Host Plant/Pest Interactions
Xylem-feeding leafhoppers and Xylella fastidiosa
The most important leafhopper vector of Xylella fastidiosa-mediated diseases in north Florida is the glassy-winged sharpshooter (Homalodisca vitripennis, formerly Homalodisca coagulata). There are two major components of this project: 1) plant/glassy-winged sharpshooter interactions and plant/X. fastidiosa interactions. There is no cure for any disease caused by X. fastidiosa (e.g. Pierce=s disease of grapevines, citrus variegated chlorosis, phony peach disease, plum leaf scald etc.). The bacterium is restricted to xylem vessels, and is transmitted from plant to plant by leafhoppers as they feed. These diseases precludes culture (e.g. V. vinifera in Florida), or results in great economic losses (e.g. peaches and plums in Florida or citrus in Brazil). The recent accidental introduction of glassy-winged sharpshooter in California now threatens the multi billion dollar grape and fruit industries, and citrus variegated chlorosis is expanding in South America, and potentially threatens the Florida citrus industry as well. Our work involving xylem chemistry-mediation of diseases caused by X. fastidiosa and vectored by leafhoppers was the subject of a United States Department of Agriculture National Research Initiative Competitive Grant, and subsequent grants from the American Vineyard Foundation, California Department of Food and Agriculture, USDA and UC Davis. We have written 30+ refereed publications on this subject from 1989 to present. We are also involved in research in Puerto Rico to discern the status of X. fastidiosa and potential leafhopper vectors in the Caribbean.
 Homalodisca vitripennis |
 Oncometopis nigrans |
Xylem fluid is extremely dilute and unique in its chemical simplicity consisting of 95 to 98 % water. The major organic constituents in xylem fluid are amino acids, organic acids and sugars. We have established an extensive database correlating the behavior (feeding, abundance etc.) and performance (survival, growth rate, instar duration, fecundity) of leafhoppers related to the chemistry of xylem fluid. Several adaptations that allow leafhoppers to subsist entirely on xylem fluid is have been elucidated. In brief, we have found that high feeding rates (10 x body weight per hour), synchronization of feeding to optimum host nutrition diurnally and seasonally, an unprecedented assimilation efficiency of monomeric compounds (99+%), and ammonotelism facilitate subsistence on xylem fluid. The concentrations of the amides (a major source of organic carbon and nitrogen) in xylem fluid are linked to adult feeding and abundance, while the balance of essential amino acids is correlated with the performance of nymphs.
 Xylella fastidiosa |
 Xylella fastidiosa traversing pit membranes of xylem vessels |
X. fastidiosa is found in a wide diversity of host plant species throughout the southern United States. Incidences of plant disease induced by X. fastidiosa are proportionally rare; expression of pathenogenic symptoms only occurs with specific combinations of host plant genotype and X. fastidiosa pathovars. Even within Vitis, disease expression is not indicative of X. fastidiosa presence, as resistant species of Vitis (i.e. Vitis rotundifolia) will often harbor the bacterium without showing PD symptomatology. The work of our laboratory and others has documented that disease expression is typically not a function of the presence of the bacteria, but rather the growth forms and the quantities of bacterial colonies. Our laboratory has developed simplified media based on xylem fluid chemistry (3G10-R and CHARD2) that allow us to test individual xylem components that may impact bacterial growth and colony formation. We have established that bacterial aggregation and biofilm formation may be the causal mechanism of X. fastidiosa diseases, as has recently been shown for a variety of other bacterial pathogens. Xylem fluid chemistry mediates many plant/X. fastidiosa/leafhopper interactions. The expected impact of this research is to establish the pivotal role of xylem chemistry to both the bacterium and the leafhopper. Ultimately we may determine the mechanisms of host plant resistance and develop management strategies to cope with these diseases and leafhopper vectors.
Thrips and tospovirus
Thrips and tospovirus have extreme economic importance on a worldwide basis, and are perhaps the most serious problem facing the tomato and pepper industry in north Florida. Very little is known about thrips nutritional requirements, or how thrips select hosts on which to feed and reproduce. Preliminary work by our laboratories and others, however, has established that many thrips species respond to increasing dietary nitrogen. This simple finding has severe implications, as we have shown that over-fertilization of crops can result in higher thrips populations and associated increases in the incidence of tospovirus. Moreover, we are beginning to understand what drives thrips host selection. We are currently developing a database examining how flower chemistry is associated with thrips feeding and reproduction. Our major focus is to better understand thrips by evaluating the importance of flower, pollen and leaf chemistry on host selection and population dynamics. Several National Research Initiative competitive grants have been submitted on this subject.
Phloem-feeding insect pests
Phloem-feeding insects (aphids, and some leafhoppers and plant hoppers) are predominant pests world-wide. We have previously examined the effects of the phloem feeding planthopper, Spissistilus festinus on nutrient partitioning of peanut, and subsequent effects on feeding behavior and performance. This insect severely girdles peanut stems and we have documented the subsequent physiological effects. Nutrient sinks are created following S. festinus girdling and feeding which impacts plant growth, and subsequently alters feeding behavior. We view it as likely that many phloem feeders may cause less noticeable nutrients sinks, as distribution for many of these pests tends to be highly clustered.
More Information
For more information on Pierce's Disease or the Glassywinged Sharpshooter, contact Pete Andersen.