Field experiments were conducted in 1998 and 1999 at Five Points, CA, in the San Joaquin Valley under irrigated conditions to study competition between four commonly grown tomato cultivars and velvetleaf and to identify cultivar characteristics associated with greater tolerance to velvetleaf. The effect of velvetleaf competition varied with both year and tomato cultivar. When grown with 5 velvetleaf plants m−1 of row, marketable yield of tomato was reduced 8% in 1998 and 60% in 1999 for cultivar H8892 and 58% in 1998 and 80% in 1999 for cultivar H9661, compared to cultivars grown in monoculture. Across velvetleaf densities, height of tomato cultivars was not reduced compared to that of cultivars grown in monoculture. However in 1999, canopy width of tomato cultivars grown with velvetleaf was less than that of cultivars grown in monoculture. At early stages of growth, the leaf area index of tomato cultivars grown with velvetleaf was less than that of cultivars grown in monoculture. Crop growth rate and aboveground dry biomass of tomato cultivars grown with velvetleaf were generally less than those of cultivars grown in monoculture. Yield loss at high weed density was similar among cultivars, whereas yield loss at low weed density varied among cultivars. Cultivar tolerance to velvetleaf varied with year. However, cultivar H8892 had low yield loss and cultivar H9661 high yield loss at low weed density in 1998 and 1999. For cultivar H8892, leaf area expansion rate was also among the greatest for both years.
Nomenclature:Tomato, Lycopersicon esculentum Mill.; velvetleaf, Abutilon theophrasti Medicus ABUTH.