Azole resistance mechanisms and population structure of the human pathogen Aspergillus fumigatus on retail plant products.

Aspergillus fumigatus is a ubiquitous saprotroph and human-pathogenic fungus that is life-threatening to the immunocompromised. Triazole-resistant A. fumigatus was found in patients without prior treatment with azoles, leading researchers to conclude that resistance had developed in agricultural environments where azoles are used against plant pathogens. Previous studies have documented azole-resistant A. fumigatus across agricultural environments, but few have looked at retail plant products. Our objectives were to determine if azole-resistant A. fumigatus is prevalent in retail plant products produced in the United States (U.S.), as well as to identify the resistance mechanism(s) and population genetic structure of these isolates. Five hundred twenty-five isolates were collected from retail plant products and screened for azole resistance. Twenty-four isolates collected from compost, soil, flower bulbs, and raw peanuts were pan-azole resistant. These isolates had the TR34/L98H, TR46/Y121F/T289A, G448S, and H147Y cyp51A alleles, all known to underly pan-azole resistance, as well as WT alleles, suggesting that non-cyp51A mechanisms contribute to pan-azole resistance in these isolates. Minimum spanning networks showed two lineages containing isolates with TR alleles or the F46Y/M172V/E427K allele, and discriminant analysis of principle components identified three primary clusters. This is consistent with previous studies detecting three clades of A. fumigatus and identifying pan-azole-resistant isolates with TR alleles in a single clade. We found pan-azole resistance in U.S. retail plant products, particularly compost and flower bulbs, which indicates a risk of exposure to these products for susceptible populations and that highly resistant isolates are likely distributed worldwide on these products.IMPORTANCEAspergillus fumigatus has recently been designated as a critical fungal pathogen by the World Health Organization. It is most deadly to people with compromised immune systems, and with the emergence of antifungal resistance to multiple azole drugs, this disease carries a nearly 100% fatality rate without treatment or if isolates are resistant to the drugs used to treat the disease. It is important to determine the relatedness and origins of resistant A. fumigatus isolates in the environment, including plant-based retail products, so that factors promoting the development and propagation of resistant isolates can be identified.

Fatal Attraction: Argiope Spiders Lure Male Hemileuca Moth Prey with the Promise of Sex.

Predator-prey coevolution, particularly chemo-ecological arms races, is challenging to study as it requires the integration of behavioral, chemical ecology, and phylogenetic studies in an amenable system. Moths of the genus Hemileuca (Saturniidae) are colorful, diurnal, and fast and often fly well above the vegetation canopy layer. However, several Hemileuca species have been reported as being captured in spider webs, specifically Argiope species (Araneidae). Female Hemileuca are known to produce mating pheromones and spiders of the Araneidae family are known to use pheromone lures to attract lepidopteran prey. We presented primarily female Argiope aurantia, which are attractive to male Anisota pellucida (Saturniidae), to different populations of Hemileuca species across the southern and western United States to categorize the homing response strength of different species of male Hemileuca. When we mapped these Argiope lure attraction strength categories onto the most recently published Hemileuca phylogeny, the behavioral patterns suggested a potential co-evolutionary arms race between predators and prey. Males of Hemileuca maia, H. grotei, and H. nevadensis (all in the same clade) appeared to have no attraction to A. aurantia, while H. magnifica and H. hera (within a different, separate clade) appeared to be strongly attracted to A. aurantia, but H. nuttalli (also within the H. hera and H. magnifica clade) displayed no attraction. Furthermore, Hemileuca eglanterina (yet a different clade) displayed strong, weak, and no attraction to A. aurantia, depending on the population. These apparent clade partitioning patterns of Argiope lure effectiveness and within-species variation in Hemileuca lure responses suggest a predator-prey coevolutionary history of measures and countermeasures.

Assessment of Prickly Sida as a Potential Inoculum Source for Sida Golden Mosaic Virus in Commercial Snap Bean Farms in Georgia, United States.

Sida golden mosaic virus (SiGMV), an obligate pathogen that infects snap beans (Phaseolus vulgaris), is known to infect prickly sida (Sida spinosa L.), which is a common weed in agricultural farms in Georgia. Prickly sida has also been reported as a suitable host of sweetpotato whitefly (Bemisia tabaci), the vector of SiGMV. Despite being a host for both SiGMV and its vector, the role of prickly sida as a reservoir and inoculum source for SiGMV in snap bean farms has not been evaluated. This study was conducted to document the occurrence of SiGMV-infected prickly sida plants and to assess its potential role as a source of SiGMV inoculum in snap bean farms. A survey of 17 commercial snap bean farms conducted in spring 2021 confirmed the presence of SiGMV-infected prickly sida in southern Georgia. In fall 2021 and 2022, on-farm field trials were conducted in four commercial farms where SiGMV-infected prickly sida plants were documented earlier as a part of survey in spring 2021. The spatial distribution and temporal patterns of adult whiteflies and SiGMV on snap bean were compared between macroplots (13.7 × 30.5 m) "with prickly sida" or "without prickly sida" that were at least 232 m apart from each other. We did not observe any consistent differences in counts of adult whiteflies between macroplots with or without prickly sida in the four commercial farms. SiGMV infection was detected earlier and with higher incidences in snap bean macroplots "with prickly sida" compared with macroplots "without prickly sida." An apparent disease gradient was observed in two of the four farms assessed. Higher SiGMV incidences were observed on the edges of macroplots "with prickly sida." These findings indicate prickly sida as a potential natural reservoir and a source for SiGMV spread in snap bean farms in southern Georgia.

Evidence of Xylella fastidiosa Infection and Associated Thermal Signatures in Southern Highbush Blueberry (Vaccinium corymbosum Interspecific Hybrids).

Xylella fastidiosa, a gram-negative bacterium vectored to plants via feeding of infected insects, causes a number of notorious plant diseases throughout the world, such as Pierce's disease (grapes), olive quick decline syndrome, and coffee leaf scorch. Detection of Xf in infected plants can be challenging because the early foliar disease symptoms are subtle and may be attributed to multiple minor physiological stresses and/or borderline nutrient deficiencies. Furthermore, Xf may reside within an infected plant for one or more growing seasons before traditional visible diagnostic disease symptoms emerge. Any method that can identify infection during the latent period or pre-diagnostic disease progress state could substantially improve the outcome of disease control interventions. Because Xf locally and gradually impairs water movement through infected plant stems and leaves over time, infected plants may not be able to effectively dissipate heat through transpiration-assisted cooling, and this heat signature may be an important pre-diagnostic disease trait. Here, we report on the association between thermal imaging, the early stages of Xf infection, and disease development in blueberry plants, and discuss the benefits and limitations of using thermal imaging to detect bacterial leaf scorch of blueberries.

Shoot Maturation Strengthens FLS2-Mediated Resistance to Pseudomonas syringae.

Temporospatial regulation of immunity components is essential for properly activating plant defense response. Flagellin-sensing 2 (FLS2) is a surface-localized receptor that recognizes bacterial flagellin. The immune function of FLS2 is compromised in early stages of shoot development. However, the underlying mechanism for the age-dependent FLS2 signaling is not clear. Here, we show that the reduced basal immunity of juvenile leaves against Pseudomonas syringae pv. tomato DC3000 is independent of FLS2. The flg22-induced marker gene expression and reactive oxygen species activation were comparable in juvenile and adult stages, but callose deposition was more evident in the adult stage than the juvenile stage. We further demonstrated that microRNA156, a master regulator of plant aging, does not influence the expression of FLS2 and FRK1 (Flg22-induced receptor-like kinase 1) but mildly suppresses callose deposition in juvenile leaves. Our experiments revealed an intrinsic mechanism that regulates the amplitude of FLS2-mediated resistance during aging. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Endogenous salicylic acid suppresses de novo root regeneration from leaf explants.

Plants can regenerate new organs from damaged or detached tissues. In the process of de novo root regeneration (DNRR), adventitious roots are frequently formed from the wound site on a detached leaf. Salicylic acid (SA) is a key phytohormone regulating plant defenses and stress responses. The role of SA and its acting mechanisms during de novo organogenesis is still unclear. Here, we found that endogenous SA inhibited the adventitious root formation after cutting. Free SA rapidly accumulated at the wound site, which was accompanied by an activation of SA response. SA receptors NPR3 and NPR4, but not NPR1, were required for DNRR. Wounding-elevated SA compromised the expression of AUX1, and subsequent transport of auxin to the wound site. A mutation in AUX1 abolished the enhanced DNRR in low SA mutants. Our work elucidates a role of SA in regulating DNRR and suggests a potential link between biotic stress and tissue regeneration.

Distribution, Diversity, and Soil Associations of Wine Grape Plant-Parasitic Nematodes in Georgia, U.S.A., Vineyards.

Wine grape (Vitis vinifera and V. vinifera hybrids) production in Georgia occurs in three distinct regions (North, West, and South) which can be characterized by sandy, sandy-loam, or sandy clay-loam soils. We studied plant-parasitic nematode (PPN) communities in 15 wine grape vineyards from the three primary growing regions to understand which nematodes are a concern and what soil characteristics are associated with their occurrence and relative abundance. Twelve genera of PPNs were detected throughout the state: Belonolaimus, Helicotylenchus, Hemicycliophora, Heterodera, Hoplolaimus, Meloidogyne, Mesocriconema, Paratrichodorus, Paratylenchus, Pratylenchus, Tylenchorhynchus, and Xiphinema. Nonmetric multidimensional scaling ordination and multirank permutation procedure identified PPN community differences and soil characteristics that were associated by region. Indicator species analysis identified Helicotylenchus, Mesocriconema, Tylenchorhynchus, and Xiphinema as statistically associated with the West while Meloidogyne and Paratrichodorus were associated with the South. Our analyses further suggested that soil texture (percent sand, percent clay, and percent silt) and the lime buffer capacity at equilibrium (LBCEQ) were associated with PPN community structure while pH was not. When focused on a single vineyard in the North, multiple logistic regression analysis suggested a statistically significant association between Meloidogyne spp. and soil characteristics, including percentages of sand, pH, and LBCEQ. Our study supports the association between soil characteristics and specific nematode genera, as well as the emergence of LBCEQ, the soil measurement with the strongest statistical association with nematode community structure and Meloidogyne presence.

Dispersal Kernel Type Highly Influences Projected Relationships for Plant Disease Epidemic Severity When Outbreak and At-Risk Populations Differ in Susceptibility.

In silico study of biologically invading organisms provide a means to evaluate the complex and potentially cryptic factors that can influence invasion success in scenarios where empirical studies would be difficult, if not impossible, to conduct. I used a disease event simulation program to evaluate whether the two most frequently used types of plant pathogen dispersal kernels for epidemiological projections would provide complementary or divergent projections of epidemic severity when the hosts in a disease outbreak differed from the hosts in the at-risk population in the degree of susceptibility. Exponential dispersal kernel simulations of wheat stripe rust (Pucciniastriiformis var trittici) predicted a relatively strong and dominant influence of the at-risk population on the end epidemic severity regardless of outbreak disease levels. Simulations using a modified power law dispersal kernel gave projections that varied depending on the amount of disease in the outbreak and some interactions were counter-intuitive and opposite of the exponential dispersal kernel projections. Although relatively straightforward, the disease spread simulations in the present study strongly suggest that a more biologically accurate dispersal kernel generates complexity that would not be revealed by an exponential dispersal gradient and that selecting a less accurate dispersal kernel may obscure important interactions during biological invasions.

Delays in Epidemic Outbreak Control Cost Disproportionately Large Treatment Footprints to Offset.

Epidemic outbreak control often involves a spatially explicit treatment area (quarantine, inoculation, ring cull) that covers the outbreak area and adjacent regions where hosts are thought to be latently infected. Emphasis on space however neglects the influence of treatment timing on outbreak control. We conducted field and in silico experiments with wheat stripe rust (WSR), a long-distance dispersed plant disease, to understand interactions between treatment timing and area interact to suppress an outbreak. Full-factorial field experiments with three different ring culls (outbreak area only to a 25-fold increase in treatment area) at three different disease control timings (1.125, 1.25, and 1.5 latent periods after initial disease expression) indicated that earlier treatment timing had a conspicuously greater suppressive effect than the area treated. Disease spread computer simulations over a broad range of influential epidemic parameter values (R0, outbreak disease prevalence, epidemic duration) suggested that potentially unrealistically large increases in treatment area would be required to compensate for even small delays in treatment timing. Although disease surveillance programs are costly, our results suggest that treatments early in an epidemic disease outbreak require smaller areas to be effective, which may ultimately compensate for the upfront costs of proactive disease surveillance programs.

Differences in distribution and community structure of plant-parasitic nematodes in pecan orchards between two ecoregions of Georgia.

In Georgia, pecans are commercially grown in the Piedmont and Coastal Plain ecoregions which are characterized by sandy-loam, sandy, and/or clay soils. If well-drained, these soils are suitable for pecan production, but the soil characteristics differ enough between ecoregions in which the plant-parasitic nematode (PPN) communities could differ substantially. We studied PPN communities in pecan orchards to evaluate the potential for ecoregion differences. In total, 11 genera (Helicotylenchus, Hemicycliophora, Heterodera, Hoplolaimus, Meloidogyne, Mesocriconema, Pratylenchus, Paratylenchus, Paratrichodorus, Tylenchorhynchs, Xiphenema) were recovered from pecan orchards in the Piedmont and Coastal Plain ecoregions. However, Non-Metric Multi-Dimensional Scaling ordination, Multi-Rank Permutation Procedure, and Indicator Species Analyses indicated that the pecan PPN communities strongly differed between ecoregions and that different genera were strongly associated with different ecoregions. For 9 of the 11 PPN genera, the maximum counts occurred in Coastal Plain locations, suggesting that the well-drained sandy soils of the Coastal Plain and comparatively ill-drained red clay soils of the Piedmont may be conducive and unfavorable for movement/reproduction of PPNs, respectively.

Positive selection and intrinsic disorder are associated with multifunctional C4(AC4) proteins and geminivirus diversification.

Viruses within the Geminiviridae family cause extensive agricultural losses. Members of four genera of geminiviruses contain a C4 gene (AC4 in geminiviruses with bipartite genomes). C4(AC4) genes are entirely overprinted on the C1(AC1) genes, which encode the replication-associated proteins. The C4(AC4) proteins exhibit diverse functions that may be important for geminivirus diversification. In this study, the influence of natural selection on the evolutionary diversity of 211 C4(AC4) genes relative to the C1(AC1) sequences they overlap was determined from isolates of the Begomovirus and Curtovirus genera. The ratio of nonsynonymous (dN) to synonymous (dS) nucleotide substitutions indicated that C4(AC4) genes are under positive selection, while the overlapped C1(AC1) sequences are under purifying selection. Ninety-one of 200 Begomovirus C4(AC4) genes encode elongated proteins with the extended regions being under neutral selection. C4(AC4) genes from begomoviruses isolated from tomato from native versus exotic regions were under similar levels of positive selection. Analysis of protein structure suggests that C4(AC4) proteins are entirely intrinsically disordered. Our data suggest that non-synonymous mutations and mutations that increase the length of C4(AC4) drive protein diversity that is intrinsically disordered, which could explain C4/AC4 functional variation and contribute to both geminivirus diversification and host jumping.

Patterns of Seed-to-Seedling Transmission of Xanthomonas citri pv. malvacearum, the Causal Agent of Cotton Bacterial Blight.

Cotton bacterial blight (CBB), caused by Xanthomonas citri pv. malvacearum, was a major disease of cotton in the United States in the early part of the twentieth century. The reemergence of CBB revealed many gaps in our understanding of this important disease. In this study, we employed a wild-type (WT) field isolate of X. citri pv. malvacearum from Georgia (U.S.A.) to generate a nonpathogenic hrcV mutant lacking a functional type-III secretion system (T3SS-). We tagged the WT and T3SS- strains with an auto-bioluminescent Tn7 reporter and compared colonization patterns of CBB-susceptible and CBB-resistant cotton seedlings using macroscopic image analysis and bacterial load enumeration. WT and T3SS- X. citri pv. malvacearum strains colonized cotton cotyledons of CBB-resistant and CBB-susceptible cotton cultivars. However, X. citri pv. malvacearum populations were significantly higher in CBB-susceptible seedlings inoculated with the WT strain. Additionally, WT and T3SS- X. citri pv. malvacearum strains systemically colonized true leaves, although at different rates. Finally, we observed that seed-to-seedling transmission of X. citri pv. malvacearum may involve systemic spread through the vascular tissue of cotton plants. These findings yield novel insights into potential X. citri pv. malvacearum reservoirs for CBB outbreaks.

Plant Pathogen Invasion Modifies the Eco-Evolutionary Host Plant Interactions of an Endangered Checkerspot Butterfly.

New plant pathogen invasions typified by cryptic disease symptoms or those appearing sporadically in time and patchily in space, might go largely unnoticed and not taken seriously by ecologists. We present evidence that the recent invasion of Pyrenopeziza plantaginis (Dermateaceae) into the Pacific Northwest USA, which causes foliar necrosis in the fall and winter on Plantago lanceolata (plantain), the primary (non-native) foodplant for six of the eight extant Taylor's checkerspot butterfly populations (Euphydryas editha taylori, endangered species), has altered eco-evolutionary foodplant interactions to a degree that threatens butterfly populations with extinction. Patterns of butterfly, larval food plant, and P. plantaginis disease development suggested the ancestral relationship was a two-foodplant system, with perennial Castilleja spp. supporting oviposition and pre-diapause larvae, and the annual Collinsia parviflora supporting post-diapause larvae. Plantain, in the absence of P. plantaginis disease, provided larval food resources throughout all butterfly life stages and may explain plantain's initial adoption by Taylor's checkerspot. However, in the presence of severe P. plantaginis disease, plantain-dependent butterfly populations experience a six-week period in the winter where post-diapause larvae lack essential plantain resources. Only C. parviflora, which is rare and competitively inferior under present habitat conditions, can fulfill the post-diapause larval feeding requirements in the presence of severe P. plantaginis disease. However, a germination timing experiment suggested C. parviflora to be suitably timed for only Washington Taylor's checkerspot populations. The recent invasion by P. plantaginis appears to have rendered the ancestrally adaptive acquisition of plantain by Taylor's checkerspot an unreliable, maladaptive foodplant interaction.

The Distribution of Onion Virulence Gene Clusters Among Pantoea spp.

Pantoea ananatis is a gram-negative bacterium and the primary causal agent of center rot of onions in Georgia. Previous genomic studies identified two virulence gene clusters, HiVir and alt, associated with center rot. The HiVir gene cluster is required to induce necrosis on onion tissues via synthesis of pantaphos, (2-hydroxy[phosphono-methyl)maleate), a phosphonate phytotoxin. The alt gene cluster aids in tolerance to thiosulfinates generated during onion tissue damage. Whole genome sequencing of other Pantoea species suggests that these gene clusters are present outside of P. ananatis. To assess the distribution of these gene clusters, two PCR primer sets were designed to detect the presence of HiVir and alt. Two hundred fifty-two strains of Pantoea spp. were phenotyped using the red onion scale necrosis (RSN) assay and were genotyped using PCR for the presence of these virulence genes. A diverse panel of strains from three distinct culture collections comprised of 24 Pantoea species, 41 isolation sources, and 23 countries, collected from 1946-2019, was tested. There is a significant association between the alt PCR assay and Pantoea strains recovered from symptomatic onion (P < 0.001). There is also a significant association of a positive HiVir PCR and RSN assay among P. ananatis strains but not among Pantoea spp., congeners. This may indicate a divergent HiVir cluster or different pathogenicity and virulence mechanisms. Last, we describe natural alt positive [RSN+/HiVir+/alt +] P. ananatis strains, which cause extensive bulb necrosis in a neck-to-bulb infection assay compared to alt negative [RSN+/HiVir+/alt -] P. ananatis strains. A combination of assays that include PCR of virulence genes [HiVir and alt] and an RSN assay can potentially aid in identification of onion-bulb-rotting pathogenic P. ananatis strains.

Assessment of Quinone Outside Inhibitor Sensitivity and Frogeye Leaf Spot Race of Cercospora sojina in Georgia Soybean.

Frogeye leaf spot (FLS), caused by the fungal pathogen Cercospora sojina K. Hara, is a foliar disease of soybean (Glycine max L. [Merr.]) responsible for yield reductions throughout the major soybean-producing regions of the world. In the United States, management of FLS relies heavily on the use of resistant cultivars and in-season fungicide applications, specifically within the class of quinone outside inhibitors (QoIs), which has resulted in the development of fungicide resistance in many states. In 2018 and 2019, 80 isolates of C. sojina were collected from six counties in Georgia and screened for QoI fungicide resistance using molecular and in vitro assays, with resistant isolates being confirmed from three counties. Additionally, 50 isolates, including a "baseline isolate" with no prior fungicide exposure, were used to determine the percent reduction of mycelial growth to two fungicides, azoxystrobin and pyraclostrobin, at six concentrations: 0.0001, 0.001, 0.01, 0.1, 1, and 10 µg ml-1. Mycelial growth observed for resistant isolates varied significantly from both sensitive isolates and baseline isolate for azoxystrobin concentrations of 10, 1, 0.1, and 0.01 µg ml-1 and for pyraclostrobin concentrations of 10, 1, 0.1, 0.01, and 0.001 µg ml-1. Moreover, 40 isolates were used to evaluate pathogen race on six soybean differential cultivars by assessing susceptible or resistant reactions. Isolate reactions suggested 12 races of C. sojina present in Georgia, 4 of which have not been previously described. Species richness indicators (rarefaction and abundance-based coverage estimators) indicated that within-county C. sojina race numbers were undersampled in this study, suggesting the potential for the presence of either additional undescribed races or known but unaccounted for races in Georgia. However, no isolates were pathogenic on 'Davis', a differential cultivar carrying the Rcs3 resistance allele, suggesting that the gene is still an effective source of resistance in Georgia.

Influence of the Environment and Vegetable Cropping Systems on Plant-Parasitic Nematode Communities in Southern Georgia.

Plant-parasitic nematodes (PPN) limit yields of vegetable production in the United States. During the spring and fall cropping seasons of 2018, 436 fields in bare ground and plastic bed cropping systems were randomly sampled from 29 counties in southern Georgia. The incidence (%), mean relative abundance, and maximum relative abundance (nematodes per 100 cm3 of soil) of the 10 different PPN genera detected in 32 vegetable crops in bare ground and plastic bed cropping systems include Meloidogyne spp. (67.3%; mean, 292; maximum, 14,144), Nanidorus spp. (49.4%; mean, 6; maximum, 136), Mesocriconema spp. (39.6%; mean, 17; maximum, 340), Helicotylenchus spp. (31.6%; mean, 20; maximum, 1152), Pratylenchus spp. (20.1%; mean, 2; maximum, 398), Rotylenchulus spp. (5.9%; mean, 1; maximum, 116), Hoplolaimus spp. (12.6%; mean, 1; maximum, 78), Heterodera spp. (2.3%; mean, <1; maximum, 60), Tylenchorhynchus spp. (0.9%; mean, <1; maximum, 12), and Xiphinema spp. (0.2%; mean, <1; maximum, 2). A nonmetric multidimensional scaling analysis indicated that most environmental and geological factors (i.e., longitude, precipitation, soil moisture, sand and silt content, and soil electrical conductivity) had no apparent relationship with nematode counts, except for latitude, soil pH, and temperature. The multirank permutation procedure followed by indicator species analysis and nonparametric Kruskal-Wallis analysis of variance indicated that Meloidogyne spp. are the predominant PPN associated with plastic beds in the south region sampled. The south region consisted mainly of commercial fields that rotated multiple vegetables crops through the same plastic beds. All other PPNs were associated with bare ground beds in the north region that are commonly rotated with row crops. This study validates that Meloidogyne spp. are the most important PPN in vegetable fields of southern Georgia and suggests that cropping systems have a greater effect on PPN population dynamics than the environment.

Indicator Species Analysis: A Useful Tool for Plant Disease Studies.

Indicator species analysis (ISA) uses indices of an organism's relative abundance and occurrence to estimate the strength of its associations with a priori groups of interest and a simple randomization test to evaluate the probability of association. Because ISA values tend to be greatest when a species is both relatively more abundant than other species in a particular group and it occurs more frequently in that same group (the expectations of a causal agent in diseased plants), ISA should be useful for identifying and narrowing the list of potential causal agents from a pool of pathogens in both emerging plant diseases and when the causal agent is unclear. Recent ISA plant disease applications suggests it may either directly identify a single causal agent from a pool of potential pathogens or narrow the pool of pathogens as candidates for pathogenicity tests in the process of fulfilling Koch's postulates. In this letter, we explain the underpinnings of ISA, summarize the known applications to plant pathosystems, offer caveats about the analysis, and suggest scenarios where ISA may be broadly applicable for plant disease studies.

Consequences of Long-Distance Dispersal for Epidemic Spread: Patterns, Scaling, and Mitigation.

Epidemics caused by long-distance dispersed pathogens result in some of the most explosive and difficult to control diseases of both plants and animals (including humans). Yet the factors influencing disease spread, especially in the early stages of the outbreak, are not well-understood. We present scaling relationships, of potentially widespread relevance, that were developed from more than 15 years of field and in silico single focus studies of wheat stripe rust spread. These relationships emerged as a consequence of accounting for a greater proportion of the fat-tailed disease gradient that may be frequently underestimated in disease spread studies. Leptokurtic dispersal gradients (highly peaked and fat-tailed) are relatively common in nature and they can be represented by power law functions. Power law scale invariance properties generate patterns that repeat over multiple spatial scales, suggesting important and predictable scaling relationships between disease levels during the first generation of disease outbreaks and subsequent epidemic spread. Experimental wheat stripe rust outbreaks and disease spread simulations support theoretical scaling relationships from power law properties and suggest that relatively straightforward scaling approximations may be useful for projecting the spread of disease caused by long-distance dispersed pathogens. Our results suggest that, when actual dispersal/disease data are lacking, an inverse power law with exponent = 2 may provide a reasonable approximation for modeling disease spread. Furthermore, our experiments and simulations strongly suggest that early control treatments with small spatial extent are likely to be more effective at suppressing an outbreak caused by a long-distance dispersed pathogen than would delayed treatment of a larger area. The scaling relationships we detail and the associated consequences for disease control may be broadly applicable to plant and animal pathogens characterized by non-exponentially bound, fat-tailed dispersal gradients.

Losing a battle but winning the war: moving past preference-performance to understand native herbivore-novel host plant interactions.

Introduced plants can positively affect population viability by augmenting the diet of native herbivores, but can negatively affect populations if they are subpar or toxic resources. In organisms with complex life histories, such as insects specializing on host plants, the impacts of a novel host may differ across life stages, with divergent effects on population persistence. Most research on effects of novel hosts has focused on adult oviposition preference and larval performance, but adult preference may not optimize offspring performance, nor be indicative of host quality from a demographic perspective. We compared population growth rates of the Baltimore checkerspot butterfly, Euphydryas phaeton, on an introduced host, Plantago lanceolata (English plantain), and the native host Chelone glabra (white turtlehead). Contrary to the previous findings suggesting that P. lanceolata could be a population sink, we found higher population growth rates (λ) on the introduced than the native host, even though some component parameters of λ were higher on the native host. Our findings illustrate the importance of moving beyond preference-performance studies to integrate vital rates across all life stages for evaluating herbivore-host plant relationships. Single measures of preference or performance are not sufficient proxies for overall host quality nor do they provide insights into longer term consequences of novel host plant use. In our system, in particular, P. lanceolata may buffer checkerspot populations when the native host is limiting, but high growth rates could lead to crashes over longer time scales.

Low relative error in consumer-grade GPS units make them ideal for measuring small-scale animal movement patterns.

Consumer-grade GPS units are a staple of modern field ecology, but the relatively large error radii reported by manufacturers (up to 10 m) ostensibly precludes their utility in measuring fine-scale movement of small animals such as insects. Here we demonstrate that for data collected at fine spatio-temporal scales, these devices can produce exceptionally accurate data on step-length and movement patterns of small animals. With an understanding of the properties of GPS error and how it arises, it is possible, using a simple field protocol, to use consumer grade GPS units to collect step-length data for the movement of small animals that introduces a median error as small as 11 cm. These small error rates were measured in controlled observations of real butterfly movement. Similar conclusions were reached using a ground-truth test track prepared with a field tape and compass and subsequently measured 20 times using the same methodology as the butterfly tracking. Median error in the ground-truth track was slightly higher than the field data, mostly between 20 and 30 cm, but even for the smallest ground-truth step (70 cm), this is still a signal-to-noise ratio of 3:1, and for steps of 3 m or more, the ratio is greater than 10:1. Such small errors relative to the movements being measured make these inexpensive units useful for measuring insect and other small animal movements on small to intermediate scales with budgets orders of magnitude lower than survey-grade units used in past studies. As an additional advantage, these units are simpler to operate, and insect or other small animal trackways can be collected more quickly than either survey-grade units or more traditional ruler/gird approaches.