Prune Brown Line Disease Occurrence in Declining `Stanley' Plum orchards in Michigan and Factors Related to Its Incidence

Abstract

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.