ARTHROPODS IN RELATION TO PLANT DISEASE Detection and Differentiation of Xylella fastidiosa Strains Acquired and Retained by Glassy-Winged Sharpshooters (Hemiptera: Cicadellidae) Using a Mixture of Strain-Specific Primer Sets H. S. COSTA, 1 A. GUZMAN, 1 R. HERNANDEZ-MARTINEZ, 2 C. GISPERT, 3 AND D. A. COOKSEY 2 J. Econ. Entomol. 99(4): 1058Ð1064 (2006) ABSTRACT Xylella fastidiosa Wells is a bacterial pathogen that causes a variety of plant diseases, including PierceÕs disease (PD) of grapevine, almond leaf scorch, alfalfa dwarf, citrus variegated chlorosis, and oleander leaf scorch (OLS). Numerous strains of this pathogen have been genetically characterized, and several different strains occur in the United States. The dominant vector in southern California is the glassy-winged sharpshooter, Homalodisca coagulata (Say) (Hemiptera: Cicadellidae). The high mobility of this insect, and its use of large numbers of host plant species, provides this vector with ample exposure to multiple strains of X. fastidiosa during its lifetime. To learn more about the ability of this vector to acquire, retain, and transmit multiple strains of the pathogen, we developed a polymerase chain reaction (PCR)-based method to detect and differentiate strains of X. fastidiosa present in individual glassy-winged sharpshooter adults. Insects were sequentially exposed to plants infected with a PD strain in grapevine and an OLS strain in oleander. After sequential exposure, a few insects tested positive for both strains (7%); however, in most cases individuals tested positive for only one strain (29% PD, 41% OLS). In transmission studies, individual adults transmitted either the PD or OLS strain of the pathogen at a rate (39%) similar to that previously reported after exposure to a single strain, but no single individual transmitted both strains of the pathogen. PD and OLS strains of X. fastidiosa remained detectable in glassy-winged sharpshooter, even when insects were fed on a plant species that was not a host of the strain for 1 wk. KEY WORDS PierceÕs disease, oleander leaf scorch, Homalodisca coagulata Xylella fastidiosa Wells is a xylem-limited bacterial plant pathogen that is primarily transmitted by leaf- hopper vectors. This pathogen causes a variety of diseases in a broad range of plant hosts, including PierceÕs disease (PD) of grapevine, almond leaf scorch, alfalfa dwarf, citrus variegated chlorosis (CVC), leaf scorch of live oak, pear leaf scorch, and oleander leaf scorch (OLS) (Hopkins 1989, Hartung et al. 1994, Purcell et al. 1999). Numerous strains of X. fastidiosa have been genetically and biologically char- acterized and compared in several studies (Chen et al. 1992, Pooler and Hartung 1995, Albibi et al. 1998, da Costa et al. 2000, Hendson et al. 2001, Almeida and Purcell 2003a). Different strains of the pathogen have different host ranges, and the complete host range for each strain is not yet known. The entire genome of a PD and a CVC strain of X. fastidiosa have been se- quenced (Simpson et al. 2000, Van Sluys et al. 2003), and draft sequences of oleander and almond strains are available (Bhattacharyya et al. 2002). Several leafhopper vectors can transmit X. fastidiosa (Almeida et al. 2005), but the dominant vector in southern California is the glassy-winged sharpshooter, Homalodisca coagulata (Say) (Hemiptera: Cicadelli- dae) (Sorensen and Gill 1996, Blua et al. 1999, Redak et al. 2004). Glassy-winged sharpshooters feed on xy- lem tissue, acquiring and transmitting the bacterium during feeding activity. The bacterium replicates in the mouthparts of the insect (Hill and Purcell 1995), and the pathogen is lost during the molting process (Purcell and Finlay 1979). However, if an insect ac- quires X. fastidiosa as an adult, it can retain the patho- gen for the remainder of its life (Severin 1949). In microscopic studies of X. fastidiosa-infected glassy- winged sharpshooters, bacteria were observed in the cibarium and the apodemal groove of the diaphragm (Brlansky et al. 1982, 1983). These data suggest a noncirculative mechanism of transmission (Purcell and Finlay 1979, Purcell 1990). The glassy-winged sharpshooter has an extremely broad range of 100 species in 31 families, and a single individual will feed on a variety of plant species during its lifetime (Turner and Pollard 1959, Blua et al. 1999, Purcell and Saunders 1999, Hoddle et al. 2003). The high mobility of this insect (Blackmer et al. 2004), and its use of a large number of host plant species, provides 1 Department of Entomology, University of California, Riverside, CA 92521. 2 Department of Plant Pathology, University of California, River- side, CA 92521. 3 University of California Cooperative Extension, 82-675 Highway 111, Rm 118, Indio, CA 92201. 0022-0493/06/1058Ð1064$04.00/0  2006 Entomological Society of America Downloaded from https://academic.oup.com/jee/article/99/4/1058/2218515 by guest on 18 October 2022