- There are over 35 known virus and virus-like diseases of sweet cherry (Prunus avium), some with potential to cause severe economic impact by reducing vegetative growth, vigor, and/or quality of fruit. Oregon is the second-ranked state for sweet cherry production in the United States with 13,000 acres and $70.8 million in production value. Statewide surveys were conducted in Oregon sweet cherry orchards to gain perspective on virus and virus-like diversity and distribution. Orchards in key production regions with suspected virus disease symptoms were sampled. Virus-specific ELISA, isothermal amplification, or qPCR were used to test for the presence of common and/or economically important sweet cherry pathogens: cherry leaf roll virus (CLRV) , cherry rasp leaf virus (CRLV), little cherry virus 2 (LChV2), plum pox virus (PPV), prune dwarf virus (PDV), prunus necrotic ringspot virus (PNRSV), tobacco ringspot virus (TRSV), tomato ringspot virus (ToRSV), and the X-Disease phytoplasma (XDP). Cherry leaf roll virus, a new virus of sweet cherry in Oregon, was found associated with enation and dieback symptoms in The Dalles. Some viruses were found in new regions, which included: Hood River (PDV, PNRSV, ToRSV) and the Umpqua Valley (PDV, PNRSV). A subsequent survey was conducted in the Mid-Columbia production region for the presence of little cherry symptoms associated with Little Cherry and X-Diseases. All symptomatic samples from The Dalles, Mosier, or Dallesport, WA, tested positive for XDP, but not LChV2. No samples from Hood River tested positive for either pathogen.
The initial statewide survey identified the presence of the dagger nematode (Xiphinema spp.) transmitted tomato ringspot virus (ToRSV) in Hood River, The Dalles, and the Grande Ronde Valley. Little is known about this virus on sweet cherry. ToRSV-infected commercial sweet cherry orchards at two locations in The Dalles were assessed for stem-pitting, foliar enations, rosetting, dieback and rapid decline symptoms. Visual assessments of symptoms on trees were made by ranking severity on a 0 to 4 scale (0 = no symptoms, 4 = most severe). Leaves were collected from a subregion of trees in the orchards for diagnostic testing using a ToRSV-specific double-antibody sandwich ELISA. A smaller section was selected for nematode sampling within the subregion. A composite of four soil cores from around each tree base was collected. Nematodes were extracted by Baermann funnel or sucrose flotation and centrifugation for identification with microscopy. Visual symptoms of enations (gall-like formations on the underside of the leaf), as well as rosetting (bunching of leaves due to shortened internodes) were found in aggregated clusters in the orchards and were positively correlated with ToRSV-infected trees. A low percentage (1 to 6%) of asymptomatic samples were also ToRSV-positive, highlighting the importance of virus testing. Dagger nematodes were present in soil near trees with and without positive ToRSV diagnoses.
Host-specificity of the ToRSV sweet cherry isolate was evaluated by interplanting various small and tree fruit hosts (apple, peach, cherry, blueberry, raspberry, and grapevine) and cucumber in a known ToRSV-infected sweet cherry orchard with dagger nematodes. After 18 months, none of the small or tree fruits tested positive for ToRSV. Cucumber bait plants did not test positive for ToRSV after one month.
Genetic diversity of ToRSV isolates collected from small and tree fruits hosts in the western United States was assessed. A ToRSV sweet cherry isolate from OR was sequenced and produced a highest match to a ToRSV isolate from grapevine (MF176959.1). Phylogenetic analyses produced two distinct clades for 22 isolates but did not produce clustering according to isolate host or geographic location.
Overall, this dissertation provides a foundation for the current understanding and management of virus and virus-like diseases of sweet cherry in Oregon and provides context for further studies into the most threatening virus and virus-like pathogens and their vectors.