DEVELOPMENT OF A PHENOLOGICAL MODEL OF AVOCADO TREE GROWTH IN CALIFORNIA

Abstract

Mature 'Hass' avocado (Persea americana Mill.) trees four rootstocks (Thomas, Topa Topa, Duke 7, or D9) were monitored from 1992 to 1996 to determine the relative timing of shoot and root growth, bloom, and carbohydrate levels, as well as the relationships between these variables and yield in southern California. Trees exhibited typical alternate bearing patterns with heavy and light crop loads (on and off years, respectively) alternating from year to year. Shoot growth occurred during two distinct flushes each year, one in spring and one in late summer. Although yield varied among rootstocks, neither the rate of shoot growth during flushes nor the total cumulative shoot growth over a season differed among rootstocks. However, cumulative shoot growth was 35 times higher in 1993, when yield was almost nil, as compared to other years. The spring growth flush accounted for the majority of total shoot growth in most years; however, the summer growth flush accounted for more of the total shoot growth during 1993. Root growth did not exhibit dormant periods as shoot growth did, but in general, root growth was greatest when shoots were not actively growing. Crop load did not appear to affect root growth patterns or intensity. Bloom occurred each year from mid-March to mid-May. Bloom did not differ among rootstocks; however, bloom in on years occurred earlier and for a longer period than the bloom in off years. Shoot starch concentrations exhibited greater fluctuations during the year than trunk starch concentrations and were highest immediately before the onset of the spring shoot flush. This information gives us insight into the relative timing of and relationships between growth events of avocado in southern California and will help growers determine the optimal timing of cultural practices.