared on the leaf surface 15 days after leaves were dipped in acetone for 1 s. Aqueous suspensions of the acetone-soluble constituents as well as authentic DVT, inhibited sporangial germination of P. tabacina (ED₁₀₀ = 25 ppm) and the antifungal activity was accounted for by DVT. When DVT was removed from a leaf surface and added back to the same leaf strip, the resistance of the leaf tissue was restored. As tobacco plants aged, their susceptibility to blue mold decreased and the quantity of DVT on the leaf surface increased. The data support a role for DVT in the resistance of tobacco against blue mold.     

Spatial and Temporal Variability of Corn Grain Yield: Site-Specific Relationships of Biotic and Abiotic Factors

Inadequate information on factors affecting crop yield variability has contributed to the slow adoption of site-specific farming (SSF). This study was conducted to determine the effects of biotic and abiotic factors on the spatial and temporal variability of irrigated corn grain yields and to derive information useful for SSF. The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil index (SI)(texture), soil NO₃–N, arthropods, and diseases on corn grain yield were investigated at Halfway, TX on geo-referenced locations. Grain yields were influenced by interrelationships among biotic and abiotic factors. Grain yields were consistently high under high water treatment, at higher elevations, and on soils with high SI (high clay and silt). Soil NO₃–N increased grain yields when water was adequate. Management zones for variable rate fertilizer and water application should, therefore, be based on information on elevation, SI, and soil NO₃–N. The effects of arthropods, diseases, and crop stress (due to drought and N) on corn grain yield were unpredictable. Spider mite (Oligonychus pratensis) and common smut (Ustilago zeae) damage occurred under hot and dry conditions in 1998. Spider mite infestations were high in areas with high soil NO₃–N. Moderate air temperatures and high relative humidity in 1999 favored southwestern corn borer (Diatraea grandiosella) and common rust (Puccinia maydis) incidences. Knowledge of conditions that favor arthropods and diseases outbreak and crop stress can improve the efficiency of scouting and in-season management of SSF. Management of SSF can be improved when effects of biotic and abiotic factors on grain yield are integrated and evaluated as a system.