Disease Note.

Mechanical Transmission of Prunus Necrotic Ringspot Virus to Young Trees of Nemaguard Peach and Nanking Cherry. J. K. Uyc-molo, USDA-ARS and University of California, Davis 95616. A. Rowhani, and C. F. Luhn, USDA-ARS and University of California, Davis 95616. Plant Dis. 80:104. Accepted for publication 30 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0104D.

Field isolates of Prunus necrotic ringspot virus (PNRSV) were obtained from almond (P. dulcis (Mill.) D. Webb) and nectarine (P. persica (L.) Batsch var. nectarina (Aiton) Maxim.) trees by sap-inoculations on cucumber seedlings (Cucumissativus L). The virus isolates were partially purified from cucumber tissues and the preparations stored at -22°C in TAE (40 mM Tris, adjusted to ph 7.8, with 1 M acetic acid, and then add EDTA to 1.0 mM) buffer-glycerol mixture. Prior to use, the virus preparations were diluted to OD260 of 5.0 with TAE buffer. One-year-old test seedlings of Nema-guard peach (P. persica (L.) Batsch var. nucipersica (Suckow) C. K. Schneid.) and Nanking cherry (P. tomenlosa Thunb.) were assayed with enzyme-linked immunosorbent assay (ELISA) (2) for PNRSV and prune dwarf virus presence, and were determined to be virus-free. Inoculations of the test seedlings were completed either by making multiple deep slashes (1) into the bark with a contaminated razor blade (cuts made through three separate 25-µl droplets of virus inoculum or TAE buffer), or by rubbing the exposed cambium surfaces on lifted bark flaps and exposed woody cylinders (three per tree) with the tip of a glass rod dipped in inocula bark flaps were repositioned onto the tree stem and tied with a rubber wrap. Also, cotyledons of cucumber seedlings were dusted with corundum abrasive, rubbed with virus inocula or buffer control, and plants recorded for symptoms development at 12 to 14 days postinoculation (PI). Based on the number of diseased cucumber plants, infectivity potentials of the virus inocula for PNRSV isolates from nectarine and almond were 50 and 85%, respectively. Cucumber plants rubbed with buffer controls remained healthy. Initial disease symptoms on the inoculated test seedlings were observed approximately 8 weeks PI and consisted of mottling and shot-holing (shock symptoms) of small clusters of leaves. With PNRSV-almond, three Nemaguard (total 10 inoculated) and three Nanking (five inoculated) test seedlings developed symptoms two trees were infected by razor-blade slashes and four by the cambial-rubbing procedure. With PNRSV-nectarine, only one razor-slashed peach tree became infected. Virus infections in all symptomatic trees were confirmed by ELISA (2) with A405 values ranging from 0.25 to 1.40 for virus-inoculated trees and 0.01 for buffer-control trees and by positive responses when diseased bud grafts were conducted on Shirofugen flowering cherry (P. serrulata Lindl), a known indicator host for PNRSV. Following tree dormancy and budbreak, all trees were reassayed by ELISA and two additional Nanking cherry tested positive: both razor-inoculated and one each of PNRSV-almond and -nectarine PNRSV isolates. These results indicate that PNRSV can be easily transmitted to perennial host plants by slash cuts in bark tissues or by rubbing inocula onto cambial surfaces of the bark and underlying woody cylinder.