First report of tomato ringspot virus infection of Krymsk 86 (Prunus cerasifera x P. persica), a hybrid rootstock, in California

Authors: REDDY, KISHOREKUMAR - University Of California, Davis; OTT, NATALIA - University Of California Agriculture And Natural Resources (UCANR); CHAVIRA, SHERIDAN - University Of California, Davis; WESTPHAL, ANDREAS - University Of California, Riverside; Sudarshana, Mysore

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2024
Publication Date: 12/3/2024
Citation: Reddy, K., Ott, N.J., Chavira, S., Westphal, A., Sudarshana, M.R. 2024. First report of tomato ringspot virus infection of Krymsk 86 (Prunus cerasifera x P. persica), a hybrid rootstock, in California. Plant Disease. 108(12):3665. https://doi.org/10.1094/PDIS-09-24-1894-PDN.
DOI: https://doi.org/10.1094/PDIS-09-24-1894-PDN

Interpretive Summary: A newly introduced rootstock Krymsk86 has been increasingly used for new plantings of prune. In the spring of 2023, an outbreak of prune brownline disease in prune trees grafted on Krymsk86 occurred in Northern California. An investigation was initiated and confirmed that the Krymsk86 rootstock was susceptible to tomato ringspot virus (ToRSV). This virus is not on the list of viruses that are required to be tested for in the registration and certification program of prunus planting material in California. This report is expected to help the growers make science-based decisions in choosing rootstock and appropriate phytosanitary certification to manage diseases caused by ToRSV.

Technical Abstract: California produces 99% of prunes (Prunus domestica) in the US, valued at $148 million and accounting for 40% of world prune production. In late spring of 2023, ‘Improved French’ prune trees in two Northern California counties, grafted on Krymsk 86 (K86; P. cerasifera x P. persica) and Lovell (P. persica), rootstocks, were found declining with pale colored canopy. The percentage of affected trees in one orchard was 3.62% (n=1,824) and 4.55% (n=1,295) in trees on K86 and Lovell, respectively. Examination of the trunk revealed a necrotic brown line at the graft union typical of prune brown line (PBL) disease as described in the 1980’s (Mircetich and Hoy, 1981; Supplementary Figure 1). PBL is caused by infection of the rootstock grafted with Prune by tomato ringspot virus (ToRSV; Nepovirus lycopersici, Family: Secoviridae). The virus is vectored by dagger nematodes (Teliz et al. 1967). Subsequent studies revealed that Marianna 2624 rootstock (P. cerasifera x P. munsoniana) was resistant to the virus (Hoy and Mircetich 1984) and hence recommended where ToRSV was endemic. However, propensity of this rootstock to produce suckers is a substantial orchard management issue. In the last decade, the rootstock K86 has gained increasing popularity because of its graft compatibility with a wide variety of Prunus crops and excellent anchorage. To confirm infection by ToRSV, RNA from leaf samples, feeder roots, and cambial tissue from bark samples from scion and rootstock of one declining tree each on K86 and Lovell, were obtained using RNeasy Plant Mini kit (www.qiagen.com), and tested by one step reverse transcription-polymerase chain reaction (RT-PCR) assay using the primers described previously (Tang et al., 2014). The reaction conditions included RT at 54oC using random hexamers and Superscript III (Thermo Fisher Sci., USA), followed by a denaturation step at 95°C for 5 min, and 35 amplification cycles of 95°C for 30 s, 55°C for 30 s, and 72°C for 45 sec; and a final extension of 72°C for 5 min. The expected size amplicon (176 bp) was visualized by agarose gel electrophoresis only from RNA obtained from the cambial scrapings of bark tissue from the rootstocks (Supplementary Figure 1 B). The amplicons were purified using a gel extraction kit (Qiagen Inc., Valencia, CA), and sent for Sanger sequencing (Azenta Life Sciences (South Plainfield, NJ, USA). BLASTn searches using the sequences revealed the presence of ToRSV in the diseased trees. RNA from cambial tissue of bark from Krymsk 86 of 10 symptomatic trees were tested and 9 of them tested positive in RT-PCR tests. In addition, extracts from bark scrapings of three symptomatic trees were also tested using immunostrips for ToRSV (www.agdia.com) and all three trees tested positive. Our results indicate that ToRSV can infect K86 and prune trees grafted on this rootstock are prone to PBL where ToRSV is endemic. This rootstock originated from the Krasnodar region of Russia, and it was never determined if this hybrid rootstock was susceptible to ToRSV or not. Even though ToRSV and its nematode vectors enjoy a wide host range that includes several crop plants and weeds, this virus is not currently part of registration & certification for phytosanitary certification of prunus species in the state of California. This is the first report of ToRSV infection of the K86 rootstock and it is important to be aware of this while adopting this rootstock in orchards where the virus is endemic.