Cross- vs. self-pollination in ‘Hass’ avocados growing in coastal and inland orchards of Southern California

Abstract

The dynamics of pollen transfer from complimentary pollinizing avocado cultivars and pollen deposition onto and within flowers of ‘Hass’ avocado trees growing in southern California was examined. Experiments were conducted during the flowering seasons of 2001, 2003, 2005, and 2006. Three commercial ‘Hass’ (an A-type avocado) orchards interplanted with complimentary, B-type cultivars were chosen in locations representing cool, humid or warm, dry “Mediterranean” conditions. Replicate cages were constructed of 40% saran shade cloth, which prevented penetration of bees and other flying insects larger than 2 mm to the ‘Hass’ flowers but permitted passage of drifting pollen. They were sown to form the rectangular cage roof, walls and floor and supported by a pipe framework. Four cages were installed over the canopies of ‘Hass’ tree branches at the humid coastal site and at the inland, dry sites during February or early March of each year. Enclosure was accomplished during floral bud development but before the beginning of floral openings. At all locations, each caged and open-pollinated partner was staged next to each other with at least two non-observed buffer trees between the pair. The conclusions of the work were that self-pollination within Stage 2 flowers was the dominant mode of pollination at all the humid, coastal and the dry, inland test sites. Moreover, it was determined that pollen transfer between cultivars was mediated by wind, and bees had a negligible role in pollen transfer despite high forage activity. Temperatures that were marginally warm enough to allow normal floral opening and closing behavior were still insufficient to allow pollen tube growth to the ovule before abscission of the flowers. These results provide the basis for understanding why growers utilizing solid block avocado plantings in some areas achieve good yields without bees.