Evidence for Noncirculative Transmission of Pierce's Disease Bacterium by Sharpshooter Leafhoppers

Abstract

Alexander H. Purcell and Allan FinlayDepartment of Entomological Sciences, University of California, Berkeley, 94720.The California Table Grape Commission and the Napa Valley Viticultural Research Fund supported this work in part.We thank Dennis Larsen for technical assistance.Accepted for publication 10 October 1978.ABSTRACTPURCELL, A. H., and A. H. FINLAY. 1979. Evidence for noncirculative transmission of Pierce's disease bacterium by sharpshooter leafhoppers.Phytopathology 69:393-395.Half of the leafhoppers (Graphocephala atropunctata) allowed acquisi- which was in close agreement with estimates for which no latent period wastion access on grapevines affected with Pierce's disease (PD) became assumed. Neither G. atropunctata nor Draeculacephala minerva retainedinfective within 2.0 hr, and there was no significant increase in acquisition infectivity after molting. The loss of infectivity after molting and lack of abeyond 24 hr. The median inoculation access period was 3.9 hr. Three of 34 latent period suggest a noncirculative mechanism of transmission of the P[)(9%) insects transmitted after I hr each for acquisition and for inoculation, bacterium by leafhoppers.Additional key words: Hordnia, Graphocephala, Draeculacephala, lucerne dwarf, alfalfa dwarf, almond leaf scorch, rickettsia-like bacteria, stylet-borne.Pierce's disease (PD) of grapevines usually is lethal to grapevines Princeton, NJ 08540) 25% WP in water at recommended rates and(Vitis vinifera); periodically it has caused serious losses to the held in a heated greenhouse. Symptoms of PD normally appearedCalifornia grape industry and it has precluded successful after 10-14 wk. Grapevines without symptoms of PD for 22-25 wkproduction of bunch grapes in the southeastern USA (5). Bacteria were recorded as free of PD.transmitted by xylem-feeding suctorial insects cause PD in grape Acquisition tests. Fifty to sixty adult and nymphal G.(2) and also are thought to cause alfalfa dwarf (7) and almond leaf atropunctata previously exposed to pretest plants for 4 days orscorch (9) diseases. longer were placed for 1, 3, 6, 12, 24, and 48-hr acquisition accessLong considered a virus disease, PD has been unusual in its periods (AAP) on PD source plants and then caged singly for a 4-vector-pathogen relationships in several respects: (i) a large but day inoculation access period (IAP) on test plants. Leafhoppersdistinct group of vectors (xylem-feeding suctorial insects such as allowed the I-hr AAP were immediately transferred to test plantssharpshooter leafhoppers of the subfamily Cicadellinae and for 1 hr and then to a second set of test plants for 4 days.spittlebugs [Cercopidae] [3]) (ii) a minimum or required latent Inoculation access period (IAP). Fifty adults and nymphs of G.period of less than 2 hr, and (iii) persistence of infectivity for long atropunctata with AAP of 2 days or more were transferred singly toperiods (perhaps permanently) in adult vectors (14). Although test plants for lAP of 1, 3, 6, 12, 24, 48, and 96 hr. One group wassharpshooter nymphs can transmit PD (14) apparently no studies transferred in the sequence of 1, 48, and 12 hr. Because ofof transtadial passage (retention of infectivity by vectors after cumulative mortality to the first group, a second group fed on themolting) have been made (12). The purpose of these studies was to same source plants was transferred in the sequence of 3, 24, 96, andinvestigate the pathway of transmission in the vector by evaluating 6 hr in order to allow all transfers to be made during daylight hours.the effects of molting and of varying acquisition and inoculation Adult D. minerva were tested for 6, 12, and 24-hr IAP.access times on transmission of the PD bacterium. A preliminary Effect of molting on vector transmission. First through fifthreport has been published (11). instar nymphs with an AAP of 48 hr or more were confinedindividually on test seedlings. Plants were inspected dailyMATERIALS AND METHODS (exceptions noted) and insects transferred approximately weekly oras soon as molting was noted. In one experiment, insects wereThe leafhopper Graphocephala atropunctata (Signoret) (= transferred to new plants every 2 days or as soon as a molt wasHordnia circellata Baker) (16) was collected from natural detected. After access to at least one test plant following molting,populations in Berkeley, CA. To test for infectivity with the PD test insects usually were transferred to a PD source for I day or in abacterium, G. atroptinctata used in acquisition tests first were few cases for 2 days and then continued on a sequence of test plants.pretested on rooted Pinot Noir cuttings for four days or longer (13).The results with naturally infective insects were not used in analyses RESULTSof acquisition data. Draeculacephala minerva Ball was colonizedon barley cultivar Atlas from the progeny of leafhoppers reared Effects of AAP and IAP. Graphocephala atropunctata. Thefrom eggs hatched in petri dishes on moist paper (4). percentage of insects that transmitted PD was correlatedGrapevines with advanced PD symptoms (6) were used as significantly (P 0.5) from eachother.