Journal of Nematology 21(4):517-523. 1989. © The Society of Nematologists 1989. Vector Capability of Xiphinema americanum sensu lato in California 1 JOHN A. GRIESBACH 2 AND ARMAND R. MAGGENTI s Abstract: Seven field populations of Xiphineraa americanum sensu lato from California's major agronomic areas were tested for their ability to transmit two nepoviruses, including the prune brownline, peach yellow bud, and grapevine yellow vein strains of" tomato ringspot virus and the bud blight strain of tobacco ringspot virus. Two field populations transmitted all isolates, one population transmitted all tomato ringspot virus isolates but failed to transmit bud blight strain of tobacco ringspot virus, and the remaining four populations failed to transmit any virus. Only one population, which transmitted all isolates, bad been associated with field spread of a nepovirus. As two California populations of Xiphinema americanum sensu lato were shown to have the ability to vector two different nepoviruses, a nematode taxonomy based on a parsimony of virus-vector re- lationship is not practical for these populations. Because two California populations ofX. americanum were able to vector tobacco ringspot virus, commonly vectored by X. americanum in the eastern United States, these western populations cannot be differentiated from eastern populations by vector capability tests using tobacco ringspot virus. Key words: dagger nematode, tobacco ringspot virus, tomato ringspot virus, nepovirus, Xiphinema americanum, Xiphinema californicum. Populations of Xiphinema americanum brownline (PBL), prunus stem pitting (PSP) Cobb, 1913 shown through rigorous test- and cherry leaf mottle (CLM) (8). Both PBL ing (23) to be nepovirus vectors include X. and PSP were transmitted with a high de- americanum sensu lato (s.1.) for tobacco gree of efficiency, whereas CLM was trans- ringspot virus (TobRSV) (5), tomato ring- mitted rarely. A North Carolina popula- spot virus (TomRSV) (1), peach rosette tion of X. americanum transmitted five mosaic virus (PRMV) (10), and cherry rasp strains of TobRSV, including the wild NC- leaf virus (CRLV) (15). Xiphinema ameri- 38, 39, 72, and 87 strains and the water- canum s.l. nepovirus vector species include melon strain from Texas (16). The vector X. rivesi for TomRSV (3,6) and X. califor- failed, however, to transmit a Peruvian iso- nicum syn. of X. americanum (Cobb) Gries- late, the eucharis mottle strain of TobRSV bach & Maggenti, 1989 (7) for TomRSV. (8). Work linking X. brevicolle with TomRSV In 1979, Lamberti and Bleve-Zacheo (11) transmission (4) was deemed inadequate suggested that the difficulty some research- because of the lack of systemic infection in ers have encountered in the transmission bait plants and (or) the lack of adequate of specific nepoviruses by "X. americanum" controls (23). might be explained by their newly pro- Members of the X. americanum group also posed species. This corresponds to level 2 have been shown to transmit different se- specificity (21), the transmission specificity rotypes of the same virus. A nematode between a nematode species and a virus, identified as X. americanum transmited three which will be referred to here as parsi- serologically distinct strains of TobRSV mony. Differential transmission of nepo- (17). Grapevine yellow vein strain was vec- viruses by geographically disparate popu- tored by a California population of X. lations could support this assumption, americanum (22). X. californicum was able to because allopatric speciation of the nema- transmit three strains of TomRSV: prune tode vectors could preclude transmission of viruses that did not co-evolve with the Received for publication 30 January 1989. nematode. Part of a Ph.D. dissertation submitted by the first author As an example, field transmission of both to the University of California, Davis, CA 95616. Plant Pathologist, Plant Division, Oregon Department of CRLV and PRMV is restricted in their geo- Agriculture, Salem, OR 97310. graphic range (Michigan and one area of s Professor, Department of Nematology, University of Cal- ifornia, Davis, CA 95616. New York and the western United States, 517 518 Journal of Nematology, Volume 21, No. 4, October 1989 respectively), when compared with North with a 17:2:1 mixture of sand : loam : or- American isolates of TobRSV and ganic matter and seeded with sudan grass TomRSV (10,18-20). Therefore with (Sorghum vulgare var. sudanence Hitch. cv. PRMV and CRLV, the potential for being Piper). This newly established sod (with vectored by restricted populations (and roots) then was cut and transplanted to the possibly species) exists. TobRSV and field soil cultures as they were collected in TomRSV, while patchy in their occur- an attempt to hasten colonization. rence, are quite widespread, indicating a Ambient glasshouse temperatures at the more cosmopolitan distribution of the vec- soil surface were 16-24 C, whereas soil tor(s). temperatures 10 cm deep were 19-22 C Additionally, Lamberti and Bleve-Zach- for the duration of the study. Cultures were eo (11) declared a restricted geographic watered to avoid major moisture level range for X. americanum, sensu stricto (s.s.), fluctuations. All virus transmission studies the type species of which was found only were conducted in the winter and spring in the eastern United States. Hence, the of 1987. vector studies not using populations from To test for the presence of viruliferous the newly restricted geographic range were nematodes in the field collections, two or not X. americanum. This establishes a con- three first-leaf cucumber (Cucumus sativa troversy regarding vector experiments L. cv. National Pickling) seedlings were performed in California, where the vectors transplanted to one pot from each site (one were identified as X. americanum and not of five cans). The bait plants (in vivo) assay X. californicum (1,15). The reported differ- were allowed to grow for 3 weeks before ences in the morphometrics of X. califor- sap was extracted in a 0.1 M phosphate nicum and X. americanum s.s. and the lack buffer, pH 7.0, and inoculated to cotyle- of documented TobRSV transmission in dons of virus-free first-leaf cucumber plants California suggest the possibility of a dif- that acted as increase hosts to amplify the ferent species on the west coast of the titer and symptoms of any virus transmis- United States. sion. Also, in the subsequent virus vector The vector capability of selected Cali- experiments, a check nematode population fornia populations, and the possibility of was included from each site to act as a sec- morphospecies vectoring different nepo- ond test against the possibility that the field- viruses were examined in this study. collected nematodes already carried a vi- rus. MATERIALS AND M~TnODS Nematodes for the virus transmission Seven field populations of Xiphinema tests were collected from the culture soil americanum s.l. from major agronomic areas with a 127-#m-pore (200 mesh) sieve and near Winters, Reedley, Parlier, Camino, placed over single-thickness tissue on a Freedom, Linden, and Calistoga, Califor- Baermann funnel in a mist chamber for no nia, were collected in the early spring of more than 12 hours. The amount of soil 1987 and were judged to be monospecific processed varied according to the original based on histograms of morphological field density of nematodes, but a minimum characteristics (7). Soil, ca. 50 liters from of 5,000 individuals ofXiphinema sp. were each site, was collected and placed in five extracted per site and immediately trans- 10-liter cans which then were placed in ferred to pre-infected acquisition and steam-sterilized soil bins in a glasshouse. mock-infected check plants growing in a Nematodes were left in their native soils (9:1 sand : loam soil mix. Sap from first-leaf to minimize potential relocation trauma. cucumbers serving as acquisition plants was The space between the cans was filled with inoculated with a 0.1 M phosphate buffer, wood chips to minimize temperature fluc- pH 7.0, from the following sources: tuations (12). Before gathering the field TobRSV-bud blight (G. Bruening), populations, groundcover flats were filled TomRSV-grapevine yellowvein (A. Go- Vector Capability of Xiphinema americanum: Griesbach, Maggenti 519 heen), TomRSV-prune brownline (A. virus transmission. The presence of chlo- Rowhani), and TomRSV-peach yellowbud rotic cotyledons, ringspots, dwarfing, and mosaic (N. Frazier). Aliquants of approx- mottling indicated transmission. Trans- imately 500 nematodes were placed on the mission was verified by gel diffusion tests root system of each virus acquisition source employing crude antisera of TobRSV from and a virus-free check plant for each pop- G. Bruening or TomRSV J. Hoy on a 1% ulation, one pot for each of five treatments agar gel. Serological cross reactions do not and seven locations for a total of 35 pots. occur between these viruses (8), so these The virus acquisition source plants and tests also provided confirmation that only check plants were potted in 0.5-liter pots the correct virus was transmitted. filled with sand and loam (9:1) and placed To assess the viability of the nematodes in wood-chip filled bins. Nematodes were transferred, each transmission series was allowed to feed on the roots of acquisition checked at the end of the 3-week period plants for 3 weeks and then were extracted for the percentage of healthy nematodes by means previously mentioned in this (23) and again at 20 weeks to check for paragraph. All transfers were made via a colonization. Nematodes were considered suction micropipette rather than hand healthy if they were able to pass through picking to speed the transferal process and the collection apparatus and if they were minimize any potential damage caused by free of obvious fungal hyphae. Extraction the contortions that the lengthy nema- procedures followed previously outlined todes undergo on a pick. protocols. The vector capability experi- Aliquants of 25 nematodes from the vi- ment was conducted only once.