Elson J. Shields

An aerobiological framework for assessing cross-pollination in maize

Abstract Maize (Zea mays L.) is one of the world’s most important crops. Until recent times, improvement in maize resulted from the manipulation and exchange of genetic information within the genus Zea. With the advent of genetic engineering, genetic information from other species is routinely incorporated into the maize genome. Since maize is a wind-pollinated outcrossing species, questions arise concerning the flow of genetic information between genetically modified (GM) and non-GM maize. Because of the rapidly accelerating introduction of GM corn into agricultural production, improved aerobiological models to predict how far and to what extent maize pollen can be transported in the atmosphere are needed today. Models should provide quantitative understanding of both short- and long-range pollen dispersal, thereby offering a means to evaluate deposition of viable maize pollen in seed production fields as well as on neighboring farms. Central to pollen dispersal is the rich interplay between physics and biology and between space and time. We present an aerobiological framework for assessing corn pollen movement in the atmosphere that could also be applied to other crops and to uncultivated species. An important aim of this paper is to spark interest in the scientific community to generate a more complete framework of corn pollen dispersal. As such this manuscript presents a multidisciplinary albeit incomplete perspective to an emerging applied problem.

Genetic Structure of Atmospheric Populations of Gibberella zeae

ABSTRACT Gibberella zeae, causal agent of Fusarium head blight (FHB) of wheat and barley and Gibberella ear rot (GER) of corn, may be transported over long distances in the atmosphere. Epidemics of FHB and GER may be initiated by regional atmospheric sources of inoculum of G. zeae; however, little is known about the origin of inoculum for these epidemics. We tested the hypothesis that atmospheric populations of G. zeae are genetically diverse by determining the genetic structure of New York atmospheric populations (NYAPs) of G. zeae, and comparing them with populations of G. zeae collected from seven different states in the northern United States. Viable, airborne spores of G. zeae were collected in rotational (lacking any apparent within-field inoculum sources of G. zeae) wheat and corn fields in Aurora, NY in May through August over 3 years (2002 to 2004). We evaluated 23 amplified fragment length polymorphism (AFLP) loci in 780 isolates of G. zeae. Normalized genotypic diversity was high (ranging from 0.91 to 1.0) in NYAPs of G. zeae, and nearly all of the isolates in each of the populations represented unique AFLP haplotypes. Pairwise calculations of Neis unbiased genetic identity were uniformly high (>0.99) for all of the possible NYAP comparisons. Although the NYAPs were genotypically diverse, they were genetically similar and potentially part of a large, interbreeding population of G. zeae in North America. Estimates of the fixation index (G(ST)) and the effective migration rate (Nm) for the NYAPs indicated significant genetic exchange among populations. Relatively low levels of linkage disequilibrium in the NYAPs suggest that outcrossing is common and that the populations are not a result of a recent bottleneck or invasion. When NYAPs were compared with those collected across the United States, the observed genetic identities between the populations ranged from 0.92 to 0.99. However, there was a significant negative correlation (R = -0.59, P < 0.001) between genetic identity and geographic distance, suggesting that some genetic isolation may occur on a continental scale. The contribution of long-distance transport of G. zeae to regional epidemics of FHB and GER remains unclear, but the diverse atmospheric populations of G. zeae suggest that inoculum may originate from multiple locations over large geographic distances. Practically, the long-distance transport of G. zeae suggests that management of inoculum sources on a local scale, unless performed over extensive production areas, will not be completely effective for the management of FHB and GER.

Horseweed (Conyza canadensis) seed collected in the planetary boundary layer

Abstract Horseweed is a winter or summer annual plant, native to North America and distributed worldwide in temperate climates. This plant is considered an important agricultural weed because it can reduce agricultural yields by 90% at high densities and becomes problematic under low-tillage agriculture. Seed production is robust with an estimated 200,000 seeds produced per plant, and seed dispersal is wind-assisted. The confirmation of glyphosate-resistant horseweed in Delaware in 2001 and the rapid spread of the resistant biotype, currently covering more than 44,000 ha, has necessitated a change in the discussion about weed dispersal. Large radio-controlled airplanes were used to sample the lower atmosphere for the presence of horseweed seeds during a 3-d period in early September 2005 in southern Delaware. The collection of multiple seeds at heights ranging from 41 to 140 m above ground level strongly suggests that horseweed seeds are entering the Planetary Boundary Layer (PBL) of the atmosphere, where long-ranged transport of aerial biota frequently occurs. With wind speeds in the PBL frequently exceeding 20 m s−1, seed dispersal can easily exceed 500 km in a single dispersal event. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq.

Quantifying Aerial Concentrations of Maize Pollen in the Atmospheric Surface Layer Using Remote-Piloted Airplanes and Lagrangian Stochastic Modeling

Abstract The extensive adoption of genetically modified crops has led to a need to understand better the dispersal of pollen in the atmosphere because of the potential for unwanted movement of genetic traits via pollen flow in the environment. The aerial dispersal of maize pollen was studied by comparing the results of a Lagrangian stochastic (LS) model with pollen concentration measurements made over cornfields using a combination of tower-based rotorod samplers and airborne radio-controlled remote-piloted vehicles (RPVs) outfitted with remotely operated pollen samplers. The comparison between model and measurements was conducted in two steps. In the first step, the LS model was used in combination with the rotorod samplers to estimate the pollen release rate Q for each sampling period. In the second step, a modeled value for the concentration Cmodel, corresponding to each RPV measured value Cmeasure, was calculated by simulating the RPV flight path through the LS model pollen plume corresponding to the ...

Night-time spore deposition of the fusarium head blight pathogen, Gibberella zeae, in rotational wheat fields

Many fungal plant pathogens rely on atmospheric motion systems for transport. The movement of these pathogens in the atmosphere is characterized by processes of liberation, drift, and deposition. We observed temporal patterns of viable spore deposition of Gibberella zeae, causal agent of fusarium head blight (FHB) of wheat, over rotational wheat fields (with no visible residues of corn or wheat, potential inoculum sources of G. zeae) in Aurora, New York, United States. Viable, airborne spores of G. zeae were collected on Petri plates containing selective medium placed 30 cm above wheat canopies. Over 40 000 viable spores of G. zeae were collected over a total of 73 day (6:00 am to 8:00 pm) or night (8:00 pm to 6:00 am) sampling periods in 3 years (2002, 2004, and 2005). The vast majority of the spores was collected at night (94% in 2002, 86% in 2004, and 82% in 2005). Viable spores were deposited in all but one sampling period spanning spike emergence through kernel milk stages of local wheat. Seven major deposition events (>50 colonies, on average, per Petri plate) were observed, all at night, and three of these were coincident with rainfall. In a single wheat field in 2004, viable spores of G. zeae were collected every 2 h throughout the night (starting at 8:00 pm) for six consecutive nights. Overall, 87% of the spores were deposited from 12:00 pm until 6:00 am, with most of the spores deposited between 4:00 am and 6:00 am. In 2005, viable spores of G. zeae were collected over variable landscape environments (two winter wheat fields, two soybean fields, one alfalfa hay field, and one fallow corn field) within 1 km2 area. Temporal patterns of viable spore deposition were virtually identical over all of the landscape environments, suggesting that spores were being deposited over kilometer distances from a well-mixed atmospheric source of inoculum. Atmospheric settling (gravitational fallout of spores) coinciding with the development of an inversion layer may explain the high degree of spore deposition at night. Vertical mixing and the presence of a mixed or turbulent layer during the daylight hours may account for the relatively low number of spores collected during the day. The deposition of spores of G. zeae primarily at night in rotational wheat fields raises some interesting questions about the origin of inoculum for epidemics of FHB. Our results suggest that spore deposition may be separated from spore release in both time and space. The cumulative exposure of wheat spikes to airborne spores of G. zeae, and the deposition of these spores mainly at night, should be considered in future prediction models and management strategies for fusarium head blight.

Calcium-regulated appressorium formation of the entomopathogenic fungusZoophthora radicans

SummaryThe fungusZoophthora radicans (Zygomycetes: Entomophthorales) requires external Ca2+ for appressorium formation but not for conidial germination. The number of appressoria formed depends on the Ca2+ concentration of the medium. At low [Ca2+] (100 pM) nuclear division and germ tube growth are significantly reduced compared to higher Ca2+ concentrations (10 and 1,000 μM). By contrast, neither external K+ nor external Cl− is needed for germination or appressorium formation. Treatment of conidia with a Ca2+-antagonist, Nd3+, and a Ca2+-channel blocker, nifedipine, inhibits appressorium formation, showing that a Ca2+ influx is required for appressorium formation. Furthermore, the partial yet saturating inhibition by nifedipine and complete inhibition by Nd3+ indicates that at least two kinds of Ca2+ channels are involved in appressorium formation. A contribution of intracellular Ca2+ to the signal transduction chain for the formation of appressoria is demonstrated by the inhibitory effect of the intracellular Ca2+ antagonist TMB-8. The calmodulin antagonists R24571, TFP, W-7, and W-5 inhibit appressorium formation at concentrations which have no effect on germination. The data presented in this paper are consistent with the hypothesis that a Ca2+/calmodulin system is involved in regulating appressorium formation. However, since the direct effects of the drugs were not specifically tested on their proposed binding sites, we leave room for alternative hypotheses that have yet to be formulated.

The aerobiology of Fusarium graminearum

Current knowledge of the aerobiology of Fusarium graminearum sensu lato is based on decades of published research documenting the processes of spore discharge, atmospheric transport, and deposition in this important pathogen of cereal crops worldwide. Spores from both local and more distant sources have been shown to cause infection in susceptible cereal crops when environmental conditions are favorable. Susceptible crops may be exposed throughout a growing season to airborne spores deposited in rain events and in night-time hours through gravitational settling. Given that spores deposited on cereal florets originate from distant as well as local sources, disease risk forecasts, based currently on weather favoring local spore production during the days before peak infection (i.e., initiation of crop flowering), might be improved by placing greater emphasis on local weather directly favoring infection at and following the time of flowering. Also, considering the genetic diversity of fungal spores introduced to local agricultural fields following atmospheric transport, crop breeders should select resistant varieties based on screening against a set of fungal isolates that represent the range of virulence observed in fungal populations across a broader geographic region. An increased understanding of the aerobiology of F. graminearum contributes to the overall knowledge of plant pathogen transport in the atmosphere.

Maize Pollen Dispersal under Convective Conditions

The widespread adoption of genetically modified (GM) crops has led to a need to better understand the atmospheric transport of pollen because of concerns over potential cross-pollination between GM and non-GM crops. Maize pollen concentrations were modeled by a modified Lagrangian stochastic (LS) model of the convective boundary layer (CBL) and were compared with concentrations measured by airborne remotely piloted vehicles (RPVs) flown from directly above to 2 km from source fields. The turbulence parameterization in an existing CBL LS model was modified to incorporate the effects of shear-driven turbulence, which has an especially large impact near the surface, where maize pollen is released. The modified model was used to calculate concentrations corresponding to the RPV flight tracks. For the most convective cases, when at least 95% of the pollen came from sources near the RPV flight track for which source strength measurements are available and the results are less sensitive to uncertainty in wind direction since most of the pollen came from directly beneath the flight track, the geometric mean of the ratio between the modeled and measured concentrations was 0.94. When cases with larger contributions from more distant fields were included, the overall geometric mean decreased to 0.43. The scatter of the measured concentrations about the modeled values followed a lognormal distribution. These results indicate that the modified model presented herein can substantially improve the description of the near-source dispersion of heavy particles released near the surface during convective conditions.

Prevalence, severity, and association of fungal crown and root rots with injury by the clover root curculio in New York alfalfa.

A survey of 61 randomly selected alfalfa fields in four physiographic regions of New York was utilized to assess the incidence, severity, and fungal flora associated with crown and root rots, as well as the incidence and severity of injury by the clover root curculio (Sitona hispidulus) and its possible association with root diseases. Five stratified random subsamples, each comprising four to nine alfalfa plants, were evaluated from each field

Multiple-Species Natural Enemy Approach for Biological Control of Alfalfa Snout Beetle (Coleoptera: Curculionidae) Using Entomopathogenic Nematodes

Abstract Multiple-species natural enemy approach for the biological control of the alfalfa snout beetle, Otiorhynchus ligustici (L.) (Coleoptera: Curculionidae), was compared with using single-species of natural enemies in the alfalfa ecosystem by using entomopathogenic nematodes with different dispersal and foraging behaviors. Steinernema carpocapsae NY001 (ambush nematode), Heterorhabditis bacteriophora Oswego (cruiser nematode), and Steinernema feltiae Valko (intermediate nematode) were applied in single-species, two-species combinations, and one three-species combination treatments at 2.5 × 109 infective juveniles per hectare. All nematode species persisted for a full year (357 d). S. carpocapsae NY001 protected the plants from root-feeding damage better than H. bacteriophora Oswego but allowed for higher larval survival than all other nematode treatments. S. feltiae Valko protected the plants better than H. bacteriophora Oswego and controlled alfalfa snout beetle larvae better than S. carpocapsae NY001. H. bacteriophora Oswego allowed for similar root damage compared with control plots but reduced larval populations better than S. carpocapsae NY001. The combination of S. carpocapsae NY001 and H. bacteriophora Oswego provided significantly better protection for the plants than the control (unlike H. bacteriophora Oswego alone) and reduced host larva survival more than S. carpocapsae NY001 alone. The combination S. feltiae Valko and H. bacteriophora Oswego could not be statistically separated from the performance of S. feltiae Valko applied alone.