Transmission Of Plant Pathogens Research Articles

Salivary enzymes are injected into xylem by the glassy-winged sharpshooter, a vector of Xylella fastidiosa

A few phytophagous hemipteran species such as the glassy-winged sharpshooter, Homalodisca vitripennis, (Germar), subsist entirely on xylem fluid. Although poorly understood, aspects of the insect’s salivary physiology may facilitate both xylem-feeding and transmission of plant pathogens. Xylella fastidiosa is a xylem-limited bacterium that causes Pierce’s disease of grape and other scorch diseases in many important crops. X. fastidiosa colonizes the anterior foregut (precibarium and cibarium) of H. vitripennis and other xylem-feeding vectors. Bacteria form a dense biofilm anchored in part by an exopolysaccharide (EPS) matrix that is reported to have a β-1,4-glucan backbone. Recently published evidence supports the following, salivation-egestion hypothesis for the inoculation of X. fastidiosa during vector feeding. The insect secretes saliva into the plant and then rapidly takes up a mixture of saliva and plant constituents. During turbulent fluid movements in the precibarium, the bacteria may become mechanically and enzymatically dislodged; the mixture is then egested back out through the stylets into plant cells, possibly including xylem vessels. The present study found that proteins extracted from dissected H. vitripennis salivary glands contain several enzyme activities capable of hydrolyzing glycosidic linkages in polysaccharides such as those found in EPS and plant cell walls, based on current information about the structures of those polysaccharides. One of these enzymes, a β-1,4-endoglucanase (EGase) was enriched in the salivary gland protein extract by subjecting the extract to a few, simple purification steps. The EGase-enriched extract was then used to generate a polyclonal antiserum that was used for immunohistochemical imaging of enzymes in sharpshooter salivary sheaths in grape. Results showed that enzyme-containing gelling saliva is injected into xylem vessels during sharpshooter feeding, in one case being carried by the transpiration stream away from the injection site. Thus, the present study provides support for the salivation-egestion hypothesis.

Induced release of a plant-defense volatile 'deceptively' attracts insect vectors to plants infected with a bacterial pathogen.

Transmission of plant pathogens by insect vectors is a complex biological process involving interactions between the plant, insect, and pathogen. Pathogen-induced plant responses can include changes in volatile and nonvolatile secondary metabolites as well as major plant nutrients. Experiments were conducted to understand how a plant pathogenic bacterium, Candidatus Liberibacter asiaticus (Las), affects host preference behavior of its psyllid (Diaphorina citri Kuwayama) vector. D. citri were attracted to volatiles from pathogen-infected plants more than to those from non-infected counterparts. Las-infected plants were more attractive to D. citri adults than non-infected plants initially; however after feeding, psyllids subsequently dispersed to non-infected rather than infected plants as their preferred settling point. Experiments with Las-infected and non-infected plants under complete darkness yielded similar results to those recorded under light. The behavior of psyllids in response to infected versus non-infected plants was not influenced by whether or not they were carriers of the pathogen. Quantification of volatile release from non-infected and infected plants supported the hypothesis that odorants mediate psyllid preference. Significantly more methyl salicylate, yet less methyl anthranilate and D-limonene, was released by infected than non-infected plants. Methyl salicylate was attractive to psyllids, while methyl anthranilate did not affect their behavior. Feeding on citrus by D. citri adults also induced release of methyl salicylate, suggesting that it may be a cue revealing location of conspecifics on host plants. Infected plants were characterized by lower levels of nitrogen, phosphorus, sulfur, zinc, and iron, as well as, higher levels of potassium and boron than non-infected plants. Collectively, our results suggest that host selection behavior of D. citri may be modified by bacterial infection of plants, which alters release of specific headspace volatiles and plant nutritional contents. Furthermore, we show in a laboratory setting that this apparent pathogen-mediated manipulation of vector behavior may facilitate pathogen spread.

Vector Transmission Efficiency of Liberibacter by Bactericera cockerelli (Hemiptera: Triozidae) in Zebra Chip Potato Disease: Effects of Psyllid Life Stage and Inoculation Access Period

Successful transmission of plant pathogens by insects depends on the vector inoculation efficiency and how rapidly the insect can effectively transmit the pathogen to the host plant. The potato psyllid, Bactericera cockerelli (Sulc), has recently been found to transmit "Candidatus Liberibacter solanacearum," a bacterium associated with zebra chip (ZC), an emerging and economically important disease of potato in several parts of the world. Currently, little is known about the epidemiology of ZC and its vector's inoculation capabilities. Studies were conducted in the field and laboratory to 1) assess transmission efficiency of potato psyllid nymphs and adults; 2) determine whether psyllid inoculation access period affects ZC incidence, severity, and potato yield; and 3) determine how fast the psyllid can transmit liberibacter to potato, leading to ZC development. Results showed that adult potato psyllids were highly efficient vectors of liberibacter that causes ZC and that nymphs were less efficient than adults at transmitting this bacterium. It was also determined that inoculation access period had little influence on overall ZC disease incidence, severity, and resulting yield loss. Moreover, results showed that exposure of a plant to 20 adult potato psyllids for a period as short as 1 h resulted in ZC symptom development. Furthermore, it was shown that a single adult potato psyllid was capable of inoculating liberibacter to potato within a period as short as 6 h, thereby inducing development of ZC. This information will help in developing effective management strategies for this serious potato disease.

Estimating Xylella fastidiosa transmission parameters: decoupling sharpshooter number and feeding period

AbstractCompared to human‐ and wildlife‐transmitted pathogens, less emphasis has been placed on developing models of plant pathogen transmission by insects. Here, we describe the transmission ecology of the bacterium Xylella fastidiosa Wells et al., the causal agent of Pierce’s disease in grapevines, by its leafhopper vectors. First, we performed a meta‐analysis of transmission studies of X. fastidiosa by its two most important vectors in the Western USA, the invasive glassy‐winged sharpshooter, Homalodisca vitripennis Germar, and the native blue‐green sharpshooter, Graphocephala atropunctata Signoret (both Hemiptera: Cicadellidae). The importance of vector number, pathogen acquisition period, and inoculation access period (IAP) for transmission differed between the two species. We fit these transmission datasets to two biologically derived transmission models, i.e., a binomial and a Poisson probability model. The Poisson model provided substantially better fit and produced estimates of H. vitripennis transmission efficiency that were dramatically lower than for G. atropunctata. We also conducted a separate pair of experiments that decoupled vector number from IAP. These experiments supported the results of the meta‐analysis. Interestingly, high vector loads not only increased transmission rate, but also shortened X. fastidiosa incubation period in grapevines. This work provides quantitative estimates of transmission of an economically important pathogen that is analogous to risk models for arthropod‐vectored human and wildlife diseases. In addition, this work suggests that heterogeneous vector loads may accelerate the disease cycle, increasing the potential for secondary spread in vineyards.

Insect Transmission of Plant Pathogens

MUCHOF BIOLOGICAL RESEARCHCONDUCTED TODAY ismultidisciplinary, partly because of the availability of ever simpler to use molecular tools and the realization that ecological systems are complex and multitrophic. Although there is a general trend toward academic specialization and cooperation, some Þelds still require that scientists be aware of the “bigger picture.” We consider that the study of insect-transmitted plant pathogens is one of those Þelds, where insect, pathogen, and plant are inseparably linked. In this triangle of interactions, researchers focus their interests on questions at the molecular and cellular to epidemiological levels. Throughout the years, plant vector entomologists have addressed a variety of important pest problems, often linking entomologists and plant pathologists for the development of improved and sustainable disease control strategies. In this issue of the Annals of the Entomological Society of America, the Cc subsection (Arthropods in Relation to Plant Disease) of the Society honors two of its members, Lowell R. “Skip” Nault and Alexander H. “Sandy” Purcell. Skip and Sandy have been recognized as two of the ESAOs preeminent plant vector entomologists. They were also partly responsible for training and motivating a new generation of scientists involved in vectorÐphytopathogen relationships. What made their contribution, recognized by tribute symposia at recent ESA meetings, so remarkable was their ability to study problems as multitrophic interactions, incorporating aspects of diverse research areas into their work as needed. Their contributions ranged from disease epidemiology and modeling to insect biology; from insect microbial ecology to bacterial genomics. The interests of Skip and Sandy merged in what might be labeled “ecology of insectborne plant diseases.” In fact, they have written or coauthored several articles on the topic. To understand the ecology of such diseases, one must know how insects transmit plant disease agents and then incorporate that knowledge into vector ecology, crop management practices, and disease development and spread. This holistic approach to vector research, difÞcult to establish for various reasons, led to numerous academic and applied breakthroughs by both researchers. Skip was an advisor, mentor, collaborator, and colleague tomanywhowandered throughhis door at one time or another during his tenure at The Ohio State University. Originally an aphidologist out of SylvesterOs laboratory, Skip saw the light and began working with leafhopper-transmittedmollicutes andviruses. In the course of his 30-yr career at the Ohio Agricultural Research and Development Center in Wooster, OH, Skip not only focused on vectorÐpathogen relationships but also ventured into new areas such as leafhopper communication, feeding behavior, and ecology, primarily through his many students and postdoctoral workers. In all, Skip trained 18 masters and doctoral students and published more than 120 refereed articles in various scientiÞc journals. A great thinker, Skip liked to synthesize new ideas and hypotheses to test. Among his most lasting contributions are his early work with aphid alarm pheromones,DalbulusÐmaize evolution concepts, and the synopses of plant virus modes of transmission. During his 30 yr as a faculty member at the University of California, Berkeley, Sandy incited interest of students, collaborators, and colleagues on various topics associated with vector-borne plant diseases. The never-ending interest he had toward all areas of science and his willingness to discuss academic questions and ideas have motivated students and colleagues worldwide. Sandy headed research that identiÞed the etiological agent of PierceOs disease of grape and other plant diseases, the bacterium Xylella fastidiosa; studied its vector transmission characteristics; and improved our understanding on the ecology and epidemiology of this pathogen. Plant pathogenic mollicutes (phytoplasmas and spiroplasmas) and their interactions with their leafhopper hosts also were continuous components of his research. Another line of SandyOs research was insect symbiosis, primarily facultative symbionts of the pea aphid, Acyrthosiphon pisum (Harris). Thewide range of tools used by Sandy to address unique ecological questions also was a hallmark of his contributions. Members of the Cc subsection have submitted manuscripts representing many, but by no means all, research areas worked on by these two researchers. Themanuscripts alsodemonstrate thebreadthofplant vector entomology and its importance in linking entomology and plant pathology in search of betterways to control insect-borne plant diseases. Finally, we are grateful to all those who served as reviewers for these manuscripts. Their thoughtful comments were essential for improvements in the Þnal presentation. 1 Department of Plant and Environmental Protection Sciences, University of Hawaii at Manoa, Honolulu, HI 96822. 2 Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74078. 3 USDAÐAPHIS, 41-650 Ahiki Street, Waimanalo, HI 96795.

Impact of pymetrozine on glassy-winged sharpshooter feeding behavior and rate of Xylella fastidiosa transmission

Pymetrozine is a compound that interferes with insect feeding and interrupts transmission of plant pathogens. The glassy-winged sharpshooter, Homalodisca coagulata Say (Hemiptera, Cicadellidae), is a vector of Xylella fastidiosa, the foregut-borne, propagative bacterium that causes Pierce's disease of grapevine. In this study, we recorded the behavioral response of H. coagulata to plants treated by soil drench with pymetrozine using time-lapse photography, quantified the reduction in liquid excreta produced by H. coagulata fed on pymetrozine-treated plants, and evaluated pymetrozine effectiveness in reducing transmission rate in grapevines. H. coagulata feeding on plants treated with 0.015 mg of pymetrozine was disrupted by decreasing the number of contacts made with the grapevine by more than 50% and by increasing movements away from the stem by more than 5-fold. Excreta production by H. coagulata was significantly reduced on plants treated with 0.015 or 0.0075 mg of pymetrozine. Contrary to the expected outcome, the mean number of X. fastidiosa-infected plants actually increased in the pymetrozine treatments relative to the controls.

Impact of pymetrozine on glassy-winged sharpshooter feeding behavior and rate of Xylella fastidiosa transmission.

Pymetrozine is a compound that interferes with insect feeding and interrupts transmission of plant pathogens. The glassy-winged sharpshooter, Homalodisca coagulata Say (Hemiptera, Cicadellidae), is a vector of Xylella fastidiosa, the foregut-borne, propagative bacterium that causes Pierce's disease of grapevine. In this study, we recorded the behavioral response of H. coagulata to plants treated by soil drench with pymetrozine using time-lapse photography, quantified the reduction in liquid excreta produced by H. coagulata fed on pymetrozine-treated plants, and evaluated pymetrozine effectiveness in reducing transmission rate in grapevines. H. coagulata feeding on plants treated with 0.015 mg of pymetrozine was disrupted by decreasing the number of contacts made with the grapevine by more than 50% and by increasing movements away from the stem by more than 5-fold. Excreta production by H. coagulata was significantly reduced on plants treated with 0.015 or 0.0075 mg of pymetrozine. Contrary to the expected outcome, the mean number of X. fastidiosa-infected plants actually increased in the pymetrozine treatments relative to the controls.

Dynamics of Seed Transmission of Plant Pathogens

Dynamics of Seed Transmission of Plant Pathogens