A rating system for leaf pubescence is needed to characterize cotton cultivars having intermediate densities of leaf trichomes. We propose a system that is based on visual examination of the relative density, distribution and length of abaxial trichomes. Our objectives were to evaluate trichome density associated with this proposed leaf pubescence rating system, to evaluate the variation of trichomes on leaves located at different positions on the main stem, and to compare leaf pubescence ratings of cultivars made over locations and years. Number of trichomes (defined as total number of trichomes and trichome branches) associated with each leaf rating was determined from leaves sampled over three years. Leaves of contrasting cultivars were also sampled from the top, middle and bottom of plants in 1995 and 2001. Pubescence of each leaf was rated both years, and trichomes were counted in the 2001 tests. Leaf pubescence of cultivars in state variety tests was rated over locations and years. Differences in density of trichomes between leaf pubescence ratings were found. Pubescence ratings and trichome counts were highest for upper canopy leaves. Trichomes became less dense as leaves enlarged, then tended to abscise as leaves aged, therefore leaf pubescence should be rated using the youngest, fully expanded main-stem leaves. Within each field test, leaf pubescence ratings varied significantly among cultivars and were consistent for cultivars at different locations and across years. These data indicate that the rating system has a morphological basis and can be used to characterize the leaf pubescence of cotton cultivars.
