WERA020: Virus and Virus-Like Diseases of Fruit Trees, Small Fruits, and Grapevines.
Statement of Issues and Justification
The membership of WERA-0020 represents specialty crops (fruit and nut trees, berries, grapes and pineapple) that occupy a significant portion of U.S. agriculture. In 2008, these perennial specialty crops had a farm gate value of $18 billion (NASS, 2009). The impact of this production on the U.S. economy is further amplified by the multiplier effect through agriculture-related industries such as spray, fertilizer, equipment, transportation, marketing and processing. However, diseases caused by viruses and virus-like agents can have devastating impacts on fruit production. Examples can be found in all sectors of perennial specialty crop production, but it can be illustrated by the sweet cherry industry on the west coast. In the 1940s, severe disease problems plagued Washington and California growers. Serious virus diseases affected 7.1% of all sweet cherry trees in Washington State and 35% of these trees were removed each year in an attempt to stop disease spread and to restore production. This level of tree removal translates to a loss to growers of more than 50% of the total farm gate value of production arising from the cost of tree removal, replanting and lost fruit yield. The unsustainable level of income loss was addressed by the creation of clean plant programs, and by research projects that provide a better understanding of the pathogens and the vectors that transmit them. These programs significantly reduce damage by viruses. It is estimated that the loss of value of the sweet cherry crop due to viruses was down to approximately 1.9% in 2003. Nevertheless, diseases caused by virus and virus-like organisms continue to be significant factors affecting tree fruit and berry industries in the United States and Canada and new disease threats continue to emerge. Further efforts are needed to help the affected industries remain competitive in the international market place. This project contributes to that goal by providing a forum for information exchange at annual meetings and by establishing contacts that encourage communication and collaboration for containing the spread of debilitating diseases.
Crop loss due to virus-induced disease is dynamic: management strategies are developed for some diseases while other diseases emerge. The world-wide movement of plant products exposes specialty crop production areas to new disease pressures. Rapid changes in disease pressure are also being imposed by changes in populations of the insects that transmit disease. Transient or long term changes in weather patterns and changes in horticultural practices are resulting in vector population shifts, and the rapid movement of goods around the globe can quickly disseminate insect vectors into new production areas. This is exemplified by some examples of disastrous introductions over the past twenty years. Globally, the aphid-transmitted plum pox virus is the most economically important virus of stone fruits. The discovery of this virus in North America for the first time in the U.S. in 1999 and in Canada in 2000 had an immediate impact on the tree fruit and nursery industries. As of 2010, more than $120 million and $180 million have been spent in the U.S. and Canada, respectively, in attempts to detect, eradicate, and/or manage the virus. Yet the source of the introduction into North America remains unknown. The accidental introduction of the glassy winged sharp shooter into California in 1990 caused devastation of the grape industry in the southernmost counties of the state by transmitting the bacterial agent causing Pierces disease that existed in the area. Citrus greening is one of the more serious diseases of citrus. The vector of this disease, Asian citrus psyllid, was introduced into the southeast U.S. in 1998 and is now present in more than a dozen states. The bacterial pathogen that causes citrus greening was detected in 2005 and action is being aggressively pursued to prevent the psyllid vector from spreading the disease beyond its current limits. Other disease issues evolved over many years. Citrus tristeza virus (CTV) is a major limitation to citrus production worldwide and efforts to control this disease in California and Arizona have been critical to maintaining the viability of this fruit tree industry in the western United States. Raspberry bushy dwarf virus causes serious yield and quality losses in Rubus spp. and Blueberry scorch carlavirus is emerging as a major disease problem for blueberry production in the Pacific Northwestern and Northeastern U.S. Western X disease, pear decline, and peach yellow leafroll are diseases caused by phytoplasmas and seriously reduce the production of stone and pome fruits in the western region of the U.S. These and other phytoplasmas cause diseases that are also associated with the eastern peach industry (yellows, rosette, and red suture). They occur erratically but are devastating when present. In Europe, the geographic range of the leafhopper vector of the phytoplasma that causes Bois noir disease of grapevines has expanded northward dramatically and increased incidence of the disease quickly followed. Disease caused by phytoplasma may spread in the new world following a similar course as global climatic conditions shift.
WERA-0020 is uniquely positioned to provide rapid access to expertise to address the changing landscape of diseases caused by viruses and virus-like agents. WERA-0020 brings together research, extension, and state and federal regulatory activities on economically important fruit crops so these activities are sustained despite a critical mass of researchers that is woefully insufficient for the needs of the industries. WERA-0020 is effective in providing information and solutions to disease problems and transferring technologies between states and across provincial borders. Many of the solutions for the detection, identification, and management of viral diseases are the same in all of the crops represented by WERA 0020. Open discussion between members and guests of all facets of virus disease control generate and stimulate appropriate approaches for solving disease management problems.
The Farm Bill - H.R. 6124 Food, Conservation, and Energy Act of 2008 established the National Clean Plant Network (NCPN), a partnership among clean plant centers and state and federal regulatory agencies to provide propagation material of perennial specialty crops throughout the U.S. Thus, the goals of WERA-0020 and the NCPN are parallel and assist specialty crop management to reduce the economic impact of virus threats. WERA-0020 is assisting the NCPN by locating and maintaining voucher samples of many diseases in research collections and by identifying pathogens of regional concern for consideration by the NCPN. The research produced by the WERA-0020 membership contributes directly to improved diagnostic capabilities of clean plant centers functioning within the NCPN and developing strategies for mitigating the negative impacts of virus and virus-like diseases.
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