Insect Transmission of Plant and Human Pathogens to Plants

Non-Technical Summary: This project primarily addresses insect transmission of plant pathogens, but the mechanisms can be very similar to those of vector-animal pathogen systems. Insect transmitted plant pathogens account for a significant proportion of the plant disease epiphytotics experienced in the United States (Agrios 2005). Most of these are viruses, but several are caused by bacterial agents. Many of these pathogens are native to the United States or were introduced but are now considered widespread. However, there are pathogens that are not endemic to the US and, if introduced, could have serious economic impact on US agriculture. Two insect-transmitted pathogens,Candidatus Liberibacter africanus and americanus (two causal agents of citrus greening disease) and Xylella fastidiosa, citrus variegated chlorosis strain (Bove and Ayres 2007) are currently on the United States Select Agent List. Other insect-transmitted disease pathogens such as grapevine yellows and citrus stubborn are considered serious threats because due to new host ranges or as re-emerging diseases. Most vectors of plant pathogens belong to the Order Hemiptera (true bugs, aphids, leafhoppers, planthoppers, whiteflies, etc.). The unique ability of these piercing-sucking insects to direct stylet movement enables acquisition and inoculation of plant pathogenic bacteria and viruses from and into tissues deep within the plant (Nault 1990, Backus and Walker 2000). This project will specifically address two issues important to phytopathogen transmission: 1) plant penetration activities during the feeding process which result in transmission of phytopathogens and 2) ecology of vector populations as they interact with plants in the field. This information is critical to the development of plant disease control strategies whether they are chemical control of the pathogen vector, cultural control by reducing vector habitats, or development of host plant resistance. <P> Approach: EPG analysis of hemipteran probing behavior is already in place. A. tristis probing activity is currently being documented, characterized and quantified. EPG comparisons of A. tristis probing activities on cucurbits of different parental ancestry will be carried out using methods previously established and accepted. Similar comparisons and ultimately development of resistance indices will be compiled for other hemipteran vectors of plant disease pathogens, particularly those that pose an imminent threat to the US agricultural economy as invasive species or select agents. This work will be done using the EPG equipment housed within the Department of Entomology and Plant Pathology, Oklahoma State University and in cooperating laboratories as required. Competent field vectors of X. fastidiosa will be collected from riparian habitats near and away from commercial grape vineyards and tree nurseries using standard collection methods. Using molecular detection techniques (q PCR, plating) and traditional transmission tests, populations of sharpshooter leafhoppers and spittle bugs harboring X.f will be identified and the information used to develop risk assessment guidelines for grape and tree growers. Identification of factors important to fly attraction to lettuce and other fresh produce is currently underway using modified fly transmission tests. A large, Plexiglas flight chamber was constructed to study fly orientation and movement to lettuce and spinach using standard protocols. A number of filth fly species, enteric bacterial pathogens, and target plant crop species will be examined. Impact of fly defecation and regurgitation spots on E. coli O157:H7 colonization of the leaf phyllosphere will be determined by fluorescent and confocal microscopy. This work will be carried out in laboratories in Noble Research Center and OSU Imaging Center. Fly forensics will be accomplished by using molecular tools (T-RFLP, q-PCR, sequencing) to compile bacterial profiles in laboratories located in Noble Research Center. Matching these profiles to known bacterial sources will provide stronger, more robust evidence of the origin of fly populations.