Efficacy studies on prebloom canopy applications or boron and/or urea to Hass avocados in California

Results of four bloom studies (two glasshouse and two field experiments) demonstrated the efficacy of applying boron or urea sprays to 'Hass ' avocado inflorescences during early expansion (cauliflower stage) but prior to full panicle expansion and anthesis. Anatomical analysis of the flowers provided evidence that the boron prebloom spray increased the number of pollen tubes that reached the ovule (p<0.05) and also increased ovule viability, but to a lesser degree than urea. The urea prebloom spray increased ovule viability compared to boron-treated or untreated flowers (p<0.05). Urea also increased the number of pollen tubes that reached the ovule (p<0.05), but to a lesser degree than boron. However, combining boron and urea resulted in double pistils (p<0.05), even when the urea was applied 8 days after the boron. The efficacy of using boron and/or urea as a prebloom canopy spray to increase fruit set and yield is under continued testing in commercial 'Hass ' avocado orchards in southern and coastal California with low, moderate, and high leaf boron concentrations, for years when anthesis, pollination, fertilization and fruit set occur under optimal versus less than optimal climatic conditions, and for both "on " and "off years of the alternate bearing cycle. The objective of the research is to determine, on the basis of benefit to cost, under what conditions this management strategy should or should not be utilized in California. 1