My research focuses on understanding viruses of small fruit crops, developing diagnostic tests for viruses, understanding the epidemiology of viruses to develop control strategies, developing resistance to viruses of small fruit crops through genetic engineering and identifying resistance genes to be used in breeding programs when engineering resistant plants.
Characterization and control of virus diseases of small fruit crops.
The Pacific Northwest is a major producer of small fruit crops, with combined small fruit acreage over 130,000 for the states of Idaho, Oregon and Washington . There are thousands of growers producing small fruit crops, dozens of processing/packing companies, and over 700 wineries in the region. The yield and quality of small fruit crops are adversely affected by viruses that are transmitted by soil-borne and aerial vectors. Some of these viruses can be limiting factors in production when they occur in single infections but mixed infections are more common in production fields and often result in various nondescript "decline" symptoms that can range from minor to 100% crop loss. In addition, there are many uncharacterized diseases of these crops that are caused by "graft transmissible agents" believed to be viruses.
Since these crops are vegetatively propagated, production of virus-free plants requires effective control strategies during the 2-4 years the plants are propagated for increase in nurseries before they are planted in production fields. Improved virus detection and identification techniques are needed to improve testing for quarantine, certification programs and identification of inoculum sources. Current detection protocols for uncharacterized viruses are time consuming, requiring graft indexing onto several indicators which takes from several months to years to complete.
During the past 5 years we have made an effort to develop diagnostics for viruses known to cause diseases in strawberry, red and black raspberry and blackberry. In the case of strawberry we now have laboratory based tests for 22 of the known viruses reported in strawberry with only strawberry pseudo-mild yellow edge virus and strawberry latent C still lacking sequence information. Our approach has been to purify double stranded RNA from infected plants and use this as starting material for cloning and sequencing. This has been very successful for virus of strawberry and Rubus spp. but we have had very limited success with dsRNA extraction from blueberry and cranberry. With this strategy we have identified and characterized several new viruses in strawberry.
In the case of Rubus species, with collaborators in Arkansas, North Carolina, Germany and Scotland, we have identified four new viruses in blackberry and two each in red and black raspberry and developed RT-PCR tests for each of these viruses. There are several emerging virus diseases of Rubus spp . in the US, including the decline of black raspberry in the Pacific Northwest and Midwest, the decline and death of blackberry in the South and Southeast caused by a virus complex and an increasing problem with crumbly fruit in red raspberry and blackberry in all production areas.
The identification of Strawberry pallidosis associated virus as part of a decline complex in strawberry in California was surprising as this was thought to be a minor virus previously. Recent work in collaboration with colleagues at Salinas, CA, has shown that this virus along with at least two related viruses are transmitted by whiteflies in strawberry and a component of a complex of aphid and whitefly transmitted viruses that caused a severe decline in strawberry in California from 2002 to 2004. A similar decline of strawberry in the Pacific Northwest , which has been a severe problem since 2000 is caused by a complex of four aphid-transmitted viruses. The whitefly transmitted viruses were rarely found in the Pacific Northwest .
Blueberry scorch virus was first found in the Fraser Valley of British Columbia in the summer of 2000 and has since been identified in cranberry . Since then it has been determined that multiple strains of the virus occur in B.C. and the resulting disease is much more severe than in Oregon and Washington . The severe strain(s) of this virus poses a threat to the blueberry industry in the PNW. Understanding the vectors of these severe strains and identification of native vegetation that may serve as hosts are needed to develop an appropriate control strategy, which is especially important in B.C. and northern Washington . Efforts to eradicate this virus from Oregon are underway.
Raspberry bushy dwarf (RBDV) has been a reemerging virus in the Pacific Northwest and wherever Rubus species are grown over the past 15 years. In 1997, RBDV was found for the first time in 'Marion' blackberry in the Pacific Northwest . 'Marion' has been a widely grown cultivar in this area since the early 1960's. The affect of RBDV on 'Marion' is dramatic, drupelet number, fruit size and yield was reduced by 35%, 25% and 40%, respectively, but did not affect cane number or length. The virus also causes a general chlorosis in the leaves of infected plants that is not expressed early in the season, but is very obvious by early July. The incidence of RBDV in 'Marion' blackberry has been increasing and several fields have been removed due to problems with fruit quality. We have developed RBDV resistant 'Meeker' red raspberry through genetic engineering. Of 249 transgenic lines evaluated, 24 lines exhibited some level of resistance. The resistance of the best line has withstood repeated graft inoculation and six years of exposure to extreme disease pressure in a field situation where 202 of 202 wild-type plants were infected after three years.
Work on a serious disease of black raspberry (Rubus occidentalis) in Oregon demonstrated the presence of Black raspberry necrosis virus in declining plants. Plants showing mosaic symptoms declined rapidly and in many cases died over a span of 2-4 years. The virus has two RNAs and is most similar to Strawberry mottle virus . It is transmitted by the large raspberry aphid (Amphorphora agathonica) and the green peach aphid (Myzus persicae). A diagnostic test based on polymerase chain reaction (PCR) was developed and used to identify the virus in transmission studies with the two aphids listed above and in alternate hosts. The virus infects red raspberry, commercial and wild blackberry and surprisingly bracken fern symptomlessly, which causes problems for growers trying to control the virus.