David G. Schmale

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.

Fusarium head blight.

The first symptoms of Fusarium head blight include a tan or brown discoloration at the base of a floret within the spikelets of the head. As the infection progresses, the diseased spikelets become light tan or bleached in appearance (Figure 1). The infection may be limited to one spikelet, but if the fungus invades the rachis the entire head may develop symptoms of the disease. The base of the infected spikelets and portions of the rachis often develop a dark brown color. When weather conditions have been favorable for pathogen reproduction, the fungus may produce small orange clusters of spores or black reproductive structures called perithecia on the surface of the glumes. Infected kernels are often shriveled, white, and chalky in appearance (Figure 2). In some cases, the diseased kernels may develop a red or pink discoloration.

Spatial patterns of viable spore deposition of Gibberella zeae in wheat fields

ABSTRACT An increased understanding of the epidemiology of Gibberella zeae will contribute to a rational and informed approach to the management of Fusarium head blight (FHB). An integral phase of the FHB cycle is the deposition of airborne spores, yet there is no information available on the spatial pattern of spore deposition of G. zeae above wheat canopies. We examined spatial patterns of viable spore deposition of G. zeae over rotational (lacking cereal debris) wheat fields in New York in 2002 and 2004. Viable, airborne spores (ascospores and macroconidia) of G. zeae were collected above wheat spikes on petri plates containing a selective medium and the resulting colonies were counted. Spores of G. zeae were collected over a total of 68 field environments (three wheat fields during 54 day and night sample periods over 2 years) from spike emergence to kernel milk stages of local wheat. Spatial patterns of spore deposition were visualized by contour plots of spore counts over entire fields. The spatial pattern of spore deposition was unique for each field environment during each day and night sample period. Spore deposition patterns during individual sample periods were classified by spatial analysis by distance indices (SADIE) statistics and Mantel tests. Both analyses indicated that the majority (93%) of the spore deposition events were random, with the remainder being aggregated. All of the aggregated patterns were observed during the night. Observed patterns of spore deposition were independent of the mean number of viable spores deposited during individual sample periods. The spatial pattern for cumulative spore deposition during anthesis in both years became aggregated over time. Contour maps of daily and cumulative spore deposition could be compared with contour maps of FHB incidence to gain insights into inoculum thresholds and the timing of effective inoculum for infection.

The forcible discharge distance of ascospores of Gibberella zeae

Ascospores of Gibberella zeae are transported through the atmosphere to the spikes of wheat and barley and the silks of corn ears, where they may cause fusarium head blight and gibberella ear rot on susceptible cultivars. The first process in ascospores becoming airborne is the liberation or forcible discharge from perithecia. We measured the distance that ascospores of G. zeae were forcibly discharged in still air, and related this distance to the conditions necessary for transport into the atmosphere. Ascospores were discharged inside small glass chambers to distances ranging from <1 mm to nearly 10 mm away from culture surfaces. On average, ascospores from 6-day-old perithecia were discharged 4.6 mm, and those from 12-day-old perithecia, 3.9 mm. A large percentage of spores were discharged distances sufficient to surpass the laminar boundary layer of air that exists in the field during daylight hours. Since it takes less than about 3 s for a discharged ascospore of G. zeae to settle to the ground in still air, it is unlikely that a significant fraction of discharged spores surpass the laminar boundary layer and become airborne in conditions typically encountered at night. Quantitative spore-dispersal models pinpointing the timing and magnitude of ascospore release could potentially be used to estimate the relative risk of infection from local and more distant sources of inoculum of G. zeae.

Monitoring the Long-Distance Transport of Fusarium graminearum from Field-Scale Sources of Inoculum

The fungus Fusarium graminearum causes Fusarium head blight (FHB) of wheat. Little is known about dispersal of the fungus from field-scale sources of inoculum. We monitored the movement of a clonal isolate of F. graminearum from a 3,716 m2 (0.372 ha) source of inoculum over two field seasons. Ground-based collection devices were placed at distances of 0 (in the source), 100, 250, 500, 750, and 1,000 m from the center of the clonal sources of inoculum. Three polymorphic microsatellites were used to identify the released clone from 1,027 isolates (790 in 2011 and 237 in 2012) of the fungus. Results demonstrated that the recovery of the released clone decreased at greater distances from the source. The majority (87%, 152/175 in 2011; 77%, 74/96 in 2012) of the released clone was recaptured during the night (1900 to 0700). The released clone was recovered up to 750 m from the source. Recovery of the released clone followed a logistic regression model and was significant (P < 0.041 for all slope term scenarios) as a function of distance from the source of inoculum. This work offers a means to experimentally determine the dispersal kernel of a plant pathogen, and could be integrated into management strategies for FHB.

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.

Lagrangian coherent structures are associated with fluctuations in airborne microbial populations

Many microorganisms are advected in the lower atmosphere from one habitat to another with scales of motion being hundreds to thousands of kilometers. The concentration of these microbes in the lower atmosphere at a single geographic location can show rapid temporal changes. We used autonomous unmanned aerial vehicles equipped with microbe-sampling devices to collect fungi in the genus Fusarium 100 m above ground level at a single sampling location in Blacksburg, Virginia, USA. Some Fusarium species are important plant and animal pathogens, others saprophytes, and still others are producers of dangerous toxins. We correlated punctuated changes in the concentration of Fusarium to the movement of atmospheric transport barriers identified as finite-time Lyapunov exponent-based Lagrangian coherent structures (LCSs). An analysis of the finite-time Lyapunov exponent field for periods surrounding 73 individual flight collections of Fusarium showed a relationship between punctuated changes in concentrations of Fusarium and the passage times of LCSs, particularly repelling LCSs. This work has implications for understanding the atmospheric transport of invasive microbial species into previously unexposed regions and may contribute to information systems for pest management and disease control in the future.

Local Distance of Wheat Spike Infection by Released Clones of Gibberella zeae Disseminated from Infested Corn Residue

Knowledge of the movement of Gibberella zeae (Fusarium graminearum) from a local source of inoculum in infested cereal debris is critical to the management of Fusarium head blight (FHB) of wheat. Previous spatial dissemination and infection studies were unable to completely distinguish the contributions of released inocula from those of background inocula. Clones of G. zeae were released and recaptured in five wheat fields in New York and Virginia in 2007 and 2008. Amplified fragment length polymorphisms were used to track and unambiguously identify the released clones in heterogeneous populations of the fungus recovered from infected wheat spikes collected at 0, 3, 6, and ≥24 m from small-area sources of infested corn residues. The percent recovery of the released clones decreased significantly at fairly short distances from the inoculum sources. Isolates of G. zeae recovered at 0, 3, 6, and ≥24 m from the center of source areas shared 65, 19, 13, and 5% of the genotypes of the released clones, respectively. More importantly, the incidence of spike infection attributable to released clones averaged 15, 2, 1, and <1% at 0, 3, 6, and ≥24 m from source areas, respectively. Spike infection attributable to released clones decreased an average of 90% between 3 and 6 m from area sources of inoculum, and the spike infection potential of inocula dispersed at this range did not differ significantly from background sources. Our data suggest that FHB field experiments including a cereal debris variable should incorporate debris-free borders and interplots of at least 3 m and preferably 6 m to avoid significant interplot interference from spores originating from within-field debris.

Path planning for efficient UAV coordination in aerobiological sampling missions

This paper is concerned with the coordinated flight of two autonomous UAVs to be used for aerobiological sampling of biological threat agents above agricultural fields. The periodic sampling task involves two phases: a sampling interval and an initialization interval. During the sampling interval, both vehicles must employ their aerobiological sampling devices and follow a precise ground track in the presence of sustained winds. During the initialization interval, the vehicles move to their respective initial states to begin the next sampling interval. To maximize the volume of air sampled by the UAVs during an individual sampling mission, the initialization interval must be as short as possible. The paper provides a simple, geometric method for generating candidate time optimal paths in steady winds, based on Dubins¿ well-known results for minimum time paths of bounded curvature. The approach is used to generate paths for both UAVs, which must coordinate their motion along their respective paths in order to avoid collision. The described methods were tested during an aerobiological sampling experiment focusing on the plant pathogen Phytophthora infestans.

Highways in the Sky: Scales of Atmospheric Transport of Plant Pathogens

Many high-risk plant pathogens are transported over long distances (hundreds of meters to thousands of kilometers) in the atmosphere. The ability to track the movement of these pathogens in the atmosphere is essential for forecasting disease spread and establishing effective quarantine measures. Here, we discuss the scales of atmospheric dispersal of plant pathogens along a transport continuum (pathogen scale, farm scale, regional scale, and continental scale). Growers can use risk information at each of these dispersal scales to assist in making plant disease management decisions, such as the timely application of appropriate pesticides. Regional- and continental-scale atmospheric features known as Lagrangian coherent structures (LCSs) may shuffle plant pathogens along highways in the sky. A promising new method relying on overlapping turbulent back-trajectories of pathogen-laden parcels of air may assist in localizing potential inoculum sources, informing local and/or regional management efforts such as conservation tillage. The emergence of unmanned aircraft systems (UASs, or drones) to sample plant pathogens in the lower atmosphere, coupled with source localization efforts, could aid in mitigating the spread of high-risk plant pathogens.