2019 ASHS Annual Conference

Landscape rose production is a $200 million dollar/year industry in the United States. Roses, like other landscape ornamentals, are typically grown in production fields with close spacing (6-12 inches) to allow for the maximum amount of plants in an area. While this close planting situation is ideal for mass production, it can allow for the rapid spread of diseases in production fields. One disease that is troubling rose growers in much of the country is Rose Rosette Disease (RRD). This disease is a viral disease caused by Rose rosette virus (RRV), which has a primary mode of plant to plant transmission through an arthropod vector, Phyllocoptes fructiphilus, an eriophyid mite. However, research has shown that there are other modes of transmission for RRV, such as grafting. Another mode of RRV transmission that has been suggested since the 1950’s is the movement of the virus through root contact or root grafting. Root transmission has been demonstrated in roses using other viral diseases. After the discovery of RRV in rose production fields in 2016-2017, there were questions of potential root transmission. Our study utilized 100 plants from this production field: 10 sets of 10 adjacent plants. Each set contained five plants (spaced at 7, 14, 21, 28 and 35 inches) radiating from a RRV positive plant. These plants were harvested from the field, prepared as bare rooted transplants, and were potted in individual pots and monitored over a one year period. Plants were regularly monitored for mites and preventative miticide applications were used. No mites were found during the experiment. Of 100 plants, 17 developed RRD symptoms and had positive PCR tests within six months. At the termination of this study, RRV was detected via PCR in 56 additional asymptomatic plants, suggesting that plants may have been infected prior to harvest. Many positive plants were 35 inches from the initial plant, leading us to question the RRV transmission method. Natural mite distribution is random in a field, so it is unlikely that mites were evenly disbursed. Also, field data collected suggests that eriophyid mite populations were aggressively managed, suggesting the possibility of alternate transmission types. We are exploring root transmission of RRV; understanding all modes of RRV transmission will allow producers to make more informed decisions in developing production planting designs. Such information may also provide guidance to how many adjacent plants should be rogued when an infected plant is discovered.