Relationships Between the Pathogen Olpidium virulentus and Viruses Associated with Lettuce Big-Vein Disease.

Lettuce big-vein disease (LBVD) is a widespread problem in commercial lettuce producing areas. It was first reported in 1934 in California. In the United States in 2009, it was estimated that 100% of all commercial lettuce production was concentrated in California (in early spring) and Arizona (in winter). The disease also occurs worldwide, particularly in regions in Australia, Europe, Brazil, and New Zealand, and at high altitudes in subtropical regions. LBVD often results in decreased yield from infected plants, mainly in winter growing seasons. Symptoms of this disease are the obvious chlorophyll clearing, which causes the big-vein leaf symptoms from which the disease gets its name, along with crinkled leaves and a reduced head size, which is often oblong in shape. LBVD is associated with a complex of two viruses, Lettuce big-vein associated virus (LBVaV) and Mirafiori lettuce big-vein virus (MLBVV). Motile zoospores of Olpidium virulentus vector LBVaV and MLBVV to the roots of healthy plants and, in the absence of susceptible hosts, its resting spores retain the ability to harbor these viruses for decades in infested soil and in dry roots for 39 months. The ability of viruliferous resting spores to persist in production areas for such extended periods of time, and also the motility of viruliferous zoospores, pose significant difficulties for the management and/or control of the disease.