Transmission of Xylella fastidiosa by Carneocephala fulgida, and its survival in Bermuda grass (Cynodon dactylon) Robin Choi Abstract The ability of Bermuda grass (Cynodon dactylon) to retain the bacteria, Xylella fastidiosa, can influence the spread Pierce's disease (P.D.). The fate of strains of the bacterium Xylella fastidiosa that causes Pierce's disease of grapevines was investigated in Bermuda grass. The P.D. strain, STL, was first mechanically inoculated into Bermuda grass, and was recovered at 1 week, 3 weeks, and 6 weeks after inoculation in the inoculated node only. Following STL, 4 other strains of X. fastidiosa (Mederios, SJV, Comm Creek, Preston) were recovered after mechanical inoculation at 3 weeks after inoculation. Three of these strains were recovered again after 6 weeks (Comm Creek, Mederios, Preston). Again detection only occurred in the inoculated node. Insect transmission efficiency of X. fastidiosa, by the RHSS (Carneocephala fulgida) was also investigated. Seven groups of RHSS' out of 40 groups successfully transmitted the bacterium from infected grape to healthy grape. The seven groups of RHSS had to ability to retain the bacterium after feeding on Bermuda grass.Introduction Xylella fastidiosa is a bacterium that is known to be the cause of Pierce's disease (PD), a lethal disease of grapevines (Davis et al. 1978). The bacterium can infect most commercial European grape species, yet some hybrids with American wild grape species are tolerant or resistant to X. fastidiosa. X. fastidiosa is not limited to grapevines. It infects, usually without symptoms, a wide variety of plant species (Freitag 1951). Studies have shown that in coastal California, the incidence of Pierce's disease of grapevines is high along riparian areas adjacent to vineyards. In the California central valley, P.D. is most intense near bayfields and pastures. (Goodwin 1992, Hill 1995, Purcell 1975). X. fastidiosa resides in the xylem of host plants, thus any piercing, sap-feeding insect is a potential vector (Freitag 1954). One of these vector insects is the redheaded sharpshooter (RHSS) Carneocephala fulgida. The RHSS feeds primarily on Bermuda grass (Cynodon dactylon) (Hewitt et al. 1942), but does have the ability to feed on grapevines. Bermuda grass is a common weed throughout California. It is a low-growing perennial herb that has been reported as one of the world's 10 worst weeds, having the capacity to grow virtually anyplace with mild winters (Mitich 1989). It has been shown that many plants common to the Californian riparian area can harbor X. fastidiosa, yet show no symptoms of disease (Purcell 1999). X. fastidiosa's ability to reside in numerous plants increases the potential for insect vectors to transmit the bacterium to commercial vineyards. With Bermuda grass in close proximity to the vineyards, questions can be raised about its impact in the transmission, spread and retention of X. fastidiosa. Bermuda grass is highly sensitive to frost, and low temperatures (Satorre 1996). In the winter Bermuda grass remains inactive, and sprouts again in the spring when the temperature is warmer. It is during spring months that X. fastidiosa has been found to be more persistent after insect inoculation in grapes Vitis vinifera (Purcell 1981). However, if either grapevines or Bermuda grass plants are infected with X. fastidiosa, there is potential for its spread and retention no matter the season. In a previous study X. fastidiosa had been recovered from Bermuda grass by laboratory vector transmission and by field vector recovery (Freitag 1951). Laboratory vector transmission entailed allowing infective RHSS to feed on Bermuda grass, removing them, and then allowing non-infective RHSS to feed. Field vector recovery entailed evaluating RHSS after caging them on field populations of Bermuda grass. The method of detection for this experiment wasquestionable. Detection was observed by allowing potentially infected adults to hatch eggs, and allowing the nymphs to feed on healthy grape or alfalfa. Since grape and alfalfa plants display symptoms of bacterial infection, positive detection was determined through symptoms on these plants. Yet there was no clear method of distinguishing if infected adults were actually infective. Also there was room for error when transferring insects to healthy plants. One single leafhopper that was infective could have contaminated a healthy plant that otherwise would have proven to be negative for the bacterium. In another study it was found that Bermuda grass did not retain any X. fastidiosa, when tested by culture and ELISA at the inoculation point 6 to 12 weeks after inoculation (Hill and Purcell 1995). These experiments used 2 strains of X. fastidiosa, and possibly were tested too late after inoculation. Further experiments showed that different PD strains of X. fastidiosa infected Bermuda grass (Purcell Unpublished data). According to a later study conducted by Purcell and Saunders, populations of X. fastidiosa are highest in most plant species within 3 to 6 weeks of inoculation (Purcell 1999). Our objectives were to: 1. Determine populations of X. fastidiosa over time in Bermuda grass after mechanical and vector inoculation. 2. Determine if nine different strains of X. fastidiosa differed in their ability to reside in Bermuda grass. 3. Determine if the RHSS could recover X. fastidiosa from infected grape and Bermuda grass. Methods Systemic growth in Bermuda grass To determine if growth of X. fastidiosa was systemic in Bermuda grass, stems were evaluated at two places, once in the inoculated node and then in the next distal node. Testing at two different places on a single node enabled for detection of growth and spread of X. fastidiosa. Stems were evaluated at 1, 3, 6 and 9 weeks to determine the survial of X. fastidiosa in Bermuda grass. Since X. fastidiosa had not been detected past 9 weeks (Hill and Purcell 1995, Purcell unpublished data), these intervals provide new information to the growth and spread of the bacterium in Bermuda grass. The population of Bermuda grass used inthis experiment was cloned from a single plant, which tested negative for the bacterium. All insect-rearing plants were maintained in greenhouses, constantly ventilated with charcoal-filtered air. 25 two-inch plastic pots each with four Bermuda grass cuttings were planted 3 weeks prior to inoculation. Each stem was inoculated
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