Twenty-one declining `Stanley' prune (Prunus domestica L.) commercial orchards in the southwestern, west-central, and northwestern regions of Michigan's lower peninsula were surveyed for prune brown line disease, associated with tomato ringspot virus (TmRSV). Fifty trees from each orchard were examined for a brown line and pitting-grooving symptoms beneath the bark at the graft union. Inner bark and cambium were taken at the graft union for ELISA testing for TmRSV. Dagger nematodes (Xiphinema americanum Cobb 1913) (the vector) were extracted from soil samples and enumerated. Dandelions (a TmRSV weed host) also were tested for TmRSV. Information on orchard cultural practices and orchard histories was compiled. The percentage of trees ELISA-positive for TmRSV ranged from 4% to 82%, with a mean of 27.9%. The percentage of orchards in the northwestern, west-central, and southwestern regions in which TmRSV was detected by ELISA was l8.0%, 32.3%, and 35.1%, respectively. There was a strong positive correlation between the percentage of trees with a brown line at the graft union and the percentage of trees in which TmRSV was detected at each location. The brown line symptom is a good indicator for the presence of TmRSV, but graft-union pitting and grooving did not correlate strongly with the presence of the virus. TmRSV was detected in dandelion plants in 63% of the orchards tested. Dandelion densities, which ranged from <0.5 to 10/m2, did not correlate positively with percentage of ELISA-positive trees. Numbers of dagger nematodes ranged from 0 to 132 per cm3 of soil. Vector nematode populations correlated positively with ELISA-positive trees from southwestern Michigan, but not in the other two regions. Orchard age, which ranged from 6 to 22 years, did not seem to relate to the percentage of trees in which TmRSV was detected, nor did the source of the plant material used to establish the orchards. Both `Myrobalan' and peach rootstocks were heavily infected. Preplant and at-planting applications of fenamiphos as a strip treatment were ineffective in preventing infection. We believe that TmRSV is endemic in Michigan orchard soils and that the virus is not being introduced to new orchards through the use of infected planting material.
D.C. Ramsdell, V.A. Adler and C.R. Kesner
K.V. Kommineni, J.M. Gillett and D.C. Ramsdell
In a greenhouse study, fifty 1-year-old `Stanley'/`Myrobalan 29C' plum (Prunus sp.) trees were inoculated with tomato ringspot nepovirus (ToRSV) by either nematode inoculation or slash inoculation to compare how inoculation effects the onset of the prune brown line (PBL) disease. In six tests (over 2 years), slash-inoculated trees had a higher percentage of ToRSV infection than nematode-inoculated trees when root and bark samples were tested by enzyme-linked immunosorbent assay (ELISA). ELISA differences between the two treatments were significant by chi-square analysis. None of the ToRSV positives by ELISA developed a brown line at the graft union. In a second experiment, five rootstock (`Myrobalan 29C', `Marianna 4001', `Marianna 2624', `Marianna GF8-1', and `St. Julian 655-2') and five scion (`Carolyn Harris', `New York 58.900.12', `Stanley', `Valor', and `70031') combinations (total combination = 25) were established in a field plot in Traverse City, Mich., and infected with ToRSV by slash and nematode inoculation. All five rootstocks were infected, with incidences of 40% to 60% ToRSV infections after 3 years. `Marianna 2624' had the lowest incidence of PBL (5%) compared to `Myrobalan 29C', which had the highest incidence (30%). The scion 70031 in combination with either `Myrobalan 29C' or `Marianna 4001' rootstocks, produced PBL in 100% of the trees. ToRSV was detected by ELISA and northern hybridization assays. ELISA consistently detected more positives when root or bark tissues were tested, and northern hybridization assay consistently detected more positives when rootstock sucker leaves were used.
Jean Beard Baker, J.F. Hancock and D.C. Ramsdell
One-year-old rooted microshoots and 2-year-old rooted hardwood blueberry cuttings (Vaccinium corymbosum L.) were inoculated with Phomopsis vaccinii Shear using stem flap, stem freeze, needle pierce, and leaf tear wounding techniques. The needle pierce was the simplest method that produced high infection rates. Nine northern-adapted cultivars were placed in a factorial experiment to measure their infection resistance. Microshoots and hardwood cuttings of `Elliott' and `Bluetta' survived the longest and had the lowest mortality rate. Phomopsis vaccinii was reisolated successfully from inoculated shoots of all cultivars.
Theresa Acquaah, D.C. Ramsdell and J.F. Hancock
To determine if blueberry shoestring virus (BBSSV) is absent in the southern United States due to resistance of cultivars, we mechanically and rub-inoculated 1-year-old rooted microshoots of nine cultivars representing southern rabbiteye (Vaccinium ashei Reade), southern highbush (hybrids of V. corymbosum and V. darrowi Camp), and northern highbush (V. corymbosum L.). Leaves were sampled from plants, and enzyme-linked immunosorbent assay screened for the presence of virus over 15 months. Only a few individuals were infected after aphid inoculation, but many northern and southern cultivars became infected after mechanical inoculation. Northern highbush `Elliot' (50%) and `Blueray' (46.3%) had the highest infection rates, followed by rabbiteye `Climax' (36.3%) and the southern highbush `O'Neal' (12.5%). The lowest rates of infection were found in southern highbush `Georgiagem' (2.5%), `Misty' (2.5%), rabbiteye `Brightwell' (0.0%), and northern highbush `Bluecrop' (2.5%). Since many southern cultivars were infected by the disease, resistance likely has not excluded BBSSV from the southern United States.
J.F. Hancock, P.W. Callow, S.L. Krebs, D.C. Ramsdell, J.R. Ballington, M.J. Lareau, J.J. Luby, G.P. Pavlis, M.P. Pritts, J.M. Smagula and N. Vorsa
Flower bud and leaf samples collected from a wide range of native North American Vaccinium populations were tested for the presence of blueberry shoestring virus (BBSSV) using the enzyme-linked immunosorbant assay. The highest disease incidence was found in Michigan (14%), although a few positive samples also were found in Virginia, New Jersey, Maine, Ontario, and Quebec. Of seven species tested, only V. corymbosum L. and V. angustifolium Ait. were infected with BBSSV.